The Science Journal of the American Association for Respiratory Care

1995 OPEN FORUM Abstracts

PRESSURE CONTROLLED INVERSE RATIO VENTILATION (PC-IRV) IS INEFFECTIVE EARLY IN ADULT RESPIRATORY DISTRESS SYNDROME (ARDS) INDUCED BY OLEIC ACID

Timothy B. Op't Holt. Ed.D., R.R.T., University of South Alabama, Mobile, AL. and Tom Clanton, Ph.D, The Ohio State University, Columbus, OH.

INTRODUCTION: Since ARDS was first described in 1967, mortality has been high, 50% or greater, depending on the study reviewed and the etiology of the symptoms. Volume ventilation with PEEP is a mainstay of support, which may lead to barotrauma. It is suggested that practitioners reduce peak airway pressure during ventilation, substituting instead pressure controlled inverse ratio ventilation (PC-IRV) to support oxygenation and ventilation without the hazard of barotrauma. We examined the effects of PCIRV on extravascular lung water (EVLW), pulmonary mechanics, hemodynamics, and gas exchange, We hypothesized that prophylactic or early treatment of ARDS with PC-IR Vwould result in a significant reduction in lung injury (as measured by changes in EVLW and compliance) and improvement in gas exchange (as measured by shunt and P(A-a)O_2). MATERIALS AND

Methods: Hourly measurements were obtained in two groups of anesthetized dogs wherein ARDS was induced by Oleic acid, 0.15ml/kg. The control group (n=6) received assist/control ventilation to maintain blood gases within normal limits. FiO_2 was adjusted to keep PaO_2>60 mm Hg. PC-IRV at an I:E ratio of 2:1 was instituted in the experimental group (n=8) in the control mode. Inspiratory pressure was set to deliver a tidal volume of 15 ml/kg, allowing mean airway pressure to vary. PEEP remained at 5 cmH_2O. Carbon dioxide retention was treated by increasing the ventilatory rate. Intrinsic PEEP was not measured systematically. When it was measured, it was 1-2 cm H_2O. Dopamine was administered to keep mean BP >=80 mm Hg.

Results: See table. Fluid administration was higher inthe PC-IRV group in an attempt to increase the wedge pressure to > 12 mm Hg prior to PC-IRV, maintain mean arterial BP >= 80 mm Hg, and to deliver dopamine. There were no significant differences (using unpaired t-tests) in other parameters (C_{static}, PIP, P(A-a)O_2, P/F, Qs/Qt, PaCO_2/Ve) between the two groups at hour seven.

Conclusions: In this dog model of ARDS, there was no apparent benefit from PC-IRV, early in ARDS before static compliance fell to 20 mL/cm H_2O, Even with a significantly greater fluid intake in the PC-IRV group, EVLW was not significantly greater than that in the A/C group. PC-IRV and the concurrent increase in mean Paw had detrimental effects on Qt and arterial BP, requiring inordinate amounts of dopamine and fluid. This study was limited by a non-constant mean Paw, which could have been accomplished if tidal volume were decreased, recently suggested. PC-IRV did not as we had hoped, improve oxygenation (as others have found in more advanced lung disease) or decrease EVLW. There was no relationship between the gain in body water and the gain in EVLW. Based on these data, PC-IRV was not beneficial early in the course of ARDS.

Results

PAP PAOP Qt Paw meanDO_2 EVLWIVfluid

mm Hg mm Hg L/min cm H_2OmlO_2/min g H_2O/KgL

A/C 18.9±1.4 4.0±1.8 3.5±0.611.6±2.0599.7±94.9 13.4±0.9 2.9±0.9

PC-IRV 26.1±4.7 9.3±3.3 1.8±0.517.9±4.2364.2±116.313.1±2.2 4.7±0.9

p=.005 .007 .0001 .006 .003 .16 .004

This research partially funded by the Bremer Foundation.

OF-95-013

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