The aADN2 in critically ill patients: is it necessary to measure PaN2?

Arterial hypoxemia and hence an increased venous admixture (QVA/QT) in the critically ill patient are mainly due to ventilation-perfusion mismatching in low VA/Q lung regions and/or to true intrapulmonary right-to-left shunt (QS/QT). In 16 intensive care patients the low VA/Q component was evaluated by determining the arterio-alveolar nitrogen partial pressure difference (aADN2). Employing a two compartment model of the lung, the fractional blood flow to a perfused and open, gas-filled but non-ventilated compartment (VA/Q = O) which represents all low VA/Q areas was calculated (QO/QT) from the aADN2 data. These results were compared with continuous ventilation-perfusion distributions obtained by using the multiple inert gas elimination technique. There was a close relationship between QO/QT and the perfusion fraction to lung regions with VA/Q ratios less than 0.6 (r = 0.855, p less than 0.001). The difference QVA/QT-QO/QT yielded a close estimation of the QS/QT value computed according to the SF6 retention (r = 0.952, p less than 0.001). We conclude that the measurement of the aADN2 and the calculation of the QO/QT together with the assessment of QVA/QT allow to estimate the contribution of low VA/Q areas and true right-to-left shunt to arterial hypoxemia at maintenance oxygen partial pressures.