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.