The Science Journal of the American Association for Respiratory Care
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
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|
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.