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

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.

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.

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.

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.