The Science Journal of the American Association for Respiratory Care

2000 OPEN FORUM Abstracts

A COMPARISON OF TOTAL PATIENT WORK OF BREATHING (TPWOB) BETWEEN VOLUME SUPPORT (VS) AND ASSISTED VOLUME CONTROL (VC) VENTILATION IN A LUNG MODEL

David L. Vines, MHS, RRT, David C. Shelledy, PhD, RRT. The University of Texas Health Science Center, San Antonio, Tx

PURPOSE: VC has been thought to provide the lowest work of breathing (WOB) in spontaneously breathing patients as compared to other modes of mechanical ventilation. To test this assumption, we compared TPWOB between VS with a variable inspiratory flow and VC with fixed inspiratory flows of 40 L/min (VC-40) and 60 L/min (VC-60) using a two-compartment mechanical lung model (Michigan Instruments Inc., Grand Rapids, MI) to simulate spontaneous breathing. METHOD: WOB was first measured on lung B alone at tidal volumes of 200, 300, 400, and 500 with a peak flow of 60 L/min and a sine wave flow pattern using the Ventrak 1550 Respiratory Mechanics Monitoring System (Novametrix Medical Systems, Inc., Wallingford CT). Then WOB was measured at these volumes for lung B to drive lung A at normal compliance (0.05 L/cmH2O) and resistance (2.7 cmH2O/L/sec), decreased compliance (0.02 L/cmH2O), and increased resistance (17.6 cmH2O/L/sec) while lung A received assistance from either VS, VC-40 or VC-60 at set tidal volumes of 400, 600, and 800 mL. TPWOB was calculated by subtracting WOB for lung B alone from the WOB for lung B to drive lung A. [TPWOB= WOB(BA) -- WOBB]

Results:
TPWOB during VS (0.379 ± 0.260 J/L) was significantly lower (p< 0.000001) than VC-40 (0.896 ± 0.199 J/L) or VC-60 (.614 ± 0.195 J/L). VC-60 had a significantly lower TPWOB than VC-40. Peak inspiratory pressure during VS (18 ± 10 cmH2O) was significantly less (p< 0.004) than VC-60 (25 ± 9 cmH2O) but not VC-40 (21± 9 cmH2O). There was no difference between the modes in exhaled spontaneous tidal volume or mean airway pressure. When spontaneous effort exceeded set tidal volume, TPWOB increased in all modes (VS- 0.942 J/L, VC-40- 1.059 J/L, and VC-60- 0.818 J/L). CONCLUSION: VS results in a significantly lower TPWOB compared to VC at a fixed inspiratory flow of 40 or 60 L/min during varying inspiratory volumes and changing lung mechanics as long as inspiratory effort does not exceed set tidal volume.

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