The Science Journal of the American Association for Respiratory Care

1999 OPEN FORUM Abstracts

THE ROLE OF PRESSURE-RISE TIME (PRT) AND PRESSURE SUPPORT (PS) LEVEL ON INSPIRATORY TIME (TI) AND DUTY CYCLE (TI/TTOT).

RH Kallet MS RRT, M Diaz RRT, JA Alonso RRT, M Siobal RRT, JA Katz MD, JD Marks MD. Department of Anesthesia. University of California, San Francisco at San Francisco General Hospital, 1001 Potrero Ave. San Francisco, CA. 94110.

Introduction: PRT regulates the rate-rise in PS to a pressure plateau (PPLAT) by limiting the acceleration of inspiratory flow ([Vdot]I). PRT may increase TI both by delaying and decreasing peak [Vdot]I, thereby lowering the [Vdot]I level needed to terminate inspiration. Alterations in TIand TI/TTOT also may represent a respiratory drive response to changes in [Vdot]I resulting from manipulations in PRT. (1) We examined the effects of varying PS and PRT on TI and TI/TTOT at two levels of simulated drive.

Methods: A Hamilton Veolar set in volume ventilation with a modified sine wave, a rate = 26 and a TI/TTOT = 0.40 powered a work of breathing lung model (2). A VT of 300 and 600 mL created two levels of simulated drive. A Drager E-2 was set at PS levels of 10 and 30 cmH2O above 5 cmH2O of PEEP. PRT was set at 0, 0.2 0.4 and 0.6 sec. Lung compliance was 35 mL/cmH2O and airway resistance was 8 cmH2O/L/sec. TI, time to peak [Vdot]I during inspiration (TIPF) and TI/TTOT were measured with a Novametrix Ventrak monitor utilizing Analysis Plus software.

Results: Low VT Demand: At low PS, increasing PRT did not alter TI (0.74 sec) or TI/TTOT(.32). At high PS, TI increased to 1.05 sec and TI/TTOT increased to.46. Increasing PRT increased TI further to 1.28 sec and increased TI/TTOT to .56. At low PS TIPF remained at 0.2 sec when PRT increased. PPLAT was achieved by lowering peak [Vdot]I and altering flow contour to a sine wave. At high PS, TIPF matched pre-set PRT and PPLAT. High VT Demand: At low PS, increasing PRT did not increase TI (.82), TI/TTOT (.36) TIPF. varied (.21-.35) but PPLAT matched PRT. At high PS, TI increased to 1.04; TI/TTOT increased to .45. Increasing PRT increased TI further to 1.15 sec and TI/TTOT to 0.50. At high PS, TIPF matched pre-set PRT and PPLAT.

Conclusions: TI and TI/TTOT increase independently of respiratory drive at high level PS and prolonged PRT. Low VT demand coupled with high levels of PS and PRT cause the ventilator's timing to exceed simulated effort, which potentially may promote patient-ventilator dyssynchrony.

1. Brochard L: Pressure-support ventilation: Still a simple mode? Intensive Care Med. 1996; 22: 1137-1138.

2. Katz JA, Kraemer RW, Gjerde GE: Inspiratory work and airway pressure with continuous positive airway pressure delivery systems. Chest 1985; 88: 519-526.

OF-99-098

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