The Science Journal of the American Association for Respiratory Care

2000 OPEN FORUM Abstracts

EFFECTS OF PEAK PRESSURE DURING THE PRESSURE-VOLUME CURVE MEASUREMENT IN AN ARDS SHEEP MODEL

Muneyuki Takeuchi, MD, Khaled A. Sedeek, MD, Robert M. Kacmarek, PhD RRT. Respiratory Care, Massachusetts General Hospital and Harvard Medical School, Boston, MA

The use of static PV curves have been recommended for the proper setting of mechanical ventilation. It is well accepted that a previous volume history must be established prior to measurement of the PV curve, however the effect of the volume history along with the maximum pressure established during the PV curve has not been explored in detail.

Methods:
Stable severe lung injury was created by saline lavage in 6 sheep (25kg-32kg). Lavage was repeated until the PaO2 < 80 mmHg during volume ventilation, VT 12 mL/kg, PEEP 5 cmH2O and FIO2 1.0. After 60 min of stabilization, hemodynamics and blood gases were measured. After which 4 PV curves were sequentially obtained with peak pressure (PIP) of 40, 50, 60 and 40 cmH2O performed 20 minutes apart. In addition, the ventilator was adjusted to increase the tidal volume to insure the same pressure for 1 minute prior to the measurement. Quasi-static inflation and deflation limbs were obtained by the supersyringe technique. Measured volume was corrected for temperature, humidity, gas compression, O2 consumption and CO2 production as described by Gattinoni (1). Pflex on the inflation limb, true inflection point (Pinf-d) and point of maximal curvature on the deflation limb (Pmc-d) were calculated by Venegas's mathematical sigmoidal equation (2).

Results:
There were no differences in hemodynamics or blood gases before and after the measurements. In 3 of 6 sheep, Pflex at PIP 40 could not be determined mathematically (therefore Pflex at PIP40 were excluded from statistics) but Pflex clearly appeared when PIP was raised to 50 or 60.

PIP40 PIP50 PIP60 PIP40-2
Pflex 19.2 ± 1.9 22.4 ± 1.7 24.0 ± 1.6 # 21.5 ± 3.6
Pinf-d 6.1 ± 4.2 13.4 ± 2.7 * 16.7 ± 2.7 * 7.8 ± 4.8
Pmc-d 29.1 ± 5.8 33.6 ± 5.6 * 36.3 ± 5.0 * 31.7 ± 5.8
Values are mean ± SD (cmH2O). * p<0.05 vs. PIP40, # p<0.05 vs. PIP50.

Conclusion: Previous volume history and peak pressure during the PV curve determination affected the values for Pflex, Pinf-d and Pmc-d in an ARDS sheep model. Furthermore, if peak pressure is inadequate, respiratory system mechanics maybe misleading.
References: (1) Gattinoni L, et al. ICM 13: 19-25, 1987. (2) Venegas JG, et al. JAP 84: 389-395, 1998.

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