Arterial blood gas derived variables as estimates of intrapulmonary shunt in critically ill children

Oxygen transport data, prospectively collected from 52 critically ill children, were analyzed to determine whether any derived variable accurately estimated intrapulmonary shunt (Q̇sp/Q̇t). Arterial hemoglobin saturation was more closely correlated with Q̇sp/Q̇t than was PaO₂, alveolar-arterial oxygen gradient, arterial mixed venous oxygen difference (C[a-v̄]o₂), arterial/alveolar oxygen ratio, and the ratio of PaO₂ to inspired oxygen (FIO₂) (r = 0.8, P < .0001). When C(a-v̄)o₂ was normal, hemoglobin saturation became a very accurate (r = 0.96) assessment of Q̇sp/Q̇t. We conclude that various blood gas derived variables do not accurately reflect Q̇sp/Q̇t in critically ill children. In these patients, a pulmonary artery catheter is needed to accurately assess intrapulmonary shunt.