The Science Journal of the American Association for Respiratory Care

2008 OPEN FORUM Abstracts


Eduardo Mireles-Cabodevila1, Robert L. Chatburn2

Background: Adaptive pressure control (APC) is a mode where inflation pressure (IP) is adjusted by the ventilator to achieve a target tidal volume (VT). APC seems appropriate in patients with lung injury because the VT can be regulated while promoting inspiratory flow synchrony. However, IP is reduced as patient effort increases, which may or may not be clinically appropriate. The purpose of this study was to evaluate the relation between ventilator work output and patient effort in APC.

Methods: An ASL 5000 lung simulator (Ingmar Medical Inc.) was set at compliance = 0.025 L/cmH20 and resistance = 10 cmH20/L/s. Muscle pressure (Pmus) was a sine wave (20% inspiration, 5% hold, 20% release) that increased from 0-25 cmH2O in steps of 5 cmH2O. APC modes tested were: Auto Flow (Dräger Evita XL); VC+ (Puritan Bennett 840); APV (Hamilton Galileo); PRVC (Siemens Servo-i and Siemens Servo 300). The target VT was set at 320 mL (IP = 0 cmH2O, Pmus = 15 cmH2O) to allow delivery of a realistic VT as the simulated patient demanded more volume. All measurements were obtained from the simulator.

Results: Patient WOB increased from 0 - 1.96 J/L through the step increase in Pmus. Target VT was maintained as long as the Pmus was below 10 cmH20. VT then increased linearly with increased Pmus. Ventilators showed 3 patterns of behavior (see figure 1) in response to an increase in Pmus: 1) Ventilator WOB (WOBv) decreased at the same rapidity as Pmus increased but plateau at Pmus = 10 by delivering a minimum pressure support level of 6 - 7 cmH20; 2) WOBv decreased as in 1 to Pmus = 10 cm H2O but then decreased at a much slower rate; 3) WOBv gradually decreased to 0 J/L as Pmus increased.

Conclusion: Adaptive pressure control algorithms differ among ventilators in their response to increasing patient effort. Notably, some ventilators allow the patient to assume all of the WOB and some provide a minimum level of WOB regardless of patient effort.