Impact of cardiac output and alveolar ventilation in estimating ventilation/perfusion mismatch in ARDS using electrical impedance tomography.

INTRODUCTION

Electrical impedance tomography (EIT) can be used to assess ventilation/perfusion (V/Q) mismatch within the lungs. Several methods have been proposed, some of them neglecting the absolute value of alveolar ventilation (V) and cardiac output (Q). Whether this omission results in acceptable bias is unknown.

METHODS

Pixel-level V/Q maps of 25 ARDS patients were computed once considering (absolute V/Q map) and once neglecting (relative V/Q map) the value of Q and V. Previously published indices of V/Q mismatch were computed using absolute V/Q maps and relative V/Q maps. Indices computed with relative V/Q maps were compared to their counterparts computed using absolute V/Q maps.

RESULTS

Among 21 patients with ratio of alveolar ventilation to cardiac output (V/Q) > 1, relative shunt fraction was significantly higher than absolute shunt fraction [37% (24-66) vs 19% (11-46), respectively, p < 0.001], while relative dead space fraction was significantly lower than absolute dead space fraction [40% (22-49) vs 58% (46-84), respectively, p < 0.001]. Relative wasted ventilation was significantly lower than the absolute wasted ventilation [16% (11-27) vs 29% (19-35), respectively, p < 0.001], while relative wasted perfusion was significantly higher than absolute wasted perfusion [18% (11-23) vs 11% (7-19), respectively, p < 0.001]. The opposite findings were retrieved in the four patients with V/Q < 1.

CONCLUSION

Neglecting cardiac output and alveolar ventilation when assessing V/Q mismatch indices using EIT in ARDS patients results in significant bias, whose direction depends on the V/Q ratio value.