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We present the design and testing of a 16-channel analog amplifier for processing neural signals. Each channel has the following features: (1) variable gain (70-94 dB), (2) four high pass Bessel filter poles (f(-3 dB)=445 Hz), (3) five low pass Bessel filter poles (f(-3 dB)=6.6 kHz), and (4) differential amplification with a user selectable reference channel to reject common mode background biological noise. Processed signals are time division multiplexed and sampled by an on-board 12-bit analog to digital converter at up to 62.5k samples/s per channel. The board is powered by two low dropout voltage regulators which may be supplied by a single battery. The board measures 8.1 cm x 9.9 cm, weighs 50 g, and consumes up to 130 mW. Its low input-referred noise (1.0 microV(RMS)) makes it possible to process low amplitude neural signals; the board was successfully tested in vivo to process cortically derived extracellular action potentials in primates. Signals processed by this board were compared to those generated by a commercially available system and were found to be nearly identical. Background noise generated by mastication was substantially attenuated by the selectable reference circuit. The described circuit is light weight and low power and is used as a component of a wearable multichannel neural telemetry system. more...

We present the design, testing, and evaluation of a 16 channel wearable telemetry system to facilitate multichannel single unit recordings from freely moving test subjects. Our design is comprised of (1) a 16-channel analog front end board to condition and sample signals derived from implanted neural electrodes, (2) a digital board for processing and buffering the digitized waveforms, and (3) an index-card sized 486 PC equipped with an IEEE 802.11b wireless ethernet card. Digitized data (up to 12 bits of resolution at 31.25k samples/s per channel) is transferred to the PC and sent to a nearby host computer on a wireless local area network. Up to 12 of the 16 channels were transmitted simultaneously for sustained periods at a range of 9 m. The device measures 5.1 cm x 8.1 cm x 12.4 cm, weighs 235 g, and is powered from rechargeable lithium ion batteries with a lifespan of 45 min at maximum transmission power. The device was successfully used to record signals from awake, chronically implanted macaque and owl monkeys. more...

To analyse the information content of multi-unit signals, cases of two and three superimposed neural responses to a stimulus were simulated. The multi-unit Shannon information rates were compared with those of the constituent single-unit spike trains and also with those of corresponding labelled line signals. The mutual information in the multi-unit response was found to depend on the degree of similarity in the attribute-specific information of the single-unit spike trains. As long as the units were encoding similar attribute-specific information, the multi-unit signal had greater information than any of the individual units. If the units' responses were confounded by different amounts, the multi-unit response could not recover the maximal single-unit information. Labelled line and pooled responses had similar mutual information when the information in the constituent units was similar. However, the normalized gain in information of a labelled line response over a pooled response increased from 27% to 84%, relative to the maximum single-unit information rate, as the difference in constituent single-unit information increased to 100%. Information in single-unit responses is not completely lost when multiple units are superimposed. There are cases in which the union of single-unit spike trains can fully conserve single-unit information and even reduce the effect of confounding information present in the individual responses. More information is available in pairs of responses when the identities of the units are maintained. Summed responses can be used at low informational cost in redundantly encoding clusters of neurons. Higher information losses are incurred as the encoding becomes more independent. more...

We have developed dual lumen probes incorporating a forward-viewing matrix array transducer with an integrated working lumen for delivery of tools in real-time 3-D (RT3-D) interventional echocardiography. The probes are of 14 Fr and 22 Fr sizes, with 112 channel 2-D arrays operating at 5 MHz. We obtained images of cardiac anatomy and simultaneous interventional device delivery with an in vivo sheep model, including: manipulation of a 0.36-mm diameter guidewire into the coronary sinus, guidance of a transseptal puncture using a 1.2-mm diameter Brockenbrough needle, and guidance of a right ventricular biopsy using 3 Fr biopsy forceps. We have also incorporated the 22 Fr probe within a 6-mm surgical trocar to obtain apical four-chamber ultrasound (US) scans from a subcostal position. Combining the imaging catheter with a working lumen in a single device may simplify cardiac interventional procedures by allowing clinicians to easily visualize cardiac structures and simultaneously direct interventional tools in a RT3-D image. more...

We have developed, manufactured, and tested two analog CMOS integrated circuit "neurochips" for recording from arrays of densely packed neural electrodes. Device A is a 16-channel buffer consisting of parallel noninverting amplifiers with a gain of 2 V/V. Device B is a 16-channel two-stage analog signal processor with differential amplification and high-pass filtering. It features selectable gains of 250 and 500 V/V as well as reference channel selection. The resulting amplifiers on Device A had a mean gain of 1.99 V/V with an equivalent input noise of 10 microV(rms). Those on Device B had mean gains of 53.4 and 47.4 dB with a high-pass filter pole at 211 Hz and an equivalent input noise of 4.4 microV(rms). Both devices were tested in vivo with electrode arrays implanted in the somatosensory cortex. more...

We describe preliminary experiments on controlling in vivo atrial fibrillation using a closed-loop feedback protocol that measures the dynamics of the right atrium at a single spatial location and applies control perturbations at a single spatial location. This study allows investigation of control of cardiac dynamics in a preparation that is physiologically close to an in vivo human heart. The spatial-temporal response of the fibrillating sheep atrium is measured using a multi-channel electronic recording system to assess the control effectiveness. In an attempt to suppress fibrillation, we implement a scheme that paces occasionally the cardiac muscle with small shocks. When successful, the inter-activation time interval is the same and electrical stimuli are only applied when the controller senses that the dynamics are beginning to depart from the desired periodic rhythm. The shock timing is adjusted in real time using a control algorithm that attempts to synchronize the most recently measured inter-activation interval with the previous interval by inducing an activation at a time projected by the algorithm. The scheme is "single-sided" in that it can only shorten the inter-activation time but not lengthen it. Using probability distributions of the inter-activation time intervals, we find that the feedback protocol is not effective in regularizing the dynamics. One possible reason for the less-than-successful results is that the controller often attempts to stimulate the tissue while it is still in the refractory state and hence it does not induce an activation. (c) 2002 American Institute of Physics. more...

This article investigates the quantitative predictive capabilities of region-specific models by comparing experimental electrograms obtained from in vivo mapping of the ventricular free wall with those obtained through simulation of a region specific three-dimensional bidomain model that incorporates measured fiber orientations. Epicardial electrograms were recorded from canine left ventricles during and after unipolar pacing using a 528-channel electrode plaque. Fiber directions throughout the tissue were estimated from diffusion-weighted MRI and from pace mapping. Electrograms were computed in the bidomain model with experimentally derived properties during paced activations at the same spatiotemporal resolution as those recorded experimentally. Epicardial potentials from model and experiment were directly compared, and sensitivities of these comparisons to reference electrode location and to the choice of material properties were analyzed. The comparisons performed here demonstrate, that (1) the stimulus artifact can be used to estimate the in vivo myocardial fiber architecture, (2) the correlation between simulated and experimental electrograms decreases with increasing pacing depth, and (3) the quantitative comparisons between bidomain model and experimental data are sensitive to both the description of the fiber architecture, and the location of the unipolar reference electrode, but relatively insensitive to moderate changes in the bidomain conductivities. more...

We investigated the feasibility of the guidance of pacemaker lead implantation using the pacemaker lead stylet as an acoustic wave-guide combined with real time 3D ultrasound imaging. In one approach, with a 2.5 MHz transducer coupled to a stylet of a pacemaker lead, we used the stylet as a transmitter to track the vibrating tip in a 3D ultrasound scan. In another approach, we connected the stylet to a piezoelectric actuator vibrating in the range 0.5-5 kHz so that the tip of the stylet was imaged using the color Doppler feature of the real time 3D ultrasound scanner. In both approaches, tracking of the isolated stylet showed good accuracy. However, neither approach offered sufficient signal-to-noise ratio to detect the vibration within the lumen of an intact pacemaker lead. more...

The authors tested the feasibility of real-time three-dimensional intracardiac echocardiography for guidance of interventional electrophysiological studies. The three-dimensional scanner uses a matrix array ultrasound transducer of 64 channels operating at 5 MHz in a 12 Fr catheter. The system features real-time three-dimensional image rendering and produces up to 60 volumetric scans per second. Using an open-chest sheep model, real-time three-dimensional images of anatomic landmarks were obtained, including the pulmonary veins and coronary sinus, which are of value in electrophysiological procedures. In vivo radio frequency ablation procedures in the right ventricle were also monitored, which yielded lesions of high image contrast. more...

Using catheter-mounted two-dimensional array transducers, we have obtained real-time three-dimensional color flow and spectral Doppler ultrasound images in phantoms. We have constructed several transducers operating at 5 MHz with up to 137 channels inside a 12 French catheter lumen. The transducer configuration may be side scanning or beveled with respect to the long axis of the catheter lumen. We have also included six electrodes to acquire simultaneous electrocardiograms. We have measured Doppler signals in two phantoms, a string phantom and a pulsatile flow phantom. Using an open chest sheep model, we inserted the catheter into the cardiac chambers to study the utility of in vivo 3D intracardiac Doppler echocardiography. more...

Cardiac procedures rely on fluoroscopy for catheter guidance and visualization. However, fluoroscopy provides poor contrast of myocardial structures and exposes both the patient and health care providers to ionizing radiation. As an alternative to fluoroscopy, real-time three-dimensional (3-D) ultrasound imaging has the potential to provide a safe means for tracking catheter position in 3-D while simultaneously imaging the heart's anatomy. A method is described for locating a catheter-mounted transducer in the 3-D ultrasound imaging field. The distance from the imaging transducer to the catheter transducer is measured by time of flight, while the angular position is determined by a spatial crosscorrelation of the received signals with stored receive profiles. Results from simulations with 20-dB SNR demonstrated a mean accuracy of 0.22 +/- 0.13 mm at a 70-mm range. In vitro testing showed a resolution of 0.23 +/- 0.11 mm at a range of 75 mm and a resolution of 0.47 +/- 0.47 mm at a range of 97 mm. With combined catheter position and imaging, this tracking method has the potential to replace fluoroscopy and enhance interventional procedures. more...

Using catheter-mounted 2-D array transducers, we have obtained real-time 3-D intracardiac ultrasound (US) images. We have constructed several transducers with 64 channels inside a 12 French catheter lumen operating at 5 MHz. The transducer configuration may be side-scanning or beveled, with respect to the long axis of the catheter lumen. We have also included six electrodes to acquire simultaneous electrocardiograms. Using an open-chest sheep model, we inserted the catheter into the cardiac chambers to study the utility of in vivo intracardiac 3-D scanning. Images obtained include a cardiac four-chamber view, mitral valve, pulmonic valve, tricuspid valve, interatrial septum, interventricular septum and ventricular volumes. We have also imaged two electrophysiological interventional devices in the right atrium, performed an in vitro ablation study, and viewed the pulmonary veins in vitro. more...

Background: Whether or not superior mesentericoportal venous resection (SM-PVR) associated with pancreaticoduodenectomy (PD) is safe and worthwhile has not been fully confirmed. The aim of the present study was to investigate results of this surgical procedure performed for pancreatic head and periampullary neoplasms., Methods: As a first analysis, postoperative morbidity and mortality after PD with (n = 31) or without SM-PVR (n = 119) were investigated in 150 patients with pancreatic head and periampullary neoplasms. As a second analysis, rates of margin-negative resection and survival after SM-PVR (n = 21) and without SM-PVR (n = 66) were compared in 87 patients with pancreatic ductal adenocarcinoma of the pancreatic head. In these patients undergoing SM-PVR (n = 21), survival rate was investigated in patients who did (n = 13) and did not (n = 8) undergo a margin-negative resection., Results: In the first analysis, duration of surgery and volume of blood transfused perioperatively were higher in patients undergoing SM-PVR. However, mortality, morbidity rates, and mean hospital stay did not differ between patients who did undergo SM-PVR (31 patients, 3.2%, 48.4%, and 22.2 days, respectively) and who did not (119 patients, 2.5%, 47.1%, 25.9 days, respectively). No postoperative death occurred in the recent part of the present study, since 1994, in patients undergoing SM-PVR. In the second analysis of pancreatic ductal adenocarcinoma, rates of margin-negative resection and 2-year survival did not significantly differ between patients who did and did not undergo SM-PVR (62% and 22%, respectively, versus 73% and 24%). In patients undergoing SM-PVR, survival rate was significantly higher for patients undergoing a margin-negative resection (n = 13) than for patients undergoing a macroscopic or microscopic margin-positive resection (n = 8, 2-year survival = 57.1% versus 0%, P <0.05)., Conclusion: PD combined with SM-PVR can be performed safely. This surgical procedure is followed by a promising survival rate and can be recommended in order to obtain a margin-negative resection; however, candidates for SM-PVR should be carefully selected. more...

A novel three-dimensional finite element model for the study of radiofrequency ablation is presented. The model was used to perform an analysis of the temperature distribution in a tissue block heated by RF energy and cooled by blood (fluid) flow. This work extends earlier models by including true flow in place of a convective boundary condition to simulate realistic experimental conditions and to improve the prediction of blood temperatures. The effect of fluid flow on the temperature distribution, the lesion dimensions, and the ablation efficiency was studied. Three flow velocities were simulated: (i) 30, (ii) 55, and (iii) 85 mm/s. The modeling results were validated qualitatively and quantitatively with in vitro data. The correlation coefficients between the modeling and the experimental temperature measurements were 0.98, 0.97, and 0.95 for flows (i)-(iii), respectively. The slopes were 0.89, 0.95, and 1.06, and the mean root mean square differences between modeling and experimental temperature measurements were 17.3% +/- 11.6%, 15.8% +/- 13.4%, and 18.8% +/- 14.9% for flows (i)-(iii), respectively. A comparison of temperature distribution obtained with a convective boundary versus inclusion of fluid motion showed that the convective boundary resulted in a similar tissue temperature distribution, but overestimated fluid temperatures and lacked the flow asymmetry seen in the true flow model. more...

This paper presents an in vitro temperature mapping study of bovine cardiac tissue during radiofrequency ablation. The objectives were to: (i) develop a technique for measuring the spatial and temporal temperature distribution in the tissue and in the blood during ablation, and (ii) use the temperature measurements to characterize the effects of fluid flow on lesion dimensions, ablation efficiency, and temperature distributions. In vitro ablation (20 W, 60 s) of bovine cardiac tissue was performed. The tissue was placed in a saline-dextrose solution maintained at 37+/- 0.5 degrees C. The solution also irrigated the tissue surface and simulated blood flow velocities of (i) 30, (ii) 55, and (iii) 85 mm/s. Thermocouple measurements were recorded from 25 and 2 locations in the tissue and in the fluid, respectively. The lowest flow resulted in the largest lesion, the maximum tissue, fluid, and electrode temperature increases, and the highest ablation efficiency. The lesions were 5.8 +/- 0.81, 4.8 +/- 0.84, and 4.4 +/- 1.25 mm deep, and 9.3 +/- 1.07, 7.9 +/- 1.48, and 7.8 +/- 1.27 mm wide for flows (i)-(iii), respectively. The blood and tissue temperature distributions were asymmetric around the ablating electrode axis with higher temperatures on the outflow than on the inflow side. The experimental measurements were used to validate a numeric model of ablation in an accompanying paper. more...

Introduction: Studies of the electrical dynamics of cardiac tissue are important for understanding the mechanisms of arrhythmias. This study uses high-frequency pacing to investigate the dynamics of sheep atria., Methods and Results: A 504-electrode mapping plaque was affixed to the right atrium in six sheep. Cathodal pacing stimuli were delivered to the center of the plaque. Pacing period (Tp) was decreased from 275 +/- 25 msec to 75 +/- 25 msec and then increased to 230 +/- 70 msec in steps of either 5 or 10 msec. In all 21 trials in six sheep, the atrium responded 1:1 at longer Tps and 2:1 at shorter Tps. As Tp was decreased, the response switched to 2:1 at a particular Tp. Conversely, as Tp was increased, the response switched back to 1:1 at a particular Tp. Over 21 trials, the 1:1-to-2:1 and 2:1-to-1:1 transitions occurred at 119.5 +/- 18.8 msec and 130.0 +/- 19.1 msec, respectively. This hysteretic behavior yielded bistability windows, 10.5 +/- 7.2 msec wide, wherein 1:1 and 2:1 responses existed at the same Tp. In 15 trials and in all animals, idiopathic wavefronts emanating from outside the mapped region passed through the mapped region. In 13 of those trials, the idiopathic wavefronts occurred at Tps within the bistability window or within 35 msec of its upper or lower limit., Conclusion: Bistability windows and idiopathic wavefronts were observed and found to be correlated with each other, suggesting a connection between bistability and arrhythmogenesis. more...

Transvenous cardiac procedures require accurate positioning of catheters within the geometrically complex cavities of the heart. Recently, nonfluoroscopic catheter tracking technologies have been developed to quantitate the (degrees-of-freedom) three-dimensional positions of intracardiac catheters. This paper presents a projection-Procrustes method to register an animated three-dimensional (3-D) model of multiple intracardiac catheters with a single-plane fluoroscopic image. Applying the computed transformation to the catheter coordinates enables the animated 3-D model of the catheters to be viewed from the same perspective as the fluoroscopic image. Mathematical simulations show that the computed transformation parameters are sensitive to both the position errors in the 3-D catheter coordinates and to the spatial distribution of the catheter-mounted transducers. Simulations with a realistic geometric model of three catheters with four transducers per catheter showed an angular error of 1.91 degrees +/- 0.27 degree for 3-D catheter position errors of 2.0 mm. An in vitro experiment demonstrated the feasibility of the method using a water tank phantom of three catheters and fluoroscopic images taken over an 80 degrees range. The mean angular error was 0.61 degree +/- 0.48 degree. The results of this study indicate that the projection-Procrustes method is a useful tool for registering 3-D catheter tracking models to single-plane fluoroscopic images. more...

Radio-frequency (RF) catheter ablation is the primary interventional therapy for the treatment of many cardiac tachyarrhythmias. Three-dimensional finite element analysis of constant-power (CPRFA) and temperature-controlled RF ablation (TCRFA) of the endocardium is performed. The objectives are to study: 1) the lesion growth with time and 2) the effect of ground electrode location on lesion dimensions and ablation efficiency. The results indicate that: a) for TCRFA: i) lesion growth was fastest during the first 20 s, subsequently the lesion growth slowed reaching a steady state after 100 s, ii) positioning the ground electrode directly opposite the catheter tip (optimal) produced a larger lesion, and iii) a constant tip temperature maintained a constant maximum tissue temperature; b) for CPRFA: i) the lesion growth was fastest during the first 20 s and then the lesion growth slowed; however, the lesion size did not reach steady state even after 600 s suggesting that longer durations of energy delivery may result in wider and deeper lesions, ii) the temperature-dependent electrical conductivity of the tissue is responsible for this continuous lesion growth, and iii) an optimal ground electrode location resulted in a slightly larger lesion and higher ablation efficiency. more...

The critical point hypothesis for the upper limit of vulnerability (ULV) states that the site of S1 pacing should not affect the ULV S2 shock strength for a single S2 shock electrode configuration but may affect the S1-S2 interval at which sub-ULV shocks induce ventricular fibrillation (VF). Furthermore, early post-S2 activations leading to VF should arise in areas with low potential gradients of similar magnitude, regardless of the S1 site. This hypothesis was tested in 10 pigs by determining ULVs for three S1 sites [left ventricular apex (LVA), LV base (LVB), and right ventricular outflow tract (RVOT)] with one S2 configuration (LVA patch to superior vena cava catheter). T-wave scanning was performed with biphasic S2 shocks incremented from 60 V in 40-V steps and stepped up or down in 20- and 10-V steps. Activations and S2 potential gradients were recorded at 528 epicardial sites. Although shocks just below the ULV induced VF significantly earlier in the T wave when the S1 site was the RVOT than when it was the LVA or LVB, ULVs were not significantly different for the three S1 pacing sites. Early post-S2 activations arose closer to the S2 electrode for weak S2s but moved to distant low potential gradient areas as the S2 strengthened. Just below the ULV, early post-S2 activations arose in the RVOT when the S1 site was the LVA or LVB but arose along the RV base when the S1 site was the RVOT. Early site potential gradients were not significantly different just below the ULV (LVA: 8.2 +/- 4.1 V/cm; LVB: 8.6 +/- 4. 9 V/cm; RVOT: 8.7 +/- 4.4 V/cm). At the ULV, early post-S2 activations arose from the same areas but did not induce VF. The results support the critical point hypothesis for the ULV. For this S2 configuration, no single point in the T wave could be used to determine the ULV for all S1 sites. more...

Objectives: To assess the effect of skin electrode location on radiofrequency (RF) ablation lesion dimensions and energy requirements., Background: Little is known about the effects of skin electrode location on RF ablation lesion dimensions and efficiency., Methods and Results: Temperature-controlled ablation at 60 degrees C for 60 seconds was performed in six sheep. Paired lesions were created in the lateral, anterior, posterior, and septal walls of both the ventricles. For group 1 lesions, the skin electrode was positioned directly opposite the catheter tip (optimal). For group 2 lesions, we used either the standard posterior location or an anterior location if the posterior skin electrode location was used for group 1. Group 1 lesions were 5.8+/-0.8 mm deep and 9.3+/-1.9 mm wide, compared with 4.6+/-1.0 mm deep and 7.7+/-1.9 mm wide group 2 lesions (P < or = 0.001). Group 1 lesion dimensions also had less variability. A finite element model was used to simulate temperature-controlled ablation and to study the effect of skin electrode locations on lesion dimensions, ablation efficiency, and blood heating. The optimal location was 1.6 times more efficient, and the volume of blood heated to > or = 90 degrees C was 0.005 mm3 for optimal versus 2.2 mm3 for the nonoptimal location., Conclusion: Optimal skin electrode placement: (1) creates deeper and larger lesions; (2) reduces lesion size variability; and (3) decreases blood heating. more...

This paper describes a technique for tracking the three-dimensional (3-D) position of a cardiac catheter using sonomicrometry and the mathematical method of multidimensional scaling (MDS). Sonomicrometry is used to measure the distances between ultrasonic transceivers. MDS is then used to calculate the 3-D coordinates of the ultrasonic transceiver locations, including the catheter tip, from the measured distances. Feasibility of catheter tracking was initially studied using simulated data from a geometric model in which the actual coordinates of all transceivers were known. The method was then shown to be feasible in vivo by tracking a catheter-mounted piezoelectric transducer using seven reference crystals sewn to the epicardial surface of a sheep heart. Simulation results indicate that a catheter can be tracked with a root-mean-square (rms) error of 1.51 +/- 0.05 mm and an average-distance error of e = 1.06 +/- 0.27 mm using 12 reference points. In vivo results showed acceptable stress values (G < 0.05) for 95% of the data samples with an average-distance error of e = 0.52 +/- 0.66 mm. These simulation and experimental results show that sonomicrometry and MDS can be used to accurately localize the 3-D position and track the motion of a catheter tip within the heart. more...

Introduction: The role of edge effects and electrode surface area of the right ventricular (RV) transvenous lead (TVL) on defibrillation efficacy is unknown., Methods and Results: Defibrillation threshold (DFT) testing was conducted randomly in 12 dogs using ring electrode leads in an RV/SVC (superior vena cava) or RV/SVC/patch system. The leads (RV-4, RV-8t, RV-8, RV-15) had electrode surface areas of 20%, 20%, 40%, and 70%, respectively. A computer model predicted the magnitude of electrode surface current (RV-8t > RV-4 > RV-8 > RV-15) and the potential distribution (PD) at four sites: electrode surface (site a) and at 2 mm (b), 4 mm (c), and 8 mm (d) away from the surface. Despite different near-field PDs (sites a, b, c), PDs were nearly identical at site d. Resistance decreased as the surface area increased. DFT energy for the RV-15 lead was lower than the RV-4 and RV-8t. There was no difference between energy requirements for the RV-15 and RV-8 leads. No difference was found in DFT current for each lead. Comparison of the RV-8t and RV-4 leads showed no difference in DFT energy despite a lower resistance and a greater number of edges., Conclusions: Increasing the RV TVL surface area lowered the resistance. However, surface area coverages > or = 40% did not lower DFT energy. No significant change in DFT current occurred despite different predicted near-field current densities. PDs were nearly identical 8 mm from the electrode surface. Thus, the far-field current density appears to play a more important role in determining defibrillation success. more...

Objective: To determine whether considerably smaller capacitors could replace 125 microF capacitors as the standard for use in implantable defibrillators., Methods: Measured energy, impedance, voltage, and current delivered were compared at defibrillation threshold in 10 mongrel dogs for defibrillation using 75 microF and 125 microF capacitors alternated randomly. Defibrillation was attempted with biphasic shocks of comparable tilt between an endocardial lead in the right ventricular apex and a "dummy" active can of an experimental implantable device placed in the subpectoral position., Results: A reduction of capacitor size of 40% was associated with an increase in voltage of 21% and in current of 22%. With a 65% tilt, no significant differences were found between the two capacitances with respect to the impedance or energy required for defibrillation., Conclusions: Multiple advances in electrode material, electrode configuration, shock morphology, and shock polarity have reduced defibrillation energy requirements. Smaller capacitors could be used in implantable cardioverter/defibrillators without a major decrease in effectiveness. more...

Introduction: The reason for the increased defibrillation efficacy of biphasic shocks over monophasic shock is not definitely known., Methods and Results: In six anesthetized pigs, we mapped the epicardium after transvenous defibrillation shocks to compare the activation patterns following successful biphasic shocks with unsuccessful monophasic shocks of the same voltage. The heart was exposed and a 510-electrode sock with approximately 4-mm interelectrode spacing was pulled over the entire ventricular epicardium and sutured to the pericardium. Defibrillation catheters were placed in the right ventricular apex and in the superior vena cava. Paired monophasic 12 msec and biphasic 6/6 msec defibrillation shocks were given using an up-down protocol to keep shock strength between the defibrillation thresholds for the two waveforms so that the biphasic shock was successful while the monophasic shock was not. Activation fronts immediately following 60 paired shocks were recorded and analyzed by animated maps of the first derivative of the electrograms. The ventricles were divided into apical (I), middle (II), and basal (III) thirds, and early sites, i.e., the sites from which activation fronts first appeared on the epicardium following the shock, were grouped according to their location. Postshock intervals, i.e., the time from the shock until earliest epicardial activation occurred, were also determined. No ectopic activation fronts followed the shock in 20 biphasic episodes. In the other 40 paired episodes, the number of early sites was smaller after biphasic shocks than after monophasic shocks [monophasic: 198 (total), 3.3 +/- 0.9 (mean +/- SD) per shock episode; biphasic: 67, 1.1 +/- 1.0, P < 0.05]. For biphasic but not monophasic shocks, early sites were less likely to arise from the middle (II) and basal (III) thirds than from the apical third (I) [monophasic: I: 84 (42%), II: 68 (34%), III: 46 (23%); biphasic: I: 49 (73%), II: 10 (15%), III: 8 (12%), P < 0.05]. Postshock intervals were significantly shorter for monophasic shocks (54 +/- 14 msec) than for biphasic shocks (75 +/- 23 msec, P < 0.05)., Conclusion: The decreased number of activation fronts and the longer delay following the shock for the earliest epicardial appearance of those activation fronts that do occur may be responsible for the increased defibrillation efficacy for biphasic shocks. more...

Previous investigations have suggested that during ventricular fibrillation (VF) pacing stimuli are incapable of evoking propagated ventricular activations. To determine whether regional myocardial capture could be achieved during rapid pacing in VF, extracellular unipolar potentials were sampled (2 kHz) and recorded from 506 Ag-AgCl electrodes arranged in a rectangular grid (22 x 23, 1.12-mm spacing) embedded in a plaque overlying two pacing electrodes in the epicardium of the anterobasal right ventricle in pentobarbital-anesthetized pigs (25 to 30 kg, n = 6). During separate episodes of electrically induced VF, two bursts of 40 monophasic stimuli (10 mA, 2-millisecond duration) were asynchronously applied to the stimulating electrodes in either a bipolar, unipolar anodal, or unipolar cathodal mode. Evidence of regional capture was provided by (1) animating the first temporal derivative of the extracellular potentials, (2) analyzing inter-beat interval patterns, and (3) employing the Karhunen-Loeve decomposition method to quantify the repetitiveness of spatio-temporal patterns of activation. Regional capture of ventricular myocardium during VF was observed when pacing stimuli fell late in the local myocardial activation interval and when the pacing cycle length was 80% to 115% of the mean subplaque activation cycle length. When myocardial activations became phase locked to the pacing stimuli, repeatable spatiotemporal patterns of activation followed each stimulus. Poincaré sections at the plaque border revealed that during VF prior to pacing, interbeat intervals were irregular but were driven by pacing to stable fixed values at times corresponding to our qualitative declaration of regional capture. A similar correspondence was demonstrated between the time of capture, defined by direct observation of the activation patterns, and a rise in the power contained in the first two spatial modes of a Karhunen-Loeve decomposition. These data demonstrate that appropriately timed stimuli produce regional capture of fibrillating right ventricular myocardium in the pig and support the existence of an excitable gap during VF in this model. more...

Ventricular fibrillation (VF) is a fatal cardiac arrhythmia, characterized by uncoordinated propagation of activation wavefronts in the ventricular myocardium. Short-term predictions of epicardial potential fields during VF in pigs were attempted using linear techniques, and prediction accuracy was measured at various stages during sustained episodes. VF was induced in five pigs via premature electrical stimulation. Unipolar electrograms were recorded from an epicardial array of 506 electrodes in a 22 x 23 array with 1-mm spacing. Optimal spatial basis functions (modes) and time-varying weighting coefficients were found using the Karhunen-Loeve decomposition. Linear autoregressive (AR) models incorporating the dynamics of only a few spatial modes led to predicted patterns that were qualitatively similar to observed patterns. Predictions were made 0.256 s into the future, based on 0.768 s of past data, over an area of approximately 5 cm2 on the ventricular epicardium. The mean squared error of predictions varied from as much as 1.23 to as little as 0.14, normalized to the variance of the actual data. Inconsistency in long-term forcasts is partly due to the limitations of linear AR models. Changes in predictability, however, were consistent. Predictability varied inversely with spatial complexity, as measured by the mean squared error of a five-mode approximation. Predictability also increased significantly during the first minute of VF. more...

A method of defibrillation threshold determination that utilizes low-strength shocks delivered in a benign rhythm would be desirable. Because the two-dimensional epicardial potential gradient (PG) is a shock parameter that is linked to defibrillation, we examined whether the epicardial PG measured for shocks delivered in paced rhythm could be used to predict the PG for defibrillation-strength shocks delivered in ventricular fibrillation (VF). In six open-chest pentobarbital-anesthetized pigs with left ventricular apex and right atrial internal defibrillation patches, we measured the epicardial PG field for shocks delivered in paced rhythm and during VF. We determined that there was a linear relationship between epicardial PG and shock strength for shocks delivered in paced rhythm. However, prediction of the PG measured for shocks in VF from those measured in paced rhythm resulted in a statistically significant overestimation of the PG in VF. We conclude that, for equivalent strength shocks, the epicardial PG field is weaker for shocks delivered in VF. This change in the potential gradient field can have an effect on defibrillation threshold estimates that are based on shocks delivered in paced rhythm. more...

Introduction: We hypothesized that drugs which alter ventricular refractoriness or excitability produce quantifiable changes in ventricular fibrillation., Methods and Results: We used a 528-channel mapping system to quantify the effects of the beta-antagonist, propranolol, and the beta-agonist, isoproterenol, on activation patterns in ventricular fibrillation. A plaque of 506 (22 x 23) electrodes spaced 1.12 mm apart and covering about 5% of the ventricular epicardium was sewn to the anterior right ventricle in 18 pigs (30 kg). Propranolol (0.25 to 0.4 mg/kg) increased the refractory period at a right ventricular epicardial site while isoproterenol (3 to 5 micrograms/min) shortened it. Ventricular fibrillation was induced by programmed stimulation, and unipolar electrograms were recorded from the 506 plaque electrodes for 2 seconds beginning 1, 15, and 30 seconds after the onset of fibrillation. Active epicardial recording sites were identified from the first derivative of the unipolar potentials (dV/dt) detected at each electrode. Then, neighboring active sites were grouped into activation fronts by computer analysis. In six pigs the effect of repeated inductions of ventricular fibrillation was assessed by comparing ventricular fibrillation after saline with a preceding control episode of fibrillation. Each activation front excited 40% +/- 46% of the mapped region before blocking. No changes were observed with saline and multiple inductions of fibrillation. In another six pigs, ventricular fibrillation after propranolol was compared with a preceding control episode of fibrillation. Ventricular fibrillation after propranolol exhibited a decreased activation rate per epicardial recording site and fewer activation fronts per second. There was no change in the amount of tissue excited by each activation front or the number of reentry cycles per activation front compared with control. In addition, there was no change in the maximum negative dV/dt detected per activation at an epicardial site. In six pigs ventricular fibrillation during isoproterenol was compared with control episodes of ventricular fibrillation before and 45 minutes after washout of the drug. The control episodes of fibrillation were not different from each other. Compared with control, ventricular fibrillation during isoproterenol exhibited an increased activation rate per epicardial site, an increased amount of tissue excited by each activation front, and an increased maximum negative dV/dt for each activation. There was no change in the number of activation fronts per second or the number of reentry cycles per activation front compared with control., Conclusion: Quantitative analysis revealed that propranolol and isoproterenol do not have symmetrically opposite effects on ventricular fibrillation. Propranolol decreased the number of activation fronts while isoproterenol increased the amount of tissue excited by each activation front. Thus, drugs that alter ventricular refractoriness or excitability alter ventricular fibrillation. more...

Objectives: This study sought to assess the effect of passive "bystander" epicardial electrodes on defibrillation efficacy., Background: We hypothesized that an inactive epicardial patch placed in an area of low potential gradient from an endocardial electrode shock might affect defibrillation efficacy through its effects on the shock field and the underlying potential gradient., Methods: We studied the effects of an inactive 18-cm2 titanium mesh patch placed on the anterolateral left ventricular epicardium on the 50% probability of successful defibrillation. A biphasic shock with both phases 6 ms in duration was delivered between superior vena cava and right ventricular catheter electrodes 10 s after the electrical induction of ventricular fibrillation. Six dogs underwent an up/down defibrillation protocol randomized with or without the patch on the heart., Results: Mean 50% (+/-) probability point for energy doubled with the conductive patch on the heart, from 8.0 +/- 3.2 to 16.8 +/- 7.0 J (p < 0.01), and leading-edge voltage increased from 334 +/- 64 to 477 +/- 98 V (p < 0.01). Mean 50% probability points for energy and leading-edge voltage were not significantly changed when the procedure was repeated using a nonconductive patch in another six dogs as a control group. In a saline-saturated foam model, measurements from electrodes placed around and under the patch revealed a 72% mean decrease in the potential gradient in the foam under the conductive patch., Conclusions: A passive defibrillator patch can markedly increase the energy requirements for defibrillation, probably by decreasing the potential gradient under the patch. These results suggest the use of caution when passive electrodes are present, for example, when a patient receives a nonthoracotomy defibrillator system while epicardial electrodes from a previously implanted system are left in place. more...

We tested the hypothesis that peripheral afferent nerve stimulation decreases the incidence of ventricular arrhythmias induced by central nervous system stimulation. The hypothalamus in each of 24 anesthetized rabbits (12 with anterior myocardial ischemia) was electrically stimulated for 10 s with 10-min intervals between each of six consecutive stimulation episodes. The left peroneal nerves were electrically stimulated for 15 min beginning 5 min after the second hypothalamic stimulation episode in six ischemic and six nonischemic rabbits. Cardiac rhythm was monitored with the electrocardiogram lead I and atrial and ventricular electrograms. Hypothalamic stimulation alone induced ventricular arrhythmias [mean no. of arrhythmic beats occurring during 3rd and 4th hypothalamic stimulation episodes: nonischemic animals, 20 +/- 8; ischemic animals, 62 +/- 41 (P < 0.05 by unpaired t-test)]. Peroneal nerve stimulation reduced the number of arrhythmic beats induced by hypothalamic stimulation in nonischemic animals (6 +/- 5; P < 0.05 vs. without peroneal stimulation) and prevented an increase in arrhythmic beats during ischemia (21 +/- 16). Thus peripheral nerve stimulation decreased the number of ventricular arrhythmic beats induced by repeated hypothalamic stimulation in both ischemic and normal rabbit hearts and may be important in the prevention of arrhythmias in patients. more...

The effects of high voltage defibrillation shocks given to six swine were studied to determine if there is a limit to the advantage gained from increasing the shock strength. An endocardial electrode was placed in the right ventricle, and a 114-cm2 cutaneous patch was placed on the left lateral thorax. Monophasic (10 msec) and single capacitor biphasic (5/5 msec) shocks with leading edge voltages of 200, 400, 600, 800, and 990 volts (approximately 2.3-59 J) were tested. For monophasic shocks, the probability of successful defibrillation ranged from 0% at 200 V to 90% at 990 V. The incidence of postshock arrhythmia increased from 0% for successful shocks at 600 V to 67% for successful shocks at 990 V. For biphasic shocks, the probability of success peaked at 97% for the 600-, 800-, and 990-V shocks. The incidence of postshock arrhythmia increased from 8% at 400 V to 55% at 990 V. Although more postshock arrhythmias occurred at lower strengths for biphasic than for monophasic shocks, an efficacy criterion, quantifying the probability of defibrillation success and the probability that a postshock arrhythmia will not occur, was always higher for biphasic shocks. The probability of success never reached 100% for either waveform while the incidence of postshock arrhythmia increased as the shock strength increased. In conclusion, for the catheter-patch electrode configuration, increasing the shock strength does not always improve the probability of success and may increase the incidence of postshock arrhythmia. more...

Implantable cardioverter defibrillators reduce the risk of sudden cardiac death in patients with ventricular tachyarrhythmias. However, for the few patients with unacceptably high defibrillation thresholds at implantation the risk of sudden death may remain high. If a small number of defibrillation attempts are used to determine a defibrillation threshold, then a high defibrillation threshold may occur in some patients due to the probabilistic nature of defibrillation: a small percentage of shocks will fail even at optimal shock strengths. Basic investigations have suggested mechanisms for high defibrillation thresholds in other patients. The extracellular potential gradients produced by a shock correlate with ability to defibrillate and may be used to classify mechanisms for high defibrillation thresholds. Computerized mapping studies have demonstrated that extracellular potential gradient fields produced by defibrillation shocks are uneven with high gradient areas close to the electrodes and low gradient areas distant from the electrodes. A high defibrillation threshold may occur because: (1) a shock creates a subthreshold potential gradient in the low gradient areas; (2) a patient has a higher minimum potential gradient threshold than other patients; or (3) a shock leads to refibrillation in the high gradient areas. This article reviews experimental evidence to support each of these three possibilities then suggests experimental and clinical investigations that may clarify the causes of high defibrillation thresholds in patients. more...

In order to measure in vivo resistivity of tissues in the thorax, the possibility of combining anatomical data extracted from high-resolution images with multiple-electrode impedance measurements, a priori knowledge of the range of tissue resistivities, and a priori data on the instrumentation noise is assessed in this study. A statistically constrained minimum-mean-square error estimator (MIMSEE) that minimizes the effects of linearization errors and instrumentation noise is developed and compared to the conventional least-squares error estimator (LSEE). The MIMSEE requires a priori signal and noise information. The statistical constraint signal information was obtained from a priori knowledge of the physiologically allowed range of regional resistivities. The noise constraint information was obtained from a priori knowledge of the linearization error and the instrumentation noise. The torso potentials were simulated by employing a three-dimensional canine torso model. The model consists of four different conductivity regions: heart, right lung, left lung, and body. It is demonstrated that the statistically constrained MIMSEE performs significantly better than the LSEE in determining resistivities. The results based on the torso model indicate that regional resistivities can be estimated to within 40% accuracy of their true values by utilizing a statistically constrained MIMSEE, even if the instrumentation noise is comparable to the measured torso potentials. The errors obtained using the LSEE with the same linearized transfer function and level of instrumentation noise were about five times larger than those obtained using the MIMSEE. For larger measurement errors the MIMSEE performs even better when compared to the LSEE. more...

Introduction: The degree of organization in ventricular fibrillation (VF) is not known. As an objective measurement of spatial order, spatial correlation functions and their characteristic lengths were estimated from epicardial electrograms of pigs in VF., Methods and Results: VF was induced by premature stimulation in five pigs. Electrograms were simultaneously recorded with a 22 x 23 array of unipolar electrodes spaced 1.12 mm apart. Data were obtained by sampling the signals at 2000 Hz for 20 minutes immediately after the initiation of FV. Correlations between all pairs of signals were computed at various times. Correlation lengths were estimated from the decay of average correlation as a function of electrode separation. The correlation length of the VF in pigs was found to be approximately 4 to 10 mm, varying as fibrillation progressed. The degree of correlation decreased in the first 4 seconds after fibrillation then increased over the next minute., Conclusion: The correlation length is much smaller than the scale of the heart, suggesting that many independent regions of activity exist on the epicardium at any one time. On the other hand, the correlation length is 4 to 10 times the interelectrode spacing, indicating that some coherence is present. These results imply that the heart behaves during VF as a high dimensional, but not random, system involving many spatial degrees of freedom, which may explain the lack of convergence of fractal dimension estimates reported in the literature. Changes in the correlation length also suggest that VF reorganizes slightly in the first minute after an initial breakdown in structure. more...

Previous studies have shown that low internal defibrillation thresholds (DFTs) can be attained by using two pairs of electrodes and combining biphasic shocks with sequential timing. The purpose of this two-part study was to test the defibrillation efficacy of two new shock sequences, an interleaved biphasic, and a common phase sequential biphasic, that utilized two pairs of electrodes and were developed from the concept of sequential biphasic shocks. In the first part, defibrillation catheters were placed in the right ventricle and the superior vena cava of six anesthetized pigs. A small patch electrode was placed on the LV apex through a subxiphoid incision and a cutaneous patch was placed on the left thorax. The mean DFT energies for the interleaved biphasic (5.2 +/- 0.4 J) and the common phase sequential biphasic waveforms (5.4 +/- 0.4 J) were substantially less (P < 0.0001) than those for either the sequential monophasic (10.6 +/- 1.0 J) or single biphasic waveforms (9.0 +/- 1.0 J). In the second study, which used nine anesthetized pigs, the importance of phase reversal was demonstrated by the finding that the DFT energy of a common phase sequential biphasic shock (6.2 +/- 0.4 J) was much less than a common phase sequential monophasic shock (17.9 +/- 1.3 J, P < 0.0001); furthermore, the average DFT for four common phase sequential biphasic configurations (5.7 +/- 0.2 J) was much less than for a configuration that was similar except that current flow was not reversed in one phase so that no biphasic effect was present (19.7 +/- 1.2 J). The efficacy of common phase sequential biphasics was comparable to that of sequential biphasics. The effectiveness of sequential biphasics, interleaved biphasics, and common phase sequential biphasics is possibly due to two mechanisms: (A) an increase in the potential gradient during a later phase in regions that were low during the first phase, and (B) the exposure of most of the myocardium to a biphasic shock that reduces the minimum extracellular potential gradient needed to defibrillate. more...

This study determined the response of action potentials during ventricular fibrillation (VF) to timed monophasic and biphasic shocks. A floating glass microelectrode was used to record intracellularly from the anterior right ventricle in 10 open-chest dogs. After 10 seconds of electrically induced VF, 5-millisecond monophasic and 2.5/2.5-millisecond biphasic shocks or 16-millisecond monophasic and 8/8-millisecond biphasic shocks were given via mesh electrodes on either side of the microelectrode. Monophasic and biphasic truncated exponential shocks of 5 V/cm were given with coupling intervals timed from the beginning of a VF action potential to the shock ranging from 50 to 70 milliseconds in 5-millisecond increments. Each coupling interval for each waveform was tested during a different VF episode. The interval between successive activations during VF was 86 +/- 15 milliseconds (mean +/- SD). The refractory period during VF was 61 +/- 5 milliseconds for 5-millisecond monophasic shocks and 66 +/- 6 milliseconds for 2.5/2.5-millisecond biphasic shocks (P < .05). At each coupling interval, action potential duration at 50% repolarization (APD50) was significantly prolonged by the shocks compared with the mean preshock APD50 (P < .05). ADP50 duration increased significantly with increases in the coupling interval (P < .05) for both monophasic and biphasic waveforms. For all coupling intervals together, APD50 prolongation as a percent of the mean preshock APD50 was 170 +/- 55%, 192 +/- 45%, 151 +/- 44%, and 175 +/- 45% for 5- and 16-millisecond monophasic and 2.5/2.5- and 8/8-millisecond biphasic waveforms, respectively. This APD50 prolongation was greater for monophasic than biphasic shocks and was greater for longer than shorter waveforms (P < .05). Thus, during VF, (1) the refractory period for 5-V/cm truncated exponential waveforms lasting 5 milliseconds is approximately 75% of the VF activation interval; (2) the refractory period is shorter for monophasic than for comparable biphasic waveforms; (3) both monophasic and biphasic 5-V/cm shock fields cause prolongation of action potential duration; (4) prolongation of action potential duration increases as the coupling interval increases; and (5) prolongation of action potential duration is greater for monophasic shocks and for longer shock waveforms. more...

Why some biphasic waveforms defibrillate with lower energies than monophasic waveforms of similar duration is unknown. One hypothesis is that the first phase of a biphasic waveform acts as a conditioning, hyperpolarizing prepulse to prepare for defibrillation by a second depolarizing phase. To test whether the second phase of a biphasic waveform is the defibrillating phase, three monophasic waveforms, an ascending ramp (A), a square wave (S), and a descending ramp (D), were compared to three biphasic waveforms with A, S, or D in the first phase (biphasic first phase) and three biphasic waveforms with A, S, or D in the second phase (biphasic second phase). Two defibrillation thresholds for each waveform were performed in 18 open chest pigs and mean defibrillation thresholds were compared. In nine pigs 16-msec monophasic and 16/16-msec biphasic waveforms were ranked by mean current and energy at defibrillation threshold. The ranks were the same for monophasic and biphasic second phase waveforms: for mean current A < S = D and for energy A < S < D. The ranks were different for the biphasic first phase waveforms: for mean current S < A = D and for energy S < A = D. Although ranks for the 16-msec monophasic waveforms matched those for the 16/16-msec biphasic second phase waveforms, the biphasic waveforms had higher mean currents and energies at defibrillation threshold. In nine pigs defibrillation thresholds for 6-msec monophasic and 6/6-msec biphasic waveforms were ranked. For mean current the ranks were monophasic: A < S = D; biphasic first phase: A = S = D; and biphasic second phase: S = D < A. For energy the ranks were monophasic: A = S < D; biphasic first phase: A = S = D; and biphasic second phase: S = D < A. Thus, ranks for the 6-msec monophasic waveforms differed from those for the 6/6-msec biphasic second phase waveforms. For 16/16-msec biphasic waveforms, less effective for defibrillation than corresponding 16-msec monophasic waveforms, these results support the hypothesis that the second phase of a biphasic waveform defibrillates since the defibrillation efficacy of a 16/16-msec biphasic waveform is related to the defibrillation efficacy of its second phase waveshape. However, for clinically useful 6/6-msec biphasic waveforms, more effective for defibrillation than 6-msec monophasic waveforms, the hypothesis is not supported because the ability of a 6/6-msec biphasic waveform to defibrillate is unrelated to the defibrillation efficacy of its second phase waveshape. more...

To study the mechanism of defibrillation and the reason for the increased defibrillation efficacy of biphasic waveforms, the potential gradient in a 32 x 30-mm region of the right ventricle in 15 dogs was progressively lowered in four steps while a strong potential gradient field was maintained throughout the rest of the ventricular myocardium. The volume of right ventricle beneath the plaque was 10 +/- 2% of the total ventricular mass. A 10-msec monophasic (eight dogs) or 5/5-msec biphasic (seven dogs) truncated exponential shock 30% above the defibrillation threshold voltage was given via electrodes on the left ventricular apex and right atrium to create the strong potential gradient field. Simultaneously, a weaker shock with the same waveform but opposite polarity was given via mesh electrodes on either side of the small right ventricular region to cancel part of the potential difference in the region and to create one of the four levels of potential gradient fields. Shock potentials and activations were recorded from 117 epicardial electrodes in the small region, and in one dog global epicardial activations and potentials were recorded from a sock containing 72 electrodes. Each gradient field was tested 10 times for successful defibrillation after 10 seconds of electrically induced fibrillation. For both monophasic and biphasic shocks, the percentage of successful defibrillation attempts decreased (p < 0.05) as the potential gradient decreased in the small region. Defibrillation was successful approximately 80% of the time for a mean +/- SD potential gradient of 5.4 +/- 0.8 V/cm for monophasic shocks and 2.7 +/- 0.3 V/cm for biphasic shocks (p < 0.05). No postshock activation fronts arose from the small region for eight waveform when the gradient was more than 5 V/cm. For both waveforms, the postshock activation fronts after the shocks were markedly different from those just before the shock and exhibited either a focal origin or unidirectional conduction.(ABSTRACT TRUNCATED AT 400 WORDS) more...

This paper presents a boundary integral method for calculating the potential field generated by external stimulation at locations within the heart using realistic heart geometry and samples of the potential taken from the epicardial surface. This method assumes the heart is homogeneous and isotropic. To test the method we made epicardial and endocardial measurements in dogs during transthoracic pacing stimuli. From the epicardial potential measurements we predicted the endocardial potential values and compared them with the measured data. Despite the seemingly gross assumptions, the mean correlation coefficient between the measured and predicted potentials for three dogs and eleven stimulation electrode configurations was 0.985, and the mean rms error was 17%. more...

Background: Two sequential biphasic shocks delivered over separate lead configurations markedly improve defibrillation efficacy compared with a single shock alone. We investigated the effect of varying the intershock interval between sequential biphasic shocks on defibrillation., Methods and Results: Defibrillation thresholds (DFTs) were obtained in six dogs for shock separations ranging from 0.2 to 125 msec. The first shock was given from a catheter electrode in the right ventricular apex to a patch on the left lateral thorax; the second was from a small patch on the left ventricular apex to a catheter electrode in the right ventricular outflow tract. When the interval between shocks was less than or equal to 10 msec or greater than or equal to 75 and less than or equal to 125 msec, the mean DFTs were less than that previously found for the first shock by itself (4.2 versus 7.4 J, p = 0.002). At a separation of 50 msec, however, there was a marked rise in the DFT to 27 J. The mean DFT for the second shock at a delay of 50 msec was not different from the mean DFT previously found for the second shock by itself (7.2 versus 7.0 J). These results were confirmed in another six dogs using defibrillation probability-of-success curves. In 12 other dogs, probability-of-success curves were generated for delays between shocks as a percentage of the activation interval during ventricular fibrillation. Minimum defibrillation energy requirements were at two separations, 0.2 msec and 90% of the activation interval., Conclusions: The optimal intershock interval between two sequential biphasic shocks is either less than or equal to 10 msec or greater than or equal to 75 and less than or equal to 125 msec. The marked rise in the DFT at a shock separation of 50 msec, requiring more energy than that for the first shock alone, suggests that the second shock at this time delay is likely to reinduce fibrillation after it is halted by the first shock until the second shock is strong enough to defibrillate independently of the first shock. more...

Background: Defibrillation may be improved if electrode configurations can be found that create a larger and more even voltage gradient field across the heart. This study determined the magnitude of the shock gradient fields generated by four nonthoracotomy electrode configurations for defibrillation., Methods and Results: In six dogs, a catheter was inserted containing a right ventricular apical electrode (V) and a right atrial electrode (A). A cutaneous patch electrode (P) was placed on the left lateral thorax. Shock potentials were recorded simultaneously from 128 electrodes in the left ventricular and right ventricular subepicardium and subendocardium, ventricular septum, and atria. With the chest closed, 50-mA shocks were given during diastole via the following lead configurations: V----A (V, cathode; A, anode); V----P; V----A+P; and V+A----P. Potential gradients were calculated at the subepicardium and subendocardium in millivolts per centimeter per volt of shock. In most dogs, the V----A+P configuration produced higher gradients throughout the ventricles than did V----A, V----P, or V+A----P. The maximum potential gradient was smaller for the V+A----P configuration than for V----A, V----P, or V----A+P. The gradient fields for the configurations with the catheter alone or combined with P were uneven., Conclusions: It is possible to estimate shock gradient fields in three dimensions. Of the four configurations tested, V----A+P produced the highest gradients and V+A----P produced the lowest high gradient. The gradient fields were uneven throughout the ventricles. more...

Although sodium channels have been reported to be inactive after 5-10 minutes of ventricular fibrillation (VF), their state during early VF is unknown. In 12 open-chest dogs, a floating glass microelectrode was used to record intracellular action potentials from the right ventricle during pacing and during electrically induced VF. Before any drug was administered, an initial episode of VF was continuously recorded for at least 20 seconds followed by defibrillation. Recordings were made during VF episodes after superfusion for 15 minutes around the microelectrode site by low (2.8 x 10(-5) M) and high (10(-4) M) concentrations of tetrodotoxin (TTX) in five dogs, or by low (4 microM) and high (100 microM) concentrations of verapamil in another four dogs. In three dogs, VF was induced without drugs three times to determine if the effects observed in the previous dogs were caused by the drugs or by successive episodes of VF. Ten consecutive action potentials were analyzed at the onset and after 5, 10, 15, and 20 seconds of VF. Action potential amplitude and duration during paced rhythm or VF were not changed by the local perfusion of either TTX or verapamil. In the TTX group, the maximum upstroke rate of depolarization of an action potential (Vmax) during paced rhythm was 104 +/- 14 V/sec for control cycles before any drug was given, 86 +/- 15 V/sec for the low TTX concentration, and 55 +/- 14 V/sec for the high TTX concentration (p less than 0.05 versus other two). Vmax decreased from 55 +/- 32 V/sec at the beginning of VF to 37 +/- 27 V/sec after 20 seconds of VF for predrug VF, from 39 +/- 20 V/sec to 18 +/- 11 V/sec for low-dose TTX VF, and from 18 +/- 13 V/sec to 12 +/- 7 V/sec for high-dose TTX VF (p less than 0.05 among the three groups). In the dogs receiving verapamil, VF was still inducible with Vmax not significantly different from predrug VF at the onset and after 5 or 20 seconds of VF but with Vmax smaller (p less than 0.05) for verapamil than for predrug VF after 10 or 15 seconds of VF. In three dogs, Vmax was not significantly different during three successive episodes of VF when no drug was given between the episodes.(ABSTRACT TRUNCATED AT 400 WORDS) more...

Background: Potential gradient field determination may be a helpful means of describing the effects of defibrillation shocks; however, potential gradient field requirements for defibrillation with different electrode configurations have not been established., Methods and Results: To evaluate the field requirements for defibrillation, potential fields during defibrillation shocks and the following ventricular activations were recorded with 74 epicardial electrodes in 12 open-chest dogs with the use of a computerized mapping system. Shock electrodes (2.64 cm2) were attached to the lateral right atrium (R), lateral left ventricular base (L), and left ventricular apex (V). Four electrode configurations were tested: single shocks of 14-msec duration given to two single anode-single cathode configurations, R:V and L:V, and to one dual anode-single cathode configuration, (R+L):V; and sequential 7-msec shocks separated by 1 msec given to R:V and L:V (R:V----L:V). Defibrillation threshold (DFT) current was significantly lower for R:V----L:V than for the other configurations and markedly higher for L:V. Despite these differences, the minimum potential gradients measured at DFT were not significantly different (approximately 6-7 V/cm for each electrode configuration). Potential gradient fields generated by the electrode configurations were markedly uneven, with a 15-27-fold change from lowest to highest gradient, with the greatest decrease in gradient occurring near the shock electrodes. Although gradient fields varied with the electrode configuration, all configurations produced weak fields along the right ventricular base. Early sites of epicardial activation after all unsuccessful shocks occurred in areas in which the field was weak; 87% occurred at sites with gradients less than 15 V/cm. Ventricular tachycardia originating in high gradient areas near shock electrodes followed 11 of 67 successful shocks., Conclusions: These data suggest that 1) defibrillation fields created by small epicardial electrodes are very uneven; 2) achievement of a certain minimum potential gradient over both ventricles is necessary for ventricular defibrillation; 3) the difference in shock strengths required to achieve this minimum gradient over both ventricles may explain the differences in DFTs for various electrode configurations; and 4) high gradient areas in the uneven fields can induce ectopic activation after successful shocks. more...

Background: Certain biphasic waveforms defibrillate at lower energies than monophasic waveforms, although the mechanism is unknown., Methods and Results: The relative ability of monophasic and biphasic shocks to stimulate partially refractory myocardium was compared because defibrillation is thought to involve stimulating relatively refractory myocardial tissue. Shocks of 25-125 V were given during regularly paced rhythm in 11 open-chest dogs. Computerized recordings of shock potentials, and of activations before and after the shocks, were made at 117 epicardial sites. To quantify the shock field strength, the shock potential gradients were calculated at the electrode sites. Monophasic action potential (MAP) electrode recordings, obtained in five dogs, confirmed direct myocardial excitation by the shock, that is, activations beginning during the shock. Tissue was directly excited up to 4 cm from the shocking electrode, and the area directly excited increased as the shock was made stronger or given less prematurely. In six dogs, strength-interval curves for direct excitation were determined from plots of potential gradient versus refractoriness at each electrode site. The biphasic curves were located to the right of the monophasic curves by 8 +/- 4 msec, indicating a lesser ability to excite refractory myocardium. When the gradient at the directly excited border was at least 3.8 +/- 1 V/cm, conduction failed to propagate away from the directly excited zone after the shock, and MAP recordings made near the border showed a shock-induced graded response. This graded response, which prolonged repolarization, may have been responsible for the failure of conduction from the directly excited zone. Although better for defibrillating, the biphasic waveform was thus less effective than the monophasic one in exciting relatively refractory myocardium., Conclusions: These results indicated that waveform selection for defibrillation should not be guided solely by the ability of the waveform to stimulate tissue, as these two properties can be discordant. more...

Recent studies suggest that 1) electrically induced fibrillation and defibrillation involve prolongation of refractoriness by the shock in addition to stimulation and 2) biphasic waveforms are more efficient for defibrillation than are comparable monophasic waveforms. The purpose of this study was to compare prolongation of action potential duration at 50% repolarization by monophasic and biphasic shocks during paced rhythm. A floating glass microelectrode was used to record intracellularly from the anterior right ventricular epicardium in seven open-chest dogs. After 10 S1 beats paced at an interval of 350 msec, 5-msec and 2.5-msec monophasic shocks and biphasic shocks, with each phase of 2.5 msec, were given via mesh electrodes on either side of the microelectrode. The shock strength was adjusted so that the shock field, measured from eight extracellular electrodes encircling the microelectrode, was about 5 V/cm. Monophasic and biphasic S2 shocks were given starting with an S1-S2 interval of 120 msec, which was increased in 5-msec steps until an action potential was produced by the S2 shock. Both monophasic and biphasic 5 V/cm shock fields caused significant prolongation of action potential duration. The prolongation of action potential duration increased as the S1-S2 interval increased. This prolongation occurred at shorter S1-S2 intervals for 5-msec monophasic shocks than for biphasic shocks. more...

The purpose of this study was to determine a lower limit of defibrillation thresholds (DFTs) that could be used to evaluate nonthoracotomy lead configurations for implantable defibrillators. A lead configuration that consisted of a left ventricular catheter and four circumferential cutaneous patches was tested because it was hypothesized to create a relatively uniform electric field for defibrillation. In eight anesthetized dogs, three 8F defibrillating catheters with 6 cm platinum clad titanium tips were inserted into the right ventricle (R), right ventricular outflow tract (O), and left ventricle (L). Four cutaneous patch electrodes (4P), each with a surface area of 42 cm2, were placed on the left lateral, right lateral, anterior and posterior thorax. DFTs for ten lead configurations, consisting of different combinations of these electrodes, were evaluated. DFTs were determined by using a modified Purdue technique and applying a single capacitor biphasic shock with both phases 6 ms in duration after 15 sec of electrically induced fibrillation. The L(-)----4P+ configuration produced a lower DFT than R(-)----4P+ (3.2 +/- 1.6 J vs 8.0 +/- 4.2 J, P less than 0.001) with reduced current (2.6 +/- 0.7 A vs 4.1 +/- 1.2 A, P less than 0.001). Lowering the impedance by a mean of 40%, configurations that used four patches produced lower DFTs than those that used a single left lateral patch. The use of an O catheter produced lower DFTs only when used in conjunction with an R catheter.(ABSTRACT TRUNCATED AT 250 WORDS) more...

Certain biphasic waveforms require less energy to defibrillate than do monophasic pulses of equal duration, although the mechanisms of this increased effectiveness remain unclear. This study used strength-duration and percent success curves for defibrillation with monophasic and biphasic truncated exponential waveforms to explore these mechanisms. In part 1, defibrillation thresholds were determined for both high- and low-tilt waveforms. The monophasic pulses tested ranged in duration from 1.0 to 20.0 msec, and the biphasic waveforms had first phases of either 3.5 or 7.0 msec and second phases ranging from 1.0 to 20.0 msec. In part 2, defibrillation percent success curves were constructed for 6.0 msec/6.0 msec biphasic waveforms with a constant phase-one amplitude and with phase-two amplitudes of approximately 21%, 62%, 94%, and 141% of phase one. This study shows that if the first phase of a biphasic waveform is held constant and the second phase is increased in either duration or amplitude, defibrillation efficacy first improves, then declines, and then again improves. For pulse durations of at least 14 msec, the second-phase defibrillation threshold voltage of a high-tilt biphasic waveform is higher than that of a monophasic pulse equal in duration to the biphasic second phase (p less than 0.05), indicating that the previously proposed hypothesis of stimulation by the second phase is not the sole mechanism of biphasic defibrillation. These facts indicate the importance of the degree of tilt for the defibrillation efficacy of biphasic waveforms and suggest at least two mechanisms exist for defibrillation with these waveforms, one that is more effective for smaller second phases and another that becomes more effective as the second phase is increased. more...