The Science Journal of the American Association for Respiratory Care

1999 OPEN FORUM Abstracts

RELATIONSHIP OF AIRWAY PLATEAU PRESSURE (PPLAT), QUASI-STATIC RESPIRATORY SYSTEM COMPLIANCE (CRS) AND PULMONARY OXYGEN EXCHANGE FUNCTION TO THE PHYSIOLOGIC DEADSPACE TO TIDAL VOLUME RATIO (VD/VT) EARLY IN THE ACUTE RESPIRATORY DISTRESS SYNDROME (ARDS)

RH Kallet MS RRT, JA Alonso RRT, T Nuckton MD, B Daniel RRT, J-F. Pittet MD and M.A. Mathay MD, Cardiovascular Research Institute. University of California, San Francisco; Department of Anesthesia San Francisco General Hospital, 1001 Potrero Ave. San Francisco, CA 94110

Background: VT size does not alter VD/VT, but the ratio of VT/peak tracheal pressure is correlated to VD/VT.(1) Both VD/VT and the arterial oxygen tension-to-inspired oxygen concentration ratio (PaO2/FiO2) are indices of ventilation/perfusion function. We evaluated PPLAT, CRS, and PPLAT/VT(mL/kg) as indicators of lung mechanics to VD/VT. The effect of ventilation mechanics on carbon dioxide (CO2) gas exchange function in early ARDS, were assessed and compared to the relationship between VD/VT and PaO2/FiO2.

Methods: VD/VT was measured on Assist/Control ventilation in 96 patients within the first 24 hours of ARDS. An arterial blood gas was obtained during measurement of mean expired CO2 concentration with a Deltatrac metabolic monitor (2). Mean partial pressure of expired CO2 was corrected for compressible volume dilution. (2) VT was corrected for compressible circuit loss and CRS was expressed as VT ¸ [PPLAT -Positive End-Expiratory Pressure]. Simple regression analysis was done comparing PPLAT, CRS, PPLAT/VT (mL/kg) and PaO2/FiO2 to VD/VT.

Results: Mean ± standard deviation (sd) VD/VT was 0.55 (±.11) and mean VT was 10.1 (±1.2) mL/kg ideal body weight. Ventilation mechanics correlated poorly with VD/VT compared to PaO2/FiO2

PPLAT cmH2O CRS ML/ cmH2O PPLAT/VT cmH2O/mL/kg PaO2/FiO2
Mean ± sd 32.4 ± 8.6 31.1 ± 11.4 3.3 ± 0.97 144 ± 60
Correlation Coefficient 0.20 -0.32 0.24 -0.53
p 0.05 0.002 0.02 < 0.0001

Conclusions: Lung mechanics appear poorly related to VD/VT compared to PaO2/FiO2. This indicates that in early ARDS pulmonary perfusion abnormalities may be more important in CO2 gas exchange than ventilation mechanics.

1. Hedley-Whyte J, Pontoppidan H, Morris MJ: The response of patients with respiratory failure and cardiopulmonary disease to different levels of constant volume ventilation. J Clin Invest. 1966; 45: 1543-1554.

2. Lum L, Saville A, Venkataraman ST: Accuracy of physiologic deadspace measurement in intubated pediatric patients using a metabolic monitor. Crit Care Med. 1998;26: 760-754.

OF-99-096

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