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 A ) and cardiac output ( Q C ). 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 C and V A . 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 A / Q C ) > 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 A / Q C < 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 A / Q C ratio value.