Ventilatory ratio, dead space, and venous admixture in patients with acute respiratory distress syndrome

Abstract

Background: Ventilatory ratio (VR) has been proposed as an alternative approach to estimate physiological dead space. However, the absolute value of VR, at constant dead space, might be affected by venous admixture and CO₂ volume expired per minute (VCO₂).

Methods: This was a retrospective, observational study of mechanically ventilated patients with acute respiratory distress syndrome (ARDS) in the UK and Italy. Venous admixture was either directly measured or estimated using the surrogate measure Pa_O2/FiO₂ ratio. VCO₂ was estimated through the resting energy expenditure derived from the Harris-Benedict formula.

Results: A total of 641 mechanically ventilated patients with mild (n=65), moderate (n=363), or severe (n=213) ARDS were studied. Venous admixture was measured (n=153 patients) or estimated using the Pa_O2/FiO₂ ratio (n=448). The VR increased exponentially as a function of the dead space, and the absolute values of this relationship were a function of VCO₂. At a physiological dead space of 0.6, VR was 1.1, 1.4, and 1.7 in patients with VCO₂ equal to 200, 250, and 300, respectively. VR was independently associated with mortality (odds ratio [OR]=2.5; 95% confidence interval [CI], 1.8-3.5), but was not associated when adjusted for V_D/V_Tphys, VCO₂, Pa_O2/FiO₂ (OR_adj=1.2; 95% CI, 0.7-2.1). These three variables remained independent predictors of ICU mortality (V_D/V_Tphys [OR_adj=17.9; 95% CI, 1.8-185; P<0.05]; VCO₂ [OR_adj=0.99; 95% CI, 0.99-1.00; P<0.001]; and Pa_O2/FiO₂ (OR_adj=0.99; 95% CI, 0.99-1.00; P<0.001]).

Conclusions: VR is a useful aggregate variable associated with outcome, but variables not associated with ventilation (VCO₂ and venous admixture) strongly contribute to the high values of VR seen in patients with severe illness.

Keywords: ARDS; dead space; mechanical ventilation; venous admixture; ventilatory ratio.