The Science Journal of the American Association for Respiratory Care

1999 OPEN FORUM Abstracts

RELATIONSHIP OF AIRWAY PLATEAU PRESSURE (P_PLAT), QUASI-STATIC RESPIRATORY SYSTEM COMPLIANCE (C_RS) 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: V_T size does not alter V_D/V_T, but the ratio of V_T/peak tracheal pressure is correlated to V_D/V_T.(1) Both V_D/V_T and the arterial oxygen tension-to-inspired oxygen concentration ratio (PaO₂/FiO₂) are indices of ventilation/perfusion function. We evaluated P_PLAT, C_RS, and P_PLAT/V_T(mL/kg) as indicators of lung mechanics to V_D/V_T. The effect of ventilation mechanics on carbon dioxide (CO₂) gas exchange function in early ARDS, were assessed and compared to the relationship between V_D/V_T and PaO₂/FiO₂.

Methods: V_D/V_T 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) V_T was corrected for compressible circuit loss and C_RS was expressed as V_T ¸ [P_PLAT -Positive End-Expiratory Pressure]. Simple regression analysis was done comparing P_PLAT, C_RS, P_PLAT/V_T (mL/kg) and PaO₂/FiO₂ to V_D/V_T.

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

	P_PLAT cmH₂O	C_RS ML/ cmH₂O	P_PLAT/V_T 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 V_D/V_T 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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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

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