The Science Journal of the American Association for Respiratory Care

1999 OPEN FORUM Abstracts

ACCURACY OF DEAD SPACE TO TIDAL VOLUME RATIO (VD/VT) MEASUREMENTS IN ARDS.

JA Alonso RRT, RH Kallet MS RRT, T Nuckton MD, B Daniel RRT, J.F. Pittet and M.A. Mathay MD, Departments of Anesthesia and CVRI. University of California, San Francisco at San Francisco General Hospital, San 1001 Potrero Ave. San Francisco, CA 94110.

Introduction: VD/VT defines the portion of VT that does not participate in gas exchange and may parallel the degree of ventilation-to-perfusion mismatching. High degrees of ventilation to poorly perfused lung regions has been reported in ARDS. (1) Measurements of VD/VT are done by collecting expired gases with either a Douglas bag, or an indirect calorimeter. However, unless the ventilator circuit is modified, the portion of the VT that is compressed in the circuit during inspiration (compression volume or VC) will mix with the expired gases. VC will dilute the fraction of expired CO2 (FECO2) and result in overestimation of the VD/VT. Error in VD/VT measurements can be corrected by a mathematical formula that adjusts for the effects of VC. Our objective was to determine the degree of error caused by VC on VD/VT measurements in patient with in ARDS.

Methods: Fifty-four patients who met the North American/European ARDS definition (2) were enrolled. The VD/VT measurements were done within 24 hours of meeting ARDS criteria. All patients were mechanically ventilated in either pressure control or volume control ventilation with VT of 10 mL/Kg. Mean expired carbon dioxide fraction (FECO2) was measured from a stable five-minute collection period with a Deltatrac indirect calorimeter. Mean carbon dioxide tension (PECO2) was calculated by multiplying the FECO2 by the barometric pressure. Corrected PECO2 was accomplished by the formula: PECO2 ´ VT (VT/VC). The VD/VT was calculated for both corrected and uncorrected VD/VT using the Enghoff modification of the Bohr equation as given by the formula: (PaCO2 - PECO2) ¸ PaCO2.

Results: There was a significant difference between corrected and uncorrected VD/VT measurements (P<0.001). VC was 2.6 ± 0.08 mL/cm H2O. A moderate correlation was found between the degree of measurement error (VD/VT uncorrected - VD/VT corrected) and the fraction of VC to VT (r = 0.54, P<0.001)

Parameter Uncorrected VD/VT Corrected VD/VT PIP (cmH2O) PEEP (cmH2O) VT (mL) VC (mL)
Mean 0.61 0.56 39 8 644 84
±SD 0.10 0.10 9.6 2.7 109 26

Conclusion: The average error for VD/VT measurements in patients with ARDS was app 5%. The use of low-compliant ventilator systems minimizes the influence VC has on VD/VT measurements.

(1) Ralph DD, Robertson, et al. Distribution of ventilation and perfusion during positive end-expiratory pressure in the adult respiratory distress syndrome. Am Rev Respir Dis. 1985;131:54-60.

(2) Bernard GR, Artigas A, et al. 1994. The American-European consensus conference on ARDS. Am J Respir Crit Care Med. 149 (3 Pt 1): 818-824.

OF-99-065

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