The Science Journal of the American Association for Respiratory Care
BACKGROUND: Methods have been developed to measure patient trigger work, spontaneous work of breathing (WOB) and machine work of breathing during mechanical ventilation. Methods for assessment of spontaneous WOB may not include work to expand the thorax and require measurement of esophageal or pleural pressures. Further, the measurement of patient WOB during mechanically supported breaths in the presence of spontaneous breathing efforts remains problematic. One approach to assess patient WOB during a mechanically supported breath in the presence of active inspiratory efforts is to compare differences in the patient's pressure-volume curves during apneic and active breathing. Methods to assess WOB in mechanical lung models have generally been limited to the measurement of trigger work, machine work and imposed work of breathing. We developed and assessed a simple method to estimate total ?patient? work of breathing (TPWOB) while using a two-compartment lung model to simulate spontaneous breathing. If reliable, such a method could be used for research purposes to compare TPWOB during spontaneous and mechanically supported breaths in the presence of simulated spontaneous breathing in the laboratory setting. METHOD: Spontaneous breathing was simulated using a two-compartment mechanical lung model (Michigan Instruments, Inc., Grand Rapids, MI). Initially, work to passively inflate compartment A (WOBA) was determined for normal compliance and resistance (CTOT = .05 L/cmH2O, Raw = 2.7 cmH2O/L/sec), decreased compliance (CTOT = .02 L/cmH2O) and increased resistance (Raw = 17.6 cmH2O/L/sec). Measures were repeated at tidal volumes of 200 - 1200 ml in 100 ml increments with a peak flow of 60 L/min and a sine wave flow pattern provided by a Puritan Bennett 7200. WOB to inflate the test lung was determined using the VenTrak 1550 Respiratory Mechanics Monitoring System (Novametrix Medical Systems, Inc., Wallingford, CT). This procedure was repeated to determine the WOB to mechanically inflate lung model compartment B (WOBB). Spontaneous breathing was next simulated by using the Puritan Bennett 7200ae to drive compartment B while linked to compartment A (WOBA+B) and the work to inflate compartment B (WOBA+B) was measured under all test conditions. Simulated spontaneous TPWOB was then calculated as follows:
TPWOB = WOBA+B - WOBB
Calculated TPWOB was compared to measured WOBA using the t-test for dependent samples. A Pearson-product correlation and regression equation were calculated to determine the slope and intercept of the regression line for paired values for TPWOB and measured WOBA.
Results: There was a significant correlation (r = .97, p = .00001) between TPWOB calculated and WOBA measured. The regression equation was: TPWOB= .0756+ .6015