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

Abstract

Arterial hypoxemia and hence an increased venous admixture (Q˙_va/Q˙_t) in the criti-cally ill patient are mainly due to ventilation-perfusion mismatching in low V˙_a/Q˙ lung regions and/or to true intrapulmonary right-to-left shunt (Q_s/Q_T). In 16 intensive care patients the low V˙_A/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 (V˙_A/Q˙ = 0) which represents all low V˙_a/Q˙ areas was calculated (Q˙_o/Q˙_t) 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 Q˙_o/Q˙_tand the perfusion fraction to lung regions with V˙_A/Q˙ ratios less than 0.6 (r=0.855, p<0.001). The difference Q˙_va/Q˙_t-Q˙_o/Q˙_t yielded a close estimation of the Q˙_S/Q˙_T value computed according to the SF₆ retention (r= 0.952, p<0.001).

We conclude that the measurement of the aADN₂ and the calculation of the Q˙_o/Q˙_t together with the assessment of Q˙_va/Q˙_t allow to estimate the contribution of low V˙_A/Q˙ areas and true right-to-left shunt to arterial hypoxemia at maintenance oxygen partial pressures.

Key-words:

• arterial hypoxemia
• ventilation-perfusion mismatching
• right-to-left shunt
• venous admixture
• arterial-alveolar nitrogen partial pressure difference