The Science Journal of the American Association for Respiratory Care

1999 OPEN FORUM Abstracts

Pressure Rise Time (PRT) Increases Work of Breathing (WOB) during Pressure Support Ventilation (PsV)

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: PSV may lessen WOB by its initial high peak inspiratory flow ([Vdot]I). PRT regulates the rate-rise in airway pressure (PAW) by limiting the acceleration of [Vdot]I to its peak. As the timing of peak [Vdot]I is delayed to a later point in inspiratory time (TI), peak [Vdot]I decreases because lung volume (and pressure) is higher. Because spontaneous peak [Vdot]I occurs by 20% of inspiration (1), we hypothesized that increasing PRT will increase WOB.

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. Measurements were made with a BICORE CP-100 monitor. WOB was expressed in Joules/L.

Results: As PS increased for any level of PRT, WOB decreased. The exception was for PRT = 0.6 at PS = 30, where TI/TTOT increased to 0.56 and resulted in airtrapping(?). For any level of PS, WOB increased as PRT was increased. Clinically relevant differences in WOB with increased PRT only occurred at high VT demand and low PS. All differences in WOB were significant by paired t-tests using Dunn's multiple comparison test (p<.05)

WOB in Joules / L at: PRT 0 PRT 0.2 PRT 0.4 PRT 0.6
PS 10 (VT = 300) 0.04 0.08 0.08 0.13
PS 30 0.02 0.03 0.07 0.19?
PS 10 (VT = 600) 0.12 0.24 0.33 0.42
PS 30 0.04 0.09 0.13 0.16

Conclusion: PRT increases WOB at low level PS and high VT demand or when PRT markedly increases TI/TTOT beyond patient demand.

1. Silverman L, Lee G, et al: Air flow measurements on human subjects with and without respiratory resistance at several work rates. Arch Indus Hyg & Occup Med 1951; 3: 461-478

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


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