The Science Journal of the American Association for Respiratory Care

1995 OPEN FORUM Abstracts

INACCURATE STATIC INSPIRATORY PRESSURES IN MECHANICALLY VENTILATED PATIENTS WITH ACUTE LUNG INJURY.

Jonathan B Waugh^{*}, Thomas L Clanton^{**}, Timothy B Op't Holt^{**}, James E Gadek^{##03#}#. ^{*} From Cardiopulmonary Care Sciences Department, Georgia State University, Atlanta, GA. ^"{**}" From Pulmonary and Critical Care Medicine, The Ohio State University, Columbus, OH.

INTRODUCTION: Static respiratory system compliance (C_{RS}), the change in lung volume per unit of pressure change during an inspiratory hold, measured during machine breaths of mechanically ventilated patients is often used to help guide ventilator management decisions. This measurement is assumed by many clinicians to be an accurate measurement of the sum of lung and chest wall compliance only. We tested the hypothesis that the static airway pressures used to calculate respiratory system compliance of animal subjects with acute lung injury measured immediately before and after death would be significantly different.

Methods: Nineteen anesthetized mongrel dogs received phorbol myristate acetate (25-30 µg/kg), an agent used to induce permeability edema lung injury, and a continuous infusion of normal saline at 10 mL[cdot]kg^-1[cdot]hr over a 7 hour period. The animals were mechanically ventilated with a Bear I ventilator with constant flow breaths of 15 mL/kg body weight. Static airway pressures were measured at the ventilator circuit Y-piece at the end of 3 second inspiratory breath holds, within three minutes before and after death to determine if unobservable expiratory efforts were altering values.

Results: Post-death static inspiratory pressures were significantly lower and static C_{RS} measurements were higher than pre-death measurements according to paired T-test analysis (p=0.004 for both).

Vent. Plateau Press.Static C_{RS} (n=19)

(n=19) cm H_2OmL/cm H_2O

Pre-death12.9±3.1, [9, 19] 26.7±6.6, [14.9, 36.4]

Post-death 10.7±1.9, [7.5, 15]31.8±7.4, [20.8, 48.2]

(results given as mean±SD, [min, max])

This significance was obtained even though decreases in static airway pressure from pre-death to post-death of greater than 1 cm H_2O were observed in only 52.6% of the subjects.

Conclusions: The results suggest that some additional variable/s, perhaps expiratory muscle activity, contributed to the exaggeration of pre-death C_{RS} values. The influence is not consistently present in ventilator patients with lung injury, making efforts to calculate corrections difficult.

OF-95-052

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