The Science Journal of the American Association for Respiratory Care
Background: VD/VT appears unaltered over a wide range of large VT's (10-23 mL/kg).(1) We measured VD/VT during the first 24 hours of ARDS and evaluated the effect of smaller VT sizes on VD/VT.
Methods: VD/VT was measured in 96 patients with ARDS on Assist/Control ventilation at the VT used for clinical management. The fractional concentration of expired carbon dioxide (CO2) was measured with a Deltatrac metabolic monitor (2) and an arterial blood gas was obtained during 5 minutes of expired gas collection. VT was corrected for compressible circuit loss and normalized to ideal body weight. Mean partial pressure of expired CO2 was corrected for compressible volume dilution.(2) Simple regression analysis was done comparing VT to VD/VT. The mean VD/VT of patients whose VT fell outside of ± 1 mL/kg of the sample mean were compared using a Wilcoxon Signed Rank test. Alpha was set at 0.05.
Results: Mean (± standard deviation) corrected VD/VT was .55 (±.11) and the mean VT was 10.1 (±1.3) mL/kg. VT ranged from 5.9 to 13.5 mL/kg. No correlation was found between VT and VD/VT (R = 0.11; p = 0.32). Fourteen patients had a mean VT = 7.9 (± .75) mL/kg with a mean VD/VT = .57 (± .11), and 18 patients had a VT = 11.8 (± .76) mL/kg with a VD/VT = .54 (± .09). VD/VT was not different (p = .69).
Conclusions: VT size doesn't appear to influence VD/VT in early ARDS. This indicates that a lower VT may not adversely affect pulmonary CO2 gas exchange function in early ARDS.
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: Comparison with the douglas bag method. Crit Care Med. 1998;26: 760-754.