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PubMed1997

A model of the spontaneously breathing patient: applications to intrinsic PEEP and work of breathing.

Intrinsic positive end-expiratory pressure (PEEPi) and inspiratory work of breathing (WI) are important factors in the management of severe obstructive respiratory disease. We used a computer model of spontaneously breathing patients with chronic obstructive pulmonary disease to assess the sensitivity of measurement techniques for dynamic PEEPi (PEEPidyn) and WI to expiratory muscle activity (EMA) and cardiogenic oscillations (CGO) on esophageal pressure. Without EMA and CGO, both PEEPidyn and WI were accurately estimated (r = 0.999 and 0.95, respectively). Addition of moderate EMA caused PEEPidyn and WI to be systematically overestimated by 141 and 52%, respectively. Furthermore, CGO introduced large random errors, obliterating the correlation between the true and estimated values for both PEEPidyn (r = 0.29) and WI (r = 0.38). Thus the accurate estimation of PEEPidyn and WI requires steps to be taken to ameliorate the adverse effects of both EMA and CGO. Taking advantage of our simulations, we also investigated the relationship between PEEPidyn and static PEEPi (PEEPistat). The PEEPidyn/PEEPistat ratio decreased as stress adaptation in the lung was increased, suggesting that heterogeneity of expiratory flow limitation is responsible for the discrepancies between PEEPidyn and PEEPistat that have been reported in patients with severe airway obstruction.

Airway Obstruction↗
PubMed1997

Use of the rapid/shallow breathing index as an indicator of patient work of breathing during pressure support ventilation.

BACKGROUND: Measuring patient work of breathing (WOBpt) has been suggested to provide safe, aggressive weaning from mechanical ventilation. We compared WOBpt and pressure-time-product (PTP) to routine weaning parameters [breath rate (f), tidal volume (VT), frequency/tidal volume ratio (f/VT)] at different levels of pressure support ventilation (PSV). METHODS: Fifteen patients in the surgical intensive care unit requiring prolonged weaning (more than 3 days) were entered in the study. A balloon-tipped esophageal catheter was placed and position confirmed by inspection of pressure and flow waveforms. Each patient was randomly assigned to breathe with 5, 10, 15, and 20 cm H2O of PSV. After 30 minutes, 40 breaths were recorded and analyzed. Measurement of WOBpt PTP, f, VT, and f/VT were made using the Bicore CP-100 monitor. Mean values for each parameter were calculated. PTP and WOBpt were plotted against f/VT to determine correlation coefficient. RESULTS: PTP, WOBpt and f/VT decreased in a stepwise fashion as PSV was increased. The f/VT correlated most closely with WOBpt (r = 0.983) and PTP (r = 0.972). Monitoring f alone also correlated with WOBpt (r = 0.894) and PTP (r = 0.881). All patients were weaned from the ventilator (mean duration, 22 +/- 5.9 days). Nine patients required tracheostomy before final liberation from the ventilator (mean duration, 22 +/- 5.9 days). Nine patients required tracheostomy before final liberation from the ventilator. CONCLUSIONS: Direct measurement of WOBpt is invasive, expensive, and' may be confusing to clinicians. Monitoring f/VT may be useful when changing PSV during weaning.

Acute Disease↗
PubMed1987

Airway caliber and the work of breathing in humans.

Mechanical work rate of breathing was measured in five normal subjects during voluntary eucapnic hyperventilation at rates of approximately 10, 20, 40, 60, and 80 l/min before and after inhalation of 1 mg of ipratropium bromide, an anticholinergic agent. Chest wall recoil pressure was measured over a range of lung volumes in each subject and was used as the reference pressure in the calculation of work rate. There was little change in elastic or resistive work rate at rest when vagal tone was reduced by ipratropium. The mean work at 40, 60, and 80 l/min was 8.9, 17.2, and 34.0 cmH2O.l-1.s before and 5.6, 12.4 and 25.8 cmH2O.l-1.s after ipratropium. This suggests that vagal tone significantly influences the work of breathing at high ventilatory rates, such as occur during strenuous exercise.

Adult↗
PubMed1989

Additional work of breathing imposed by endotracheal tubes, breathing circuits, and intensive care ventilators.

A disadvantage of spontaneous breathing through an endotracheal tube (ETT) and connector attached to a breathing circuit and/or ventilator (breathing device) is an increase in the work of breathing. The work of breathing associated with ETT of 6 to 9-mm diameter and eight breathing devices was determined, using a lung simulator to mimic spontaneous inspiration at flow rates of 20 to 100 L/min and a tidal volume of 500 ml, at both zero end-expiratory pressure (ZEEP) and 10 cm H2O continuous positive airway pressure (CPAP). Work associated with the breathing devices alone (WCIR) ranged from -0.002 kg.m/L (Servo 900-C ventilator, 7-mm ETT, 20 L/min, ZEEP) to 0.1 kg.m/L (continuous flow circuit, 7-mm ETT, 100 L/min, CPAP), the latter representing 196% of the work of normal breathing. When the devices were attached to ETT, total apparatus work (WAPP) ranged from 0.009 kg.m/L (Mapleson-D circuit, 9-mm ETT, 20 L/min, ZEEP) to 0.25 kg.m/L (Drager EV-A, 6-mm ETT, 100 L/min, ZEEP), the latter representing 490% of the work of normal breathing. This additional work imposed by the ETT varied considerably among devices. Spontaneous breathing through modern ventilators, circuits and ETT imposes a burden of increased work, most of which is associated with the presence of the ETT and connector. Whether this burden represents an impediment to the weaning patient, or has training value for the ultimate resumption of unassisted spontaneous ventilation, remains to be determined.

Female↗
PubMed1995

Breathing frequency and pattern are poor predictors of work of breathing in patients receiving pressure support ventilation.

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.

Adult↗
PubMed1993

Relationship of oxygen consumption and cardiac output to work of breathing.

This study examined the relationship between work of breathing and estimated blood flow to and oxygen consumption by the respiratory muscles. Five subjects performed inspiratory loaded breathing and voluntary hyperpnea while ventilatory work, cardiac output, and oxygen consumption were measured. Blood flow to and oxygen consumption by the respiratory muscles were estimated by subtracting the resting from the working values of cardiac output the oxygen consumption, respectively. Loaded breathing increased cardiac output, but there was no significant correlation with work of breathing, while oxygen consumption was significantly correlated with work of breathing. During hyperpnea both cardiac output and oxygen consumption were correlated with work of breathing. Our results indicate that blood flow and oxygen consumption are increased in a regular pattern with increases in work of breathing. These results may be significant in estimating the demand of the respiratory muscles in disease and exercise.

Analysis of Variance↗
PubMed1986

The importance of the balloon reservoir volume of a CPAP system in reducing the work of breathing.

We have previously reported, that the work of breathing in spontaneously breathing patients on CPAP could be significantly reduced by increasing the volume of the balloon reservoir in the circuit of a CPAP system from 3 to 23 l. We now report a study designed to determine the optimum balloon reservoir volume for the minimization of the work of breathing. Twenty intubated, spontaneously breathing patients were connected to a CPAP system with interchangeable balloon reservoirs. In each patient the work of breathing was measured for reservoir volumes of 3, 6, 12, 18, and 24 l attached in random order, while the positive airway pressure was held constant at 10 cm H2O. The balloons were constructed of the same material and had similar compliance. Rebreathing was prevented with use of one-way valves. Significant (p less than 0.001) decreases in the work of breathing were found on increasing reservoir volumes from 3 to 6, 6 to 12, and 12 to 18 l. A less significant (p less than 0.01) decrease in the work of breathing was found between reservoirs of 18 and 24 l. Rebreathing did not occur with significantly (p less than 0.001) lower flow rates when large reservoirs were used. We conclude that a balloon reservoir of 18 l represents the best compromise between reduction in the work of breathing, utilization of low source flow, and convenience of clinical use.

Carbon Dioxide↗
PubMed2004

Effects of the components of positive airway pressure on work of breathing during bronchospasm.

INTRODUCTION: Partial assist ventilation reduces work of breathing in patients with bronchospasm; however, it is not clear which components of the ventilatory cycle contribute to this process. Theoretically, expiratory positive airway pressure (EPAP), by reducing expiratory breaking, may be as important as inspiratory positive airway pressure (IPAP) in reducing work of breathing during acute bronchospasm. METHOD: We compared the effects of 10 cmH2O of IPAP, EPAP, and continuous positive airwaypressure (CPAP) on inspiratory work of breathing and end-expiratory lung volume (EELV) in a canine model of methacholine-induced bronchospasm. RESULTS: Methacholine infusion increased airway resistance and work of breathing. During bronchospasm IPAP and CPAP reduced work of breathing primarily through reductions in transdiaphragmatic pressure per tidal volume (from 69.4 +/- 10.8 cmH2O/l to 45.6 +/- 5.9 cmH2O/l and to 36.9 +/- 4.6 cmH2O/l, respectively; P < 0.05) and in diaphragmatic pressure-time product (from 306 +/- 31 to 268 +/- 25 and to 224 +/- 23, respectively; P < 0.05). Pleural pressure indices of work of breathing were not reduced by IPAP and CPAP. EPAP significantly increased all pleural and transdiaphragmatic work of breathing indices. CPAP and EPAP similarly increased EELV above control by 93 +/- 16 ml and 69 +/- 12 ml, respectively. The increase in EELV by IPAP of 48 +/- 8 ml (P < 0.01) was significantly less than that by CPAP and EPAP. CONCLUSION: The reduction in work of breathing during bronchospasm is primarily induced by the IPAP component, and that for the same reduction in work of breathing by CPAP, EELV increases more.

Acute Disease↗
PubMed1993

Bedside measurement of work of breathing.

This paper describes the technique of measuring work of breathing, presented at the 13th International Symposium on Computers in Clinical Medicine and Anaesthesiology, Rotterdam, June 1992. Measuring work of breathing has clinical uses in the Intensive Care Unit. Oxygen consumption does not truly reflect work of breathing. Mechanical work of breathing can be measured by recording continuous pressure and flow and integrating the resultant power. This method is facilitated at the bedside with the use of a PC computer and a spreadsheet program. It is further simplified by software to measure the area under the inspiratory pressure: volume loop.

Anesthesiology↗
PubMed1976

Mechanical work of breathing derived from rib cage and abdominal V-P partitioning.

Estimates of the mechanical work of breathing derived from measurements of separate rib cage and abdominal volume displacements, each plotted against transthoracic pressure, include the elastic cost of chest wall distortion which may occur during breathing. Inspiratory work is partitioned between the diaphragm and the rib cage musculature by adding measurements of transabdominal pressure. The mechanical work of breathing derived from separate rib cage and abdominal volume-pressure (V-P) tracings (the sum of work done by the diaphragm, rib cage, and abdominal musculature) is compared with ventilatory work estimated from the Campbell diagram (which does not include any distortional work). During resting breathing the two estimates are closely comparable, consistent with little or no distortion of the chest wall during quiet breathing. As ventilation increases, the estimate developed from rib cage and abdominal tracings reveals systematically greater mechanical work than is estimated from the Campbell diagram, consistent with distortion of the chest wall from the relaxed thoracoabdominal configuration at higher levels of ventilation. At ventilations achieved during exercise, the Campbell diagram may underestimate the work of breathing by up to 25%.

Abdomen↗
PubMed1991

Effects of albuterol inhalation on the work of breathing during weaning from mechanical ventilation.

The work of breathing is a major determinant of the success of weaning from mechanical ventilation. The aim of this study was to assess whether an inhaled bronchodilator could reduce the mechanical load on the respiratory muscles and diminish the work. For this purpose, 15 intubated patients in the process of weaning from mechanical ventilation inhaled the beta 2-agonist bronchodilator albuterol via a spacer device filled with 1 mg of the drug and connected to the endotracheal tube. During spontaneous breathing, the mean work of breathing diminished significantly after albuterol, from 9.35 +/- 1.05 to 8.33 +/- 1.13 J/min (p less than 0.01), and seven patients exhibited a decrease superior or equal to 15%. This decrease resulted from a marked reduction in lung and airway resistance, from 12.0 +/- 1.7 to 9.8 +/- 1.4 cm H2O.L-1.s (p less than 0.05). No significant changes were observed in the breathing pattern, intrinsic PEEP or arterial blood gas measurements after albuterol, and peripheral cardiovascular effects were not significant. In seven patients, we were able to compare the changes that occurred after albuterol in the work of breathing during weaning from mechanical ventilation with the changes in pulmonary function induced by albuterol after extubation, as assessed by the forced oscillation method. A close correlation was found between the two types of change, further indicating that the reduction in the work of breathing was more likely to occur in patients with the largest bronchodilating effect of albuterol at baseline.(ABSTRACT TRUNCATED AT 250 WORDS)

Administration, Inhalation↗
PubMed1996

Tracheal pressure triggering a demand-flow continuous positive airway pressure system decreases patient work of breathing.

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.

Acute Disease↗
PubMed2006

Imposed work of breathing during high-frequency oscillatory ventilation: a bench study.

INTRODUCTION: The ventilator and the endotracheal tube impose additional workload in mechanically ventilated patients breathing spontaneously. The total work of breathing (WOB) includes elastic and resistive work. In a bench test we assessed the imposed WOB using 3100 A/3100 B SensorMedics high-frequency oscillatory ventilators. METHODS: A computer-controlled piston-driven test lung was used to simulate a spontaneously breathing patient. The test lung was connected to a high-frequency oscillatory ventilation (HFOV) ventilator by an endotracheal tube. The inspiratory and expiratory airway flows and pressures at various places were sampled. The spontaneous breath rate and volume, tube size and ventilator settings were simulated as representative of the newborn to adult range. The fresh gas flow rate was set at a low and a high level. The imposed WOB was calculated using the Campbell diagram. RESULTS: In the simulations for newborns (assumed body weight 3.5 kg) and infants (assumed body weight 10 kg) the imposed WOB (mean +/- standard deviation) was 0.22 +/- 0.07 and 0.87 +/- 0.25 J/l, respectively. Comparison of the imposed WOB in low and high fresh gas flow rate measurements yielded values of 1.63 +/- 0.32 and 0.96 +/- 0.24 J/l (P = 0.01) in small children (assumed body weight 25 kg), of 1.81 +/- 0.30 and 1.10 +/- 0.27 J/l (P < 0.001) in large children (assumed body weight 40 kg), and of 1.95 +/- 0.31 and 1.12 +/- 0.34 J/l (P < 0.01) in adults (assumed body weight 70 kg). High peak inspiratory flow and low fresh gas flow rate significantly increased the imposed WOB. Mean airway pressure in the breathing circuit decreased dramatically during spontaneous breathing, most markedly at the low fresh gas flow rate. This led to ventilator shut-off when the inspiratory flow exceeded the fresh gas flow. CONCLUSION: Spontaneous breathing during HFOV resulted in considerable imposed WOB in pediatric and adult simulations, explaining the discomfort seen in those patients breathing spontaneously during HFOV. The level of imposed WOB was lower in the newborn and infant simulations, explaining why these patients tolerate spontaneous breathing during HFOV well. A high fresh gas flow rate reduced the imposed WOB. These findings suggest the need for a demand flow system based on patient need allowing spontaneous breathing during HFOV.

Computer Simulation↗
PubMed1986

Excessive work of breathing during intermittent mandatory ventilation.

Difficulties were experienced in weaning a patient from a ventilator by means of intermittent mandatory ventilation (IMV). The difficulty was overcome by installing an alternative IMV system (Hudson "disposable IMV valve") through which the patient drew her spontaneous breaths. Laboratory measurements showed that the resistance of the ventilator breathing system was much higher than that of the alternative system, mainly as a result of the resistance of the humidifier. It was calculated from measurements with a preset pattern of simulated breathing that the extra, external, work of breathing through the ventilator breathing system was approximately 1.5 times the normal internal mechanical work of breathing for a normal patient; with the alternative system, the extra work was only 0.5 times the normal. It is stressed that the breathing systems of IMV ventilators should be judged with the same rigour as other systems through which the patient is required to breathe spontaneously. It is recommended that manufacturers should provide the necessary information.

Aged↗
PubMedDate not supplied

The measurement of the work of breathing for the clinical assessment of ventilator dependence.

The work of breathing was measured in 10 normal subjects and in 28 critically ill patients with acute complications of obstructive pulmonary disease treated with assisted ventilation. The measurement of the work of breathing was found to be a useful objective variable for determining the capability for independent ventilation. Dependence on mechanical ventilation was observed when the respiratory work was greater than or equal to 1.7 kg-m/min. Patients were ordinarily capable of spontaneous ventilation when the respiratory work was less than or equal to 1.0 kg-m/min. Discontinuation of assisted ventilation was likely to be successful in those patients whose respiratory work was less than or equal to 1.5 kg-m/min if the patient was able to increase and maintain his work capability by a factor of two when breathing against an inspiratory resistance of 5 cm H2O for 10 min. Failure to increase the respiratory work when breathing against an inspiratory resistance indicated limitations in respiratory mechanics and was associated with the need for prolonged assisted ventilation. The measurement of the work of breathing against an inspiratory resistance of 5 cm H2O was useful in the identification and quantification of ventilatory reserve in patients with obstructive pulmonary disease.

Adult↗
PubMed1994

Mechanical ventilators optimized for pediatric use decrease work of breathing and oxygen consumption during pressure-support ventilation.

OBJECTIVES: a) To investigate whether the patient work of breathing needed to trigger inspiration is affected by the type of ventilator delivering pressure-support ventilation for mechanically ventilated pediatric patients. b) To determine whether changes in oxygen consumption (VO2) trend with changes in work of breathing and would thus be helpful in tracking work of breathing. DESIGN: Prospective study. SETTING: Pediatric intensive care unit at a university hospital. PATIENTS: Nine mechanically ventilated patients (2 to 75 months of age). INTERVENTIONS: While maintaining a constant pressure-support ventilation level, patients were alternately supported with the Siemens Servo 900C, the Bird VIP, and the Newport Wave E200 ventilators in random order. MEASUREMENTS AND MAIN RESULTS: Work of breathing, defined as the integral of the pressure-volume curve corresponding to negative pressure, was calculated with a pulmonary monitoring system. VO2 was measured with a metabolic cart. Patient distress levels were assessed using the COMFORT scale, a behavioral scoring system. Mean values (20 breaths/patient) for measured variables with each ventilator were compared using analysis of variance and Scheffé tests, with p < .05 indicating statistical significance. The lowest VO2 (103 +/- 35 mL/min/m2) and work of breathing (24 +/- 15 g.cm/m2) were achieved with the Bird VIP ventilator and were significantly (p < .05) lower than those values obtained with either the Siemens Servo 900C (VO2 147 +/- 33 mL/min/m2; work of breathing 49 +/- 18 g.cm/m2) or the Newport Wave E200 (VO2 122 +/- 33 mL/min/m2; work of breathing 35 +/- 15 g.cm/m2). Also, the values of work of breathing and VO2 obtained using the Newport Wave E200 were significantly (p < .05) lower than those values obtained using the Servo 900C. No change in behavioral distress occurred when the ventilators were changed. In all patients, there was a clear similarity in the trends of VO2 and work of breathing. CONCLUSIONS: We conclude that VO2 and work of breathing may be reduced significantly using the latest generation of mechanical ventilators optimized for infant and pediatric use. Because work of breathing is less with the Bird VIP than the other two ventilators tested, leading to a corresponding decrease in VO2, we suggest that the Bird VIP better adapts the patient to the ventilator and may facilitate weaning from ventilatory support. We also suggest that changes in VO2 might be helpful in tracking changes in work of breathing.

Analysis of Variance↗
PubMed1994

Components of the work of breathing and implications for monitoring ventilator-dependent patients.

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.

Airway Resistance↗