The Science Journal of the American Association for Respiratory Care

2010 OPEN FORUM Abstracts

COMPARISON OF APRV AND BIPAP IN A MECHANICAL MODEL OF ARDS

Ehab Daoud, Robert L. Chatburn; Cleveland Clinic, Cleveland, OH

BACKGROUND: Airway Pressure Release Ventilation (APRV) and BiPhasic Positive Airway Pressure (BIPAP) are alternative modes to treat the difficult to oxygenate patients. While both modes are forms of pressure controlled intermittent mandatory ventilation with unrestricted spontaneous breathing, APRV is usually set with higher I:E ratios than BIPAP. No objective data are available directly comparing the two modes. The purpose of this study was to compare major parameters of ventilation between the two modes when settings are based on time constants to manage expected end expiratory lung pressure and volume. METHODS: A spontaneously breathing ARDS patient was modeled with a lung simulator (IngMar ASL 5000): compliance = 30 mL/cm H2O, resistance = 10 cm H2O/L/s, respiratory rate = 10/minute, and muscle pressure = 5 cm H2O. Ventilator settings: P High (25 cm H2O) and number of releases (10/min) were the same in both modes. T Low was set to one time constant (1t) = 0.3 seconds in APRV (I:E = 19:1) to generate predicted auto-PEEP of ยช 9 cmH2O and 5t in BIPAP = 1.5 seconds (I:E = 3:1) to minimize auto-PEEP. P Low was 0 cmH2O in APRV and 9 cmH2O in BIPAP (theoretically equal to the auto-PEEP generated with the APRV T Low). Mandatory and spontaneous minute ventilation (MV), mean airway pressure (mPaw) and total PEEP were compared with t-tests; P value of .05 was considered significant. RESULTS: The results are summarized in the table. All the spontaneous breaths fell on the P High in both modes. CONCLUSION: There is no irrefutable evidence of superiority of one mode over the other. Extreme prolongation of the T High generates higher mPaw but at the expense of MV and vice versa. The lower tidal volume with APRV was due to the lower pressure gradient (P High minus total PEEP). Total PEEP was higher than expected in APRV because: a) volume decay lagged flow decay, b) flow takes a few milliseconds to achieve peak expiratory value and c) flow decay from peak is not consistent between different time constants. Our data show that a real ventilator does not behave like a mathematical model. Setting T Low for either mode according to the respiratory system time constant results in unpredictable total PEEP. Further research is needed to identify an optimum strategy for setting T Low. The clinical significance of our strategy needs to be validated in a human trial to compare the effects on oxygenation, ventilation and hemodynamics between both modes. Sponsored Research - None