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We describe a real time, noninvasive method of estimating work of breathing (esophageal balloon not required) during noninvasive pressure support (PS) that uses an artificial neural network (ANN) combined with a leak correction (LC) algorithm, programmed to ignore asynchronous breaths, that corrects for differences in inhaled and exhaled tidal volume (VT) from facemask leaks (WOBANN,LC/min). Validation studies of WOBANN,LC/min were performed. Using a dedicated and popular noninvasive ventilation ventilator (V60, Philips), in vitro studies using PS (5 and 10 cm H2O) at various inspiratory flow rate demands were simulated with a lung model. WOBANN,LC/min was compared with the actual work of breathing, determined under conditions of no facemask leaks and estimated using an ANN (WOBANN/min). Using the same ventilator, an in vivo study of healthy adults (n = 8) receiving combinations of PS (3-10 cm H2O) and expiratory positive airway pressure was done. WOBANN,LC/min was compared with physiologic work of breathing/min (WOBPHYS/min), determined from changes in esophageal pressure and VT applied to a Campbell diagram. For the in vitro studies, WOBANN,LC/min and WOBANN/min ranged from 2.4 to 11.9 J/min and there was an excellent relationship between WOBANN,LC/breath and WOBANN/breath, r = 0.99, r(2) = 0.98 (p < 0.01). There were essentially no differences between WOBANN,LC/min and WOBANN/min. For the in vivo study, WOBANN,LC/min and WOBPHYS/min ranged from 3 to 12 J/min and there was an excellent relationship between WOBANN,LC/breath and WOBPHYS/breath, r = 0.93, r(2) = 0.86 (p < 0.01). An ANN combined with a facemask LC algorithm provides noninvasive and valid estimates of work of breathing during noninvasive PS. WOBANN,LC/min, automatically and continuously estimated, may be useful for assessing inspiratory muscle loads and guiding noninvasive PS settings as in a decision support system to appropriately unload inspiratory muscles.

Purpose: We hypothesized that non-invasively determined work of breathing per minute (WOB(N)/min) (esophageal balloon not required) may be useful for predicting extubation outcome, i.e., appropriate work of breathing values may be associated with extubation success, while inappropriately increased values may be associated with failure., Methods: Adult candidates for extubation were divided into a training set (n = 38) to determine threshold values of indices for assessing extubation and a prospective validation set (n = 59) to determine the predictive power of the threshold values for patients successfully extubated and those who failed extubation. All were evaluated for extubation during a spontaneous breathing trial (5 cmH(2)O pressure support ventilation, 5 cmH(2)O positive end expiratory pressure) using routine clinical practice standards. WOB(N)/min data were blinded to attending physicians. Area under the receiver operating characteristic curves (AUC), sensitivity, specificity, and positive and negative predictive values of all extubation indices were determined., Results: AUC for WOB(N)/min was 0.96 and significantly greater (p < 0.05) than AUC for breathing frequency at 0.81, tidal volume at 0.61, breathing frequency-to-tidal volume ratio at 0.73, and other traditionally used indices. WOB(N)/min had a specificity of 0.83, the highest sensitivity at 0.96, positive predictive value at 0.84, and negative predictive value at 0.96 compared to all indices. For 95% of those successfully extubated, WOB(N)/min was ≤10 J/min., Conclusions: WOB(N)/min had the greatest overall predictive accuracy for extubation compared to traditional indices. WOB(N)/min warrants consideration for use in a complementary manner with spontaneous breathing pattern data for predicting extubation outcome.

Background: Pressure support ventilation (PSV) should be applied so that the inspiratory muscles are unloaded appropriately. We developed a computerized advisory system that assesses the load on the inspiratory muscles to spontaneously inhale, reflected by the automatically and noninvasively measured work of breathing per minute, and tolerance for that load, reflected by spontaneous breathing frequency and tidal volume, in a fuzzy-logic algorithm that provides recommendations for setting PSV. We call this a load and tolerance strategy for determining PSV., Methods: In a clinical validation study, we compared the recommendations from our PSV advisory system to the recommendations of experienced critical-care Registered Respiratory Therapists (RRTs) for setting PSV in patients with respiratory failure. With 76 adult patients in a university medical center surgical intensive care unit receiving PSV, a combined pressure/flow sensor, positioned between the endotracheal tube and patient Y-piece, sent measurements to the PSV advisory system. We compared the advisory system's recommendations (increase, maintain, or decrease the pressure support) to the RRTs' recommendations at the bedside., Results: There were no significant differences between the RRTs' and the advisory system's recommendations (n = 109) to increase, maintain, or decrease PSV. The RRTs agreed with 91% of the advisory system's recommendations (kappa statistic 0.85, P < .001). The advisory system was very good at predicting the RRTs' pressure support recommendations (r(2) = 0.87, P < .02)., Conclusions: A load and tolerance strategy with a computerized PSV advisory system provided valid recommendations for setting PSV to unload the inspiratory muscles, and the recommendations were essentially the same as the recommendations from experienced critical-care RRTs. The PSV advisory system operates continuously and automatically and may be useful in clinical environments where experts are not always available.

Objective: To measure the imposed power of breathing (imposed work of breathing per minute) associated with spontaneous breathing through an active impedance threshold device and a sham impedance threshold device., Design: Prospective randomized blinded protocol., Setting: University medical center., Patients: Nineteen healthy, normotensive volunteers (10 males, 9 females, age range 20-56 y, mean +/- SD weight 54.8 +/- 7.7 kg for females, 84 +/- 8 kg for males)., Methods: The volunteers completed 2 trials of breathing through a face mask fitted with an active impedance threshold device set to open at -7 cm H(2)O pressure, or with a sham impedance threshold device, which was identical to the active device except that it did not contain an inspiratory threshold pressure valve diaphragm. Spontaneous breathing frequency (f), tidal volume (V(T)), exhaled minute ventilation, inspiratory pressure, and inspiratory time were measured with a respiratory monitor, and the data were directed to a laptop computer for real-time calculation of the imposed power of breathing., Results: There were no significant differences in heart rate, respiratory rate, tidal volume, and minute ventilation, with and without inspiratory impedance. For the sham and active impedance threshold device groups, respectively, the mean +/- SD imposed power of breathing values were 0.92 +/- 0.63 J/min and 8.18 +/- 4.52 J/min (p < 0.001), the mean +/- SD inspiratory times were 1.98 +/- 0.86 s and 2.97 +/- 1.1 s (p = 0.001), and the mean +/- SD inspiratory airway/mouth pressures were -1.1 +/- 0.6 cm H(2)O and -11.7 +/- 2.4 cm H(2)O (p < 0.001)., Conclusions: Breathing through an active impedance threshold device requires significantly more power than breathing through a sham device. All subjects tolerated the respiratory work load and were able to complete the study protocol.

Objective: To evaluate the operation of a continuous positive airway pressure system by using tracheal airway pressure (PT) as the control signal for system operation (i.e., tracheal pressure control)., Design: Repeated measures., Setting: University research laboratory., Subjects: Twelve anesthetized, spontaneously breathing swine., Interventions: Subjects were intubated and connected to a tracheal pressure control system (5 cm H2O continuous positive airway pressure). Varying inspiratory flow demands and degrees of partial endotracheal tube occlusion (25%, 50%, and 75%) were studied. Tracheal pressure control was compared with a conventionally controlled system (pressure from breathing circuit Y-piece [PY] used as control signal) during endotracheal tube occlusion., Measurements and Results: Imposed resistive work of breathing (work to spontaneously inhale through endotracheal tube and ventilator circuit), work by ventilation system assisting inhalation, PT, PY, tidal volume, and inspiratory flow demands were measured. As inspiratory flow demands increased (range, 0.2-2.3 L/sec), pressure assist increased automatically (range, 5-40 cm H2O) as well as work of breathing by ventilation system assisting inhalation (range, 0.2-2.5 J/L). Imposed resistive work of breathing was nullified at the lower and was negligible at the higher flow demands. During endotracheal tube occlusion with a conventionally controlled system, PY was unchanged, whereas PT decreased (up to -15 cm H2O) and imposed resistive work of breathing increased (up to 1.05 J/L). With tracheal pressure control, PY increased automatically (range, 8-52 cm H2O), whereas PT varied slightly (range, 2 to -4.6 cm H2O). Imposed resistive work of breathing was negligible (range, 0-0.2 J/L). Breathing circuit pressure (PY), not pulmonary airway pressure (PT), increased significantly during tracheal pressure control., Conclusions: Tracheal pressure control results in automatic and variable levels of pressure assist to decrease imposed resistive work of breathing under conditions of varying spontaneous inspiratory flow demands and endotracheal tube occlusion. Conventional systems are potentially flawed when PY is used as the control signal because they do not function in this manner and do not accurately assess pulmonary airway pressure.

Objectives: Triggering a ventilator "ON" at the carinal end of the endotracheal tube decreases imposed work of breathing by bypassing the resistance imposed by the breathing circuit and the endotracheal tube. We compared work of breathing during spontaneous ventilation between three methods of triggering the ventilator "ON": a) conventional pressure triggering from inside the ventilator; b) flow-by triggering; or c) tracheal pressure triggering at the carinal end of the endotracheal tube. We hypothesized that the work of breathing would be substantially decreased with tracheal pressure triggering compared with conventional pressure and flow-by methods in patients receiving continuous positive airway pressure., Design: Clinical, prospective study., Setting: University teaching hospital., Patients: Fourteen adults diagnosed with acute respiratory failure., Interventions: All patients were breathing spontaneously at an FIO2 of 0.30 to 0.40 and received 5 cm H2O of continuous positive airway pressure. Three different methods of triggering the ventilator while set in the continuous positive airway pressure mode were administered in random order., Measurements and Main Results: Real-time measurements of esophageal pressure and tidal volume were integrated with a respiratory monitor (CP-100, Bicore, Riverside, CA) that uses the Campbell diagram to calculate total work of breathing. Imposed work of breathing was calculated by integrating tidal volume with the pressure at the carinal end of the endotracheal tube. Physiologic work of breathing was calculated by subtracting imposed work of breathing from the total work of breathing. Breathing frequency, the index of rapid shallow breathing (breathing frequency/tidal volume), peak inspiratory flow rate demand, exhaled minute ventilation, and the duration of respiratory muscle contraction assessed by the ratio of inspiratory time to total cycle time were also measured. Data were analyzed by Friedman's repeated-measures analysis of variance on ranks. Alpha was set at .05 for statistical significance. Imposed work of breathing decreased to approximately zero during tracheal pressure triggering. As a result, total work of breathing decreased by approximately 40% compared with the flow-by and conventional methods. During tracheal pressure triggering only, airway pressure increased above baseline pressure to approximately 11 cm H2O, which resembled pressure-support ventilation. Also, during tracheal pressure triggering, tidal volume and peak inspiratory flow rate were significantly increased, while the pressure-time product and the index of rapid shallow breathing were significantly decreased. Hemodynamic status and oxygen saturation were not clinically affected., Conclusions: The tracheal pressure triggering of a demand-flow continuous positive airway pressure system creates an effect similar to pressure-support ventilation that significantly decreases imposed work of breathing and, thus, total work of breathing. We recommend moving the triggering site of the ventilator to the carinal end of the endotracheal tube.

Objective: To evaluate the relationships between directly measured work of breathing (WOB) and variables of the breathing pattern commonly used at the bedside to infer WOB for intubated, spontaneously breathing patients treated with pressure support ventilation (PSV)., Design: In vivo measurements of the WOB were obtained on a consecutive series of adults. Breathing frequency (f), tidal volume (VT), the index of rapid, shallow breathing (f/V T), the duration of respiratory muscle contraction expressed as the ratio of inspiratory time over total respiratory cycle time (TI/TTOT), and a breathing pattern score (applied to approximately 50% of the patients) which ranks f, VT, sternocleidomastoid muscle activity, substernal retraction, and abdominal paradox on a scale were variables of the breathing pattern were also measured. The greater the breathing pattern score, the lower the WOB and vice versa., Setting: Surgical ICUs in two university teaching hospitals., Patients: Sixty-seven adults (42 men and 25 women, aged 20 to 78 years) who had acute respiratory failure from various etiologies were studied. All patients were breathing spontaneously receiving continuous positive airway pressure and PSV., Interventions: Intraesophageal pressure (indirect measurement of intrapleural pressure) was measured with an esophageal balloon integrated into a nasogastric tube. VT was obtained by positioning a flow sensor between the "Y" piece of breathing circuit and the endotracheal tube. Data from these measurements were directed to a bedside respiratory monitor (Bicore; Allied Healthcare Products; Riverside, Calif) that calculates WOB using the Campbell diagram. Patients received PSV at levels deemed reasonable to unload the respiratory muscles. All measurements were obtained after 15 to 20 min at each level of PSV, averaged over 1 min, and then variables of the breathing pattern were regressed with directly measured values for WOB., Results: All breathing pattern variables poorly predicted WOB as evidenced by the low values for the coefficients of determination (r2). Breathing frequency correlated positively with WOB (r = 0.47, p < 0.001) and predicted or explained only 22% (r2 = .22) of the variance in WOB. VT correlated negatively and f/VT and TI/TTOT each correlated positively with WOB. However, these variables predicted only 20 to 27% of the variance in WOB. The breathing pattern score correlated negatively with WOB and predicted only 43% of the variance in WOB. A prediction model taking all variables into consideration using multiple regression analysis predicted only 50% of the variance in WOB; thus, it too was a poor to moderate predictor of WOB., Conclusion: Our data reveal that WOB should be measured directly because variables of the breathing pattern commonly used at the bedside appear to be inaccurate and misleading inferences of the WOB. The clinical implication of these findings involves the traditional and empirical practice of titrating PSV based on the breathing pattern. We do not imply that the patient's breathing pattern should be ignored, nor undermine its importance, for it provides useful diagnostic information. It appears, however, that relying primarily on the breathing pattern alone does not provide enough information to accurately assess the respiratory muscle workload. Using the breathing pattern as the primary guideline for selecting a level of PSV may result in inappropriate respiratory muscle workloads. A more comprehensive strategy is to employ WOB measurements and the breathing pattern in a complementary manner when titrating PSV in critically ill patients.

Study Objective: We evaluated the difference in work of breathing (WOB) during spontaneous ventilation with continuous positive airway pressure (CPAP) among three methods of triggering the ventilator: conventional pressure triggering, tracheal pressure triggering, and flow-by triggering., Methods: In an in vitro model of the respiratory system consisting of a bellows (lungs) in a plastic canister (chest wall), spontaneous ventilation was simulated with a piston-driven pump (respiratory muscles). Data were recorded during CPAP of 5 cm H2O (model 7200ae ventilator, Puritan-Bennett, Overland Park, Kan) at peak sinusoidal inspiratory flow rate demands of 60 and 80 L/min and airway resistances of 5 and 20 cm H2O/L/s, with the demand flow system triggered by conventional pressure, tracheal pressure, or flow. Under each condition, tidal volume, pressure-time product (PTP), WOB, and changes in intrapleural pressure (Ppl) and airway pressure were recorded in real time by means of a computerized portable respiratory monitor (model CP-100, Bicore, Irvine, Calif). The Ppl was measured from within the canister, tidal volume by positioning a flow sensor between the Y-piece of the breathing circuit and the endotracheal tube (ETT), and airway pressure from a catheter attached to the flow sensor. The WOB was calculated by the monitor in real time., Results: Changes in Ppl were greatest with conventional pressure triggering, less with flow-by triggering, and least with tracheal pressure triggering. The WOB was significantly lower (approximately 50%) with tracheal pressure triggering than with the other two methods. With tracheal pressure triggering only, an effect similar to that of pressure support ventilation (PSV) occurred, which accounted in part for the significant decrease in WOB. The PTP/breath ratio correlated strongly and was a good predictor of WOB (r2 = 0.95)., Conclusions: Compared with conventional pressure and flow-by methods, triggering with tracheal pressure decreased WOB significantly. This method of triggering may improve patient-ventilator interaction.

Objective: A new, microprocessor-controlled respiratory monitor (model CP-100 Pediatric, Bicore Monitoring Systems, Irvine, CA) that measures imposed work of breathing and a variety of respiratory parameters for pediatric patients receiving ventilatory support has recently been developed. To validate its accuracy, measurements obtained using this monitor were compared with those obtained using conventional laboratory equipment., Methods: An in vitro lung model was used to simulate spontaneously breathing pediatric patients ranging from infancy to 10 years of age. Tidal volume, respiratory rate, and peak inspiratory flow rates were simulated in a stepwise manner. Values for imposed work, tidal volume, peak inspiratory flow rate, and change in airway pressure for both methods were compared using regression analysis., Results: The coefficients of determination (r2) describing the relationships of both methods of measuring imposed work, tidal volume, peak inspiratory flow rate, and the change in airway pressure ranged from 0.99 to 1.00, and were highly significant (p < 0.001). For all measurements, bias was minimal and precision was calculated., Conclusions: Our data reveal that this pediatric respiratory monitor accurately measures imposed work of breathing, as well as tidal volume, flow rate, and airway pressure. Imposed work of breathing measurements obtained from the monitor may be used to adjust pressure support ventilation, so that the imposed work of the breathing apparatus is reduced to zero and the patient's total work of breathing is thus decreased.

Objective: To evaluate the clinical feasibility of using real-time measurements of work of breathing obtained at the bedside with a portable, commercially available respiratory monitor as an objective and quantifiable guideline for appropriately setting pressure support ventilation (PSV) to partially and totally unload the respiratory muscles in patients with respiratory failure., Design: In vivo measurements of work of breathing were performed on a consecutive series of patients after applying incremental levels of PSV., Setting: University teaching hospital in a surgical ICU., Patients: Thirty adults (18 men and 12 women, ages 20 to 77 years) who had acute respiratory failure were studied. All patients had an endotracheal or a tracheostomy tube in place and were breathing spontaneously, receiving continuous positive airway pressure and PSV., Interventions: Intraesophageal pressure (indirect measurement of intrapleural pressure) was measured with an esophageal balloon catheter positioned in the mid- to lower-third of the esophagus. Tidal volume was obtained by positioning a flow sensor between the "Y" piece of the breathing circuit and the endotracheal or tracheostomy tube. Airway pressure was measured from a catheter attached to the flow sensor. Data from these measurements were directed to the respiratory monitor (CP-100, Bicore Monitoring Systems) which calculates work of breathing performed by the patient using the Campbell diagram. Work of breathing performed by the ventilator to inflate the respiratory system was calculated by the monitor by integrating the change in airway pressure and tidal volume. Initially, the level of PSV was set to 0 cm H2O and work measurements were obtained. Pressure support ventilation was then increased until the work performed by the patient decreased to a range of 0.3 to 0.6 J/L, which corresponds to a normal range for physiologic work of breathing (ie, partial respiratory muscle unloading), and then until the work decreased to 0 J/L (ie, total respiratory muscle unloading)., Results: Work performed by the patient varied inversely (r = -0.83; p < 0.001) and work performed by the ventilator varied directly with the level of PSV (r = 0.94; p < 0.001). Work performed by the patient was 1.5 +/- 0.3 J/L at zero pressure support ventilation and decreased significantly to 0.50 +/- 0.1 J/L (p < 0.05) as the level of PSV was increased to 18 +/- 7 cm H2O. The respiratory muscles were partially unloaded under these conditions. Patient work decreased to 0 J/L and ventilator work increased when the muscles were totally unloaded at a PSV level of 31 +/- 8 cm H2O., Conclusion: We propose an objective and goal-oriented clinical approach for using PSV by directly measuring the work of breathing performed by the patient with an easy to operate, bedside respiratory monitor and then applying pressure support ventilation to decrease the work to appropriate levels. Partially or totally shifting the workload from the respiratory muscles to the ventilator is appropriate under specific clinical conditions.

Background: Computerized, yet portable, bedside pulmonary monitors that can measure work of breathing (WOB) are now commercially available; however, none accurately measures WOB. The purpose of this study was to evaluate new software designed to measure WOB by means of the Campbell diagram and to test the agreement between a monitor (Model CP-100, Bicore, Irvine CA) programmed with the new software and the conventional method of measuring WOB., Materials & Methods: Using a lung model of our own devising, we compared WOB measurements between the monitor and conventional laboratory equipment. Inspiratory flow-rate, tidal volume (VT), and resistance of the model were adjusted to produce WOB ranging from 0.80 to 3.25 J/L. Regression analysis and calculation of bias and precision were performed for these data., Results: For total, elastic, and resistive WOB, the two methods of measurement correlated strongly and positively. For all values of WOB, the correlation coefficients (r) and coefficients of determination (r2) were close to 0.99 (p < 0.0001); bias was minimal (-0.05 J/L) and precision acceptable (0.06 J/L)., Conclusion: Data from a lung model reveal that a respiratory monitor programmed with appropriate software can accurately measure total, elastic, and resistive WOB. The monitor may eventually prove useful for clinically assessing WOB, which then can be used to adjust the ventilator to optimize respiratory muscle loads.

Objectives: a) To discuss the components of the work of breathing using an established physiologic model (Campbell diagram); b) to describe the requirements of a monitor to measure work; and c) to discuss the implications and relevance for assessing the work of breathing of ventilator-dependent patients., Data Sources: Relevant articles from the medical and physiologic literature are referenced, as well as the authors' experience., Study Selection: Identified (by authors) laboratory and clinical research establishing the need and physiologic importance for correctly measuring the work of breathing., Data Extraction: A physiologic model of the various components of the work of breathing is used in conjunction with data from published literature., Synthesis: Diagrams of increasing complexity based on the Campbell diagram depict the physiologic elastic and resistive work of breathing for the lungs and chest wall under normal and abnormal conditions. Decreases in compliance and increases in airways resistance are associated with increases in elastic and resistive work, respectively. A modification of the Campbell diagram to include an additional area depicting the imposed work of the breathing apparatus is suggested; i.e., the additional resistive load imposed on the respiratory muscles by the endotracheal tube, breathing circuit, and the ventilator's demand-flow system during spontaneous breathing. Increases in physiologic and/or imposed work result in respiratory muscle loading, predisposing to increases in oxygen consumption and the development of fatigue and hypercapnia. Measuring work of breathing by integrating the area of the esophageal pressure-volume loop alone underestimates the work of breathing relative to the Campbell diagram and, therefore, should not be used. Because the site of pressure measurement and mode of ventilation influence measurements of the work of breathing as well as compliance, clinicians should be aware of these factors when interpreting measurements. Monitors that are used in clinical practice to assess the work of breathing should be able to measure pressure at the airway opening (between the Y-piece of the breathing circuit and the endotracheal tube), at the carinal end of the endotracheal tube, and in the esophagus (inference of intrapleural pressure); as well as measure flow rate and volume at the airway opening; and calculate the various components of the work of breathing based on the Campbell diagram., Conclusions: Accurate measurement of physiologic and imposed work performed by the patient are essential to assess the afterload on the respiratory muscles, diagnose specific work of breathing abnormalities, and monitor the effects of interventions to mitigate respiratory muscle loading. Work of breathing data are useful in formulating objective guidelines for setting the ventilator appropriately to optimize respiratory muscle loads, e.g., selecting an appropriate amount of pressure support ventilation to decrease the work of breathing to a specific level.

Objectives: To apply pressure-support ventilation with the goal of decreasing the imposed work of the breathing apparatus (endotracheal tube, breathing circuit tubing, and the ventilator's demand-flow system) to zero and to evaluate a clinical method of measuring the imposed work of breathing., Design: A prospective evaluation of adult and pediatric patients receiving mechanical ventilatory support., Setting: A surgical and a pediatric intensive care unit in a university hospital., Patients: Fifteen patients (11 adult and four pediatric), who were diagnosed with acute respiratory failure from various etiologies, and who were intubated and spontaneously breathing, received continuous positive airway pressure and pressure-support ventilation., Measurements and Main Results: Imposed work of the breathing apparatus was calculated by integrating pressure measured at the tracheal end of the endotracheal tube from a narrow air-filled catheter and the change in volume from a miniature pneumotachograph (flow sensor) positioned between the "Y" piece of the breathing circuit and the endotracheal tube. Pressure and volume signals were directed to a computerized, portable respiratory monitor (Bicore Monitoring Systems) that provides real-time display of the pressure-volume (work) loops and calculation of the imposed work. Imposed work was measured at 0 cm H2O pressure-support ventilation, and then incremental levels of pressure-support ventilation were applied until the imposed work decreased to zero. Imposed work decreased in a quadratic fashion after incremental levels of pressure-support ventilation (r = -.83 [r2 = .69]; p < .001). At pressure-support ventilation level of 0 cm H2O, the imposed work was 0.60 +/- 0.17 joule/L. At mean pressure-support ventilation levels of 13.5 +/- 4.8 cm H2O, imposed work decreased to 0 joule/L., Conclusions: Ideally, the imposed work of the breathing apparatus should be zero to decrease the afterload on the ventilatory muscles and, thus, the patient's work of breathing. Eliminating the imposed work is achieved using appropriate levels of pressure-support ventilation. We describe an easily applied, practical method of measuring imposed work using a commercially available, portable, bedside respiratory monitor. We recommend that all patients diagnosed with respiratory failure and compromised pulmonary mechanics and who are intubated and breathing spontaneously, receive at least a minimal level of pressure-support ventilation that results in zero breathing apparatus-imposed work of breathing.

Objectives: To compare the inspiratory imposed work of breathing during spontaneous ventilation with continuous positive airway pressure using three methods of triggering "ON" the demand-flow system of a ventilator: a) conventional pressure triggering with the pressure measuring/triggering site inside the ventilator on the exhalation limb of the breathing circuit; b) tracheal pressure triggering from the tracheal or carinal end of the endotracheal tube; and c) flow-by (flow triggered) triggering., Design: Multitrial tests under simulated clinical conditions using a mechanical lung model., Setting: A research laboratory at a university medical center., Interventions: Spontaneous breathing with continuous positive airway pressure, at peak sinusoidal inspiratory flow rate demands of 30, 60, and 90 L/min with sizes 6, 7, 8, and 9 mm internal diameter endotracheal tubes at each flow rate during conventional pressure triggering, tracheal pressure triggering, and flow-by., Measurements and Main Results: Pressures were measured at the tracheal end of the endotracheal tube, "Y" piece of the breathing circuit, and inside the ventilator on the exhalation limb of the breathing circuit. Volume measured between the endotracheal tube and lung model and pressure measured at the tracheal end of the endotracheal tube were integrated to generate pressure-volume (work) loops to calculate the inspiratory imposed work of the total breathing apparatus (i.e., endotracheal tube, breathing circuit, and ventilator). Significantly (p < .05) greater decreases in pressure during spontaneous inhalation were measured for all methods of triggering at the tracheal end of the endotracheal tube than at the Y piece or inside the ventilator. Inspiratory-imposed work was significantly lower during tracheal pressure triggering compared with conventional pressure triggering and flow-by under most conditions. For example, with a 7-mm internal diameter endotracheal tube at a peak inspiratory flow rate demand of 60 L/min, imposed work was 382% and 315% lower, respectively, during tracheal pressure triggering compared with the conventional pressure triggering and flow-by triggering methods. Under all conditions, inspiratory imposed work was lower during flow-by triggering compared with conventional pressure triggering. The smaller the internal diameter of the endotracheal tube and the greater the peak inspiratory flow rate demand, the greater the inspiratory imposed work of breathing for all methods of triggering. Under all conditions, inspiratory-imposed work was significantly greater at a peak inspiratory flow rate demand of 90 L/min than at 60 L/min, and at a peak inspiratory flow rate demand of 60 L/min than at 30 L/min., Conclusions: An endotracheal tube is a resistor in the breathing apparatus over which a pressure decrease must be developed by the patient in order to inhale spontaneously. An endotracheal tube, therefore, imposes substantial resistance and work. The results indicate that the pressure measuring/triggering site for a ventilator's demand-flow system should be at the tracheal or carinal end of an endotracheal tube so as to effectively decrease the resistance of the endotracheal tube, thus, decreasing the patient's work of breathing.

Objective: To describe the importance of measuring pressure at the tracheal end of the endotracheal tube during spontaneous breathing with continuous positive airway pressure in order to correctly assess: a) the changes in airway pressure and b) the work imposed by the breathing apparatus., Design: Multitrial tests under simulated clinical conditions using a mechanical lung model., Setting: A research laboratory at a university medical center., Interventions: Spontaneous breathing with continuous positive airway pressure, at peak sinusoidal inspiratory flow-rate demands of 30 and then 60 L/min with sizes 6, 7, 8, and 9 mm internal diameter endotracheal tubes at each flow rate., Measurements and Main Results: Pressure, flow rate, and inhaled and exhaled volumes, during simulated spontaneous ventilation with continuous positive airway pressure were measured. Pressure was measured alternately at the "Y" piece of the breathing tubing of the continuous positive airway pressure system and at the tracheal end of the endotracheal tube to calculate the work imposed by the breathing circuit, endotracheal tube, and the total breathing apparatus. Greater changes in pressure and work were measured at the tracheal end of the endotracheal tube than at the "Y" piece of the breathing tubing for all test conditions. For example, at a peak inspiratory flow-rate demand of 30 L/min when pressures measured at the tracheal end of endotracheal tubes were compared with pressures measured at the "Y"piece, the total work imposed by the breathing apparatus increased by approximately 145% with a 6-mm tube, 95% with a 7-mm tube, 50% with an 8-mm tube, and 40% with a 9-mm tube (p less than .05). Measuring pressure at the "Y" piece of the tubing results in significant underestimations of the changes in pressure and the work imposed, especially when the endotracheal tube has a small internal diameter and/or when the peak inspiratory flow-rate demand is high., Conclusions: The results indicate that pressure should be measured as close to the patient's airway as possible, i.e., at the tracheal end of the endotracheal tube, rather than using the traditional approach of measuring pressure and assessing work at the inspiratory or expiratory limbs, or "Y" piece of the breathing tubing.

To minimize work of breathing, airway pressure should not fluctuate during spontaneous breathing with continuous positive airway pressure (CPAP). However, flow resistance in the inspiratory limb of the breathing circuit and an inadequate continuous gas flow rate result in airway pressure fluctuation and increased work of breathing. Flow resistance of the expiratory pressure/exhalation valve also directly affects the level of airway pressure during spontaneous inhalation with CPAP (the greater the resistance of the valve, the greater the decrease in airway pressure and work of breathing). We compared this effect with three types of expiratory pressure valves: a threshold resistor with low resistance to flow, an inflatable balloon (mushroom) valve with moderate resistance to flow, and a variable-orifice flow resistor with a high resistance to flow. Work increased up to threefold with the balloon valve and more than tenfold with the flow resistor compared with the threshold resistor. To apply CPAP, expiratory pressure valves with low resistance to flow should be used to minimize fluctuations in airway pressure and, thus, in the work of spontaneous breathing.