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Two cases with a concealed left-sided accessory atrioventricular bypass tract are described. In both, functional longitudinal dissociation of the atrioventricular node narrowed the range of atrial premature beat coupling intervals which could initiate re-entry using the accessory pathway. In case 1 early premature atrial beats were followed by an atrioventricular nodal re-entrant echo. The atrial echo pre-empted retrograde conduction over the Kent bundle and thus limited the development of paroxysmal supraventricular tachycardia. In case 2 atrioventricular nodal conduction showed typical features ascribed to dual atrioventricular nodal pathways. In addition there was a bradycardia-related retrograde block in the concealed accessory pathway. Early premature atrial beats, because of exclusive "slow pathway" anterograde conduction, arrived at the ventricles during the period of bradycardia-dependent retrograde block and failed to initiate a macro re-entrant tachycardia. This study shows that (1) longitudinal dissociation within the atrioventricular node may limit the ability to initiate tachycardia in patients with concealed pre-excitation; and (2) discontinuous atrioventricular nodal conduction curves occasionally help to reveal bradycardia-related retrograde block in a concealed accessory pathway. [ABSTRACT FROM PUBLISHER]

Microwave tomographic approach is proposed to detect and image breast cancers. Taking into account the big difference in dielectrical properties between normal and malignant tissues, we have proposed using the microwave tomographic method to image a human breast. Because of the anatomical features of the objects, this case has to be referred to the tomography with a limited angle of observation. As a result of computer experiments we have established that multiview cylindrical configurations are able to provide microwave tomograms of the breast with a small size tumor inside. Using the gradient method, we have developed a computer code to create images of the three-dimensional objects in dielectrical properties on microwave frequencies.

A model of dielectrical properties of cellular structures of a tissue has been proposed. Cellular structures were presented as a composition of membrane covered spheres and cylinders that do not interact with each other. No restrictions were applied to the thickness of cellular membranes. The model was further generalized into a case of electrically interacting cells. The difference in dielectrical properties calculated with the model of electrically noninteracting versus interacting cells is inversely dependent on frequency. At biological values of cellular volume fraction near 0.7 (packed configuration) the difference is about 10%-15% in resistance and in epsilon' for frequencies near 0.1 MHz. Experimental data for myocardial tissue and theoretical data, for both interacting and noninteracting models, reasonably agree at frequencies of 1-100 MHz.

Right bundle branch block with coved ST-segment elevation in leads V1 through V3 is the electrocardiographic (ECG) marker of the Brugada syndrome. We describe a healthy young man with a normal baseline ECG in whom a transient Brugada pattern was observed repeatedly after recreational cocaine use. Intravenous administration of procainamide and subsequent intravenous propranolol followed by noradrenaline failed to reproduce the Brugada sign. An electrophysiologic study performed in the presence of the Brugada ECG pattern showed no inducible arrhythmias. This case illustrates that, in susceptible individuals, cocaine may provoke the Brugada sign. The clinical importance of this cocaine-induced ECG abnormality is currently unknown.

The purpose of this study was to assess whether physiologic changes in canine myocardium due to coronary blood flow reduction, ischemia, and infarction could be detected by changes in dielectrical properties. Measurements were made in the frequency range of 0.2-6.0 GHz. Percent coronary blood flow reduction was linearly related to the decrease in epsilon" at frequencies of 0.2 GHz(R =-0.997) and 1.1 GHz (R =- 0.9987). In 2 h occlusions, increased conduction time in the infarct area mirrored the temporal changes in dielectrical properties. In 2-week-old infarctions differences, in epsilon' between normal and central infarct zones were statistically significant (P<0.05) for all frequencies. For epsilon" the differences between normal and central infarct zones were also significant for all frequencies (P<0.01). In conclusion, coronary blood flow reduction, ischemia, and infarction can be detected by microwave spectroscopy and potentially can form the basis for a physiologic microwave tomographic imaging system.

The proposed dielectrical relaxation model of the myocardium in the microwave spectrum has been verified both on test solutions and on normal canine myocardium. Furthermore, the model was utilized to reconstruct the changes in tissue properties (including myocardial bulk resistance and water content) following myocardial acute ischemia and chronic infarction. It was shown that the reconstructed myocardial resistance and water content correlate dynamically with the process of the development of acute myocardial ischemic injury. In chronic cases the reconstructed resistance and water content of infarcted myocardium are significantly different from that of normal myocardium: the resistance is lower and water content is higher than in normal myocardium.

A method of image reconstruction in three-dimensional (3-D) microwave tomography in a weak dielectric contrast case has been developed. By utilizing only one component of the vector electromagnetic field this method allows successful reconstruction of images of 3-D mathematical phantoms. A prototype of the 3-D microwave tomographic system capable of imaging 3-D objects has been constructed. The system operates at a frequency of 2.36 GHz and utilizes a code-division technique. With dimensions of the cylindrical working chamber z = 40 cm and d = 60 cm, the system allows measurement of an attenuation up to 120 dB having signal-to-noise ratio about 30 dB. The direct problem solutions for different mathematical approaches were compared with an experimentally measured field distribution inside the working chamber. The tomographic system and the reconstruction method were tested in simple experimental imaging.

Background: Surgical ablation of ventricular tachycardia (VT) after myocardial infarction has been reported by different endocardial approaches. The ventriculotomy may increase mortality of the procedure., Methods and Results: We report on nine patients who suffered from recurrent VT in the late post-myocardial infarction period. Significant stenoses were detected in all patients. The mean left ventricular ejection fraction was 43.1 +/- 8.3%. Left ventricular scar (n = 9) was seen. The mean NYHA class was 2.2 +/- 0.4. Sustained VT (mean cycle length, 293 +/- 52 ms) occurred spontaneously (n = 9) and could be induced reproducibly. Catheter mapping detected a prematurity of -42 +/- 13 ms in six patients. Clinical VT was inducible during surgery in seven patients. Middiastolic potentials were detected from the epicardial surface (n = 3), and premature potentials were found (n = 8 with prematurity of -108 +/- 46 ms). Application of neodymium/yttrium/argon/ garnet (Nd:YAG) laser energy to early epicardial activation terminated the arrhythmia (n = 7). Ventriculotomy was not performed. Seven patients have been free of VT for a mean follow-up period of 17 +/- 11 months; one patient relapsed and was treated with an implantable cardioverter-defibrillator, as was a second patient with inducible VT after surgery., Conclusions: Surgical Nd:YAG laser photocoagulation of VT on the epicardial surface of the heart in post-myocardial infarction patients without ventriculotomy is safe and has a high success rate. At the present time, this method is recommended in patients with sustained and tolerated VT who need bypass surgery. This is the first report on epicardial laser ablation of VT in post-myocardial infarction VT.

Microwave tomographic imaging is one of the new technologies which has the potential for important applications in medicine. Microwave tomographically reconstructed images may potentially provide information about the physiological state of tissue as well as the anatomical structure of an organ. A two-dimensional (2-D) prototype of a quasi real-time microwave tomographic system was constructed. It was utilized to reconstruct images of physiologically active biological tissues such as an explanted canine perfused heart. The tomographic system consisted of 64 special antennae, divided into 32 emitters and 32 receivers which were electronically scanned. The cylindrical microwave chamber had an internal diameter of 360 mm and was filled with various solutions, including deionized water. The system operated on a frequency of 2.45 GHz. The polarization of the incident electromagnetic field was linear in the vertical direction. Total acquisition time was less than 500 ms. Both accurate and approximation methods of image reconstruction were used. Images of 2-D phantoms, canine hearts, and beating canine hearts have been achieved. In the worst-case situation when the 2-D diffraction model was used for an attempt to "slice" three-dimensional (3-D) object reconstruction, we still achieved spatial resolution of 1 to 2 cm and contrast resolution of 5%.

This study was undertaken to investigate the activation patterns of spontaneous ventricular arrhythmias during acute myocardial ischemia in dogs. In 14 open-chest dogs, the left anterior descending coronary artery was occluded for 2 hours. Three-dimensional activation maps were derived from 240 bipolar sites by insertion of 60 plunge needle electrodes into both ventricles and the septum. Global ventricular activation sequences were displayed in five planes in 10 dogs, whereas the high density regional activation maps of the anterior wall were displayed in four layers in 4 dogs. Three-dimensional activation maps of 95 sinus beats, 82 premature ventricular complexes (PVCs), and 210 beats of ventricular tachycardia (VT) were analyzed. Sinus beats had a uniform activation pattern with total ventricular activation times measuring 42 +/- 4 ms and 67 +/- 8 ms during baseline and ischemia, respectively (P < .05). The PVCs and VTs originated from the subendocardial and intramural layers, and activation patterns invariably suggested focal excitation. Macroeentry was not operative because (1) the breakthrough sites were always remote from the latest activation areas; (2) there was no electrical activity bridging the gap between the termination of a beat and initiation of the subsequent beat; and (3) impulse conduction was not sufficiently delayed to reexcite the area of impulse origin even though functional conduction block was frequently present. In high-density regional activation maps, fragmented activity spanning the diastolic interval was never found. In conclusion, spontaneously occurring PVCs and VTs during acute myocardial ischemia in dogs display focal excitation with no evidence of macroreentry.

To determine safety and efficacy of neodymium:YAG laser irradiation of the endocardium, temperatures at both the epicardium and the endocardium were recorded for thermal damage evaluation. A total of 48 coagulation lesions were created at power settings of 20 and 30 W in 20 open chest dogs by transcatheter endocardial laser irradiation. Tissue temperatures were monitored by epicardial thermography (Tepi), and by endocardial thermocouples at the catheter tip (Tprox) and 4 mm below the endocardial surface (Tdist). In group I the optical fiber extended 1 mm from the catheter and irradiation times ranged from 3 to 60 sec. Tepi reached > or = 57 degrees after a weighted average of 5 sec of laser irradiation (n = 44). In group II the fiber was retracted 1 mm from the catheter tip, and irradiation times were 100 to 150 sec. Tepi reached > or = 57 degrees C after a weighted average of 30 sec (n = 4). Blood vessels were recognized as heat sinks until coagulation occurred. Lesion volume showed a proportional increase with total delivered energy. From the observed timeframes in epicardial temperature rise it is suggested that total direct light absorption at the epicardium was the main contribution to Tepi, and the Nd:YAG laser can efficiently create transmural lesions. The epicardial temperatures remained below 80 degrees C in combination with the constant movement of the epicardial wall suggested safety from thermal damage to the ambient organs.

Endothelial cell injury, the disruption of the internal elastic membrane and medial damage represent important stimuli for the development of a neointima. It is unclear whether selective adventitial and medial injury also induce neointima formation. Incremental argon laser energies (11.4-180 J/cm2) were applied to the external surface of dog femoral arteries to evaluate the vascular repair of acute adventitial or medial necrosis without injury of the intima. The animals were sacrificed either one hour after the initial procedure or after an 8 week follow up period for histologic examination. Acute, and mild to moderate necrosis of the arterial wall was found above 50 J/cm2. Ablation of the internal elastic membrane or mural thrombi was not detected. Eight weeks after photocoagulation with laser energies above 50 J/cm2, a significant increase in mean wall thickness of the media was observed. The medial thickening was characterised by an accumulation of extracellular matrix and a loss of smooth muscle cells. Necrosis of adventitia and media resulted in arterial wall thickening without neointima formation. It is concluded that, in dogs, an acute, selective injury of adventitia and media stimulates the production of extracellular matrix and not the proliferation of cells. Smooth muscle cell migration and subsequently neointima formation are induced by viable smooth muscle cells when blood-borne stimuli are available.

Tissue samples ranging from 2 to 16 mm in thickness were irradiated at 1064 nm with energies ranging from 40 to 2400 J. Coagulation lesions of in vitro and in vivo experiments were subjected to temperature profiling and submitted for histology. Irreversible damage was calculated with the damage integral formalism, following the bioheat equation solved with Monte Carlo computer light-distribution simulations. Numerical temperature rise and coagulation depth compared well with the in vitro results. The in vivo data required a change in the optical properties based on integrating sphere measurements for high irradiance to make the experimental and numerical data converge. The computer model has successfully solved several light-tissue interaction situations in which scattering dominates over absorption.

Objectives: The purpose of this study was to evaluate systematically the effects of the adenosine antagonist aminophylline on resuscitation outcome in a canine model of postcardioversion nonperfusing rhythm., Background: Theoretic considerations and experimental studies indicate that myocardial adenosine accumulation during prolonged ventricular fibrillation might play a significant role in postcardioversion asystole and electromechanical dissociation. A recent uncontrolled clinical trial has suggested that the adenosine antagonist aminophylline might improve the outcome of cardiopulmonary resuscitation from refractory bradyasystolic cardiac arrest., Methods: Two placebo-controlled, randomized, blinded experimental studies were performed. In protocol 1 (20 dogs), ventricular fibrillation was induced and maintained for 7.5 min. Sixty seconds before cardioversion, dogs received 1 mg of epinephrine followed by 250 mg of aminophylline or placebo. In protocol 2 (20 dogs), dogs were cardioverted to electromechanical dissociation after 5 min of unsupported ventricular fibrillation. Sixty seconds later, all dogs received 1 mg of epinephrine followed by 250 mg of aminophylline or placebo. In both experiments, resuscitation efforts were continued until return of spontaneous circulation, or up to 30 min. The primary end point was survival to 1 h., Results: In protocol 1, 4 of 10 dogs survived in the aminophylline group, whereas 7 of 10 dogs survived in the placebo group, a nonsignificant trend toward unfavorable outcome from aminophylline. Pretreatment with aminophylline increased the number of cardioversion applications required to terminate ventricular fibrillation. In protocol 2, 5 of 10 and 6 of 10 dogs survived in the aminophylline and placebo groups, respectively., Conclusions: The results of this study suggest that aminophylline fails to improve the outcome of resuscitation from prolonged ventricular fibrillation. It does not reverse established electromechanical dissociation and may in fact increase the number of cardioversion applications required to terminate ventricular fibrillation. The rationale for conducting clinical trials with aminophylline during cardiopulmonary resuscitation is questionable.

The purpose of this study was to evaluate prospectively the efficacy and safety of mobile cardiac catheterization. Mobile cardiac catheterization was introduced into clinical practice in 1989, but there has been no systematic study of its performance and safety. A registry was established in 1989 to monitor outcomes with mobile cardiac catheterization and is reported here. Patients were screened for eligibility for mobile cardiac catheterization using the joint AHA/ACC criteria for outpatient angiography. Eligible patients underwent mobile catheterization at eight hospitals within 120 miles of the base tertiary center. Helicopter evacuation services were available at each mobile site. The indications, findings, dispositions, and complications of mobile cardiac catheterization were recorded by means of a checklist, telephone follow-up and chart review. A total of 1,001 consecutive patients were entered into the registry in the first 20 months of operation, including 436 females and 565 males aged 22 to 84 years. Angina (Canadian Classes II-IV) was the most frequent primary indication for catheterization (46.4%), followed by atypical chest pain (36.9%), or a positive exercise stress test (25.6%). Infrequent indications for catheterization included a history of myocardial infarction (5.6%), congestive heart failure (7.1%), arrhythmias (4.1%), and valvular heart disease (0.7%). Catheterization was accomplished in 99.9% of patients. Angiographically normal studies were observed in 22.8%, and mild (< or = 50%) coronary artery disease in 13.6% of patients.(ABSTRACT TRUNCATED AT 250 WORDS)

The study group included 1,553 consecutive patients from areas serviced by our mobile catheterization laboratories: 719 procedures were performed in the mobile unit at their local hospitals, 277 were performed at a tertiary hospital with less than a 24 hr hospital stay, and 557 were performed at a tertiary hospital as inpatients. The indications for mobile catheterization were predominantly atypical chest pain, angina pectoris, or positive treadmill stress test, whereas patients with less than 24 hr hospitalization at the tertiary center had their catheterization performed for additional reasons. The majority of the inpatient indications were for recent myocardial infarction or unstable angina. Using the American College of Cardiology/American Heart Association (ACC/AHA) criteria for outpatient catheterization, the mobile catheterizations were performed safely with a complication rate of only 0.7% compared to a complication rate of 3.1% for inpatients demonstrating that a low risk group of patients can be prospectively identified and catheterized safely in the mobile setting. An extremely high risk group of patients with ongoing unstable angina and recent myocardial infarction was also identified which should undergo catheterization only at a tertiary center.

A simple catheterization technique for producing suppression or block of atrioventricular (AV) conduction in dogs by neodymium:YAG (Nd:YAG) laser photocoagulation from the aortic root is described. In 14 canines, electrode catheters were percutaneously advanced to the high right atrium, to the His bundle area at the tricuspid valve (right His [RH]), and the noncoronary cusp of the aortic valve (left His [LH]). A 400 microns optical fiber was fed into the lumen of the LH catheter. Continuous wave Nd:YAG lasing at lambda = 1,064 nm at 15-20 watts was performed at the site of the largest recorded LH deflection, opposite to the RH catheter, until complete AV block (CAVB, group I, eight dogs) or persistent A-H prolongation (AHPr, group II, four dogs) occurred; in two animals, laser photoablation was unsuccessful. CAVB and AHPr were achieved in < 140 seconds of lasing. Mean lasing time was 51 +/- 41 seconds, total energy delivered, 776 +/- 601 J. There were no complications. Dogs were followed for up to 120 days. In group I, 6/6 dogs followed for at least 1 week continued to have CAVB (three AV nodal, five infra-His). In group II, 1/4 dog progressed to infra-His CAVB; 3/4 had depressed AV nodal function. AV block was not reversed by isoproterenol. Aortic root angiograms revealed intact valve function in all. Histology showed various degrees of damage and repair of AV junctional tissue. There were no valvular perforations. Transcatheter Nd:YAG laser photocoagulation of the AV junction from the aortic root is a fast, simple, and safe method for creating AV conduction block in dogs.

Objectives: The purpose of this study was to test the feasibility of selective elimination of ventriculoatrial (VA) conduction by limited laser photocoagulation of the atrioventricular (AV) node, and to analyze the histologic substrate of unidirectional retrograde block., Background: Atrioventricular node reentry requires intact retrograde conduction., Methods: Neodymium:yttrium-aluminum-garnet laser photocoagulation was performed during cardiopulmonary bypass through a right atriotomy in 15 dogs that had intact retrograde conduction before operation. Short laser pulses were delivered to an area between the coronary sinus orifice and the proximal His bundle. The end point of lasing was second-degree AV node block at a paced atrial cycle length of 250 ms., Results: Complete retrograde block developed immediately in 11 of the 15 dogs (group I), while AV conduction persisted in all 11. In 4 of the 15 dogs (group II), both AV and VA conduction remained intact. During a 3-month follow-up period, retrograde conduction remained absent in all group I dogs. Retrograde block was not reversed by isoproterenol. Anterograde AV node characteristics (Wenckebach cycle length, functional refractory period, ventricular rate during atrial fibrillation) were unchanged in five dogs and modified in six. Complete AV block did not develop. In four control dogs (group III, sham operation), anterograde and retrograde AV node characteristics were unchanged. The anterograde Wenckebach cycle lengths in groups I, II and III at 3 months measured 192 +/- 15 ms, 195 +/- 6 ms and 170 +/- 22 ms, respectively, whereas the retrograde Wenckebach cycle lengths in groups II and III measured 345 +/- 62 ms and 278 +/- 25 ms, respectively. Histologic study at 3 months in cases with unidirectional VA block showed the compact part of the AV node intact with destruction of the atrial approaches and the superficial layers of the proximal end of the node on the right side., Conclusions: 1) With limited laser photocoagulation of the proximal AV node area, VA conduction can be eliminated and anterograde AV node transmission maintained. 2) Destruction of the atrial approaches on the right side with preservation of the compact part of the AV node may result in unidirectional retrograde block.

Background: Directed surgery for the definitive treatment of drug resistant ventricular tachycardia (VT) due to coronary artery disease carries a significant operative mortality. Surgical failure to cure VT remains a problem, especially in patients without anterior left ventricular myocardial infarcts and aneurysms. A method has been developed in which Nd:YAG laser is used to photocoagulate myocardium responsible for the initiation of VT using a "sequential" approach intended to improve operative results and gain insight into the variable substrates causing VT., Methods: Under normothermic cardiopulmonary bypass, VT is induced and then extensive endocardial and epicardial mapping performed to localize and characterize that form of VT. Nd:YAG is applied to the areas of myocardium from which that form of VT originates until it disappears and is no longer inducible. Next attempts are made to induce other forms of VT and when successful, mapping and lasing repeated until finally VT is no longer inducible., Results: Fifty-one patients were operated on and have been followed for at least 1 year. Operative mortality in 12 patients with preoperative ejection fractions less than 20% was 41%; in 39 patients with ejection fractions greater than 20% operative mortality was 8%. Eighty-eight percent of the 43 operative survivors are free of recurrent sustained VT at 1 year. There have been no arrhythmic mortalities. In a group of 30 patients evaluated for epicardial VT, 9 of 14 patients with inferior infarcts without left ventricular aneurysms had at least one form of epicardial VT., Conclusions: Nd:YAG laser photocoagulation of myocardial VT using a sequential approach is a viable method that permits an ongoing study of this entity. Operative mortality remains high in patients with diffusely poor left ventricular function. Epicardial VT is frequent in patients with inferior infarcts and may account for inferior results in these patients when conventional endocardial approaches are used alone.

Angioplasty is considered as an alternative to surgical reconstruction of arteriosclerotic vessels especially since lasers and atherectomy devices have become clinically available. However, the resulting arterial injury may lead to acute thrombotic occlusion and chronic restenosis because of hyperplastic vascular repair. The purpose of this experimental study was to evaluate the consequences of thermal laser arterial injury on platelet deposition and myointimal hyperplasia in dog femoral arteries. An intraarterial, short-term prostacyclin (PGI2) infusion was given to evaluate the antithrombotic and antiproliferative effects of this drug. Severe arterial necrosis, partly carbonized and vacuolized, extending to the adventitia was induced by a transluminal heated laser probe motion. The platelet deposition after one hour was 33.62 +/- 6.56 (x 10(6)/cm2.) (mean +/- SEM) without prostacyclin, after 40 ng/kg/min prostacyclin (PGI2) 24.70 +/- 5.45 and after 400 ng/kg/min 9.3 +/- 2.26 (p less than 0.005 no PGI2 vs 400 ng/kg/min PGI2). Myointimal hyperplasia was present eight weeks after thermal laser vascular injury independent of the initially administered prostacyclin. In conclusion, acutely thrombotic and chronically hyperplastic femoral arteries were found following transluminal thermal arterial injury in dogs. Prostacyclin administration could be clinically beneficial in reducing acute vascular thrombosis following thermal angioplasty. Short-term use of this substance, however, may not prevent a hyperplastic response to angioplasty.

Neodymium:yttrium-aluminum-garnet (YAG) photocoagulation during ventricular tachycardia allows the electrophysiologic effects of the temporal and spatial sequence of energy delivery to be correlated with local activation times. A retrospective analysis was performed of the termination of 19 episodes of ventricular tachycardia for which the local diastolic activation time was known for all successful ablation sites and for 95% of all ablation sites. The mode of termination was compared with that of 26 episodes of spontaneously terminating ventricular tachycardias. Spontaneous terminations occurred without a change in cycle length (54%) or with a 7 +/- 15% change in cycle length over one to three terminal beats (46%). In contrast, laser ablation-induced terminations resulted in a 39 +/- 55% increase in cycle length over nine or more cycles. The effect of attempted laser ablation was compared with the local presystolic activation time and the local activation time expressed as a percent of the diastolic interval (end of QRS complex = 0%, onset of next QRS complex = 100%). With one exception, no tachycardia terminated at ablation sites activating less than -50 ms before the QRS complex. All 8 successful first ablation attempts and 13 of all 19 successful ablations occurred in the 35% to 50% interval of diastolic activation. All successful ablations at sites activating at greater than 50% of the diastolic interval required multiple ablation attempts. Successful ablation was performed from the epicardium in 6 and from the endocardium in 13 episodes of ventricular tachycardia. These results are most consistent with a macroreentrant mechanism with a region of high vulnerability represented by the 35% to 50% interval of diastolic activation.(ABSTRACT TRUNCATED AT 250 WORDS)

The purpose of this study was to test the feasibility of neodymium-yttrium-aluminum-garnet (Nd-YAG) laser photocoagulation of the atrioventricular (AV) node to control the ventricular rate during rapid atrial rhythms without creating AV block. In 12 dogs on normothermic cardiopulmonary bypass, short laser pulses were delivered to an area between the coronary sinus orifice and the site of the most proximally recorded His deflection until second degree AV block occurred at a paced atrial rate of 200 beats/min. Long-term effects on AV node function were followed up for 3 months. Three animals developed chronic high grade AV block. In nine animals with preserved 1:1 conduction, the mean (+/- SEM) critical atrial cycle length resulting in AV node Wenckebach periodicity increased from 183 +/- 6 to 261 +/- 24 ms (+43%), the mean RR interval during induced atrial fibrillation increased from 248 +/- 14 to 330 +/- 27 ms (+32%) and the shortest RR interval during atrial fibrillation increased from 215 +/- 11 to 275 +/- 20 ms (+28%). Laser effects were not reversed by isoproterenol infusion. Histologic examination of the irradiated area showed fibrotic changes in the AV node and fatty metamorphosis. This study suggests that 1) graded Nd-YAG laser photocoagulation of the AV node region in dogs results in long-term modification of anterograde AV node transmission properties; 2) 1:1 conduction during sinus rhythm usually remains preserved, but ventricular rate during rapid atrial rhythms is chronically reduced; and 3) progression to high grade AV block occurs in a minority of animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Atrioventricular (AV) nodal reentry requires intact retrograde ventriculoatrial (VA) conduction. The purpose of this study was to assess the contribution of various pacing and pharmacologic techniques to uncover VA conduction during apparent unidirectional VA block, and to evaluate the role of several biologic and electrophysiologic factors in concealment of retrograde conduction. Forty healthy dogs underwent catheter-electrophysiologic studies of AV and VA conduction. Group I (20 animals) had intact VA conduction. Group II (six animals) had VA dissociation with ventricular pacing initiated during sinus rhythm, but the presence of VA conduction was confirmed by isoproterenol infusion or by premature ventricular stimulation. In group III (14 animals), the above techniques failed to uncover VA conduction. Eight of 14 group III animals underwent thoracotomy and crushing or freezing of the sinoatrial (SA) node. Ventricular pacing initiated during sinus standstill was associated with 1:1 VA conduction in each experiment. VA conduction time and retrograde Wenckebach cycle length, both in the baseline state and during isoproterenol infusion, were significantly longer in the eight animals in group III than in those in group I. Age, gender, weight, breed, sinus cycle length, and anterograde AV conduction properties were not significantly different between groups I, II, and III. The data suggest that (1) in normal dogs, complete unidirectional VA block probably does not exist; (2) in the presence of anterograde input to the AV node, even sophisticated pacing and pharmacologic maneuvers may fail to uncover the presence of VA conduction; (3) although anterograde input is essential for concealment of VA conduction, the phenomenon is more closely associated with depressed retrograde than with anterograde AV nodal characteristics.(ABSTRACT TRUNCATED AT 250 WORDS)