25, 16, 26
Native cardiac output can be estimated during extracorporeal support, but every method rests on an assumption of complete mixing that is frequently false. Differential hypoxia - on the venous side as much as the arterial - invalidates every derived cardiac output, which places the Harlequin physiology upstream of all of them. Cross-check any derived value against an independent echocardiographic estimate before acting on it.
The shunt-fraction method, which turns off sweep gas to create a right-to-left shunt
DOI 10.1002/ehf2.14804
The modified Fick principle for estimating native cardiac output during extracorporeal support
A published letter summarising the authors' own programme of work; animal and bench data rather than patient validation; no head-to-head comparison of the competing methods exists. Indexed passage only. [VERIFICATION REQUIRED]
The authors argue the shunt-fraction method is a special case of their general modified Fick solution, restricted to zero sweep gas, and concede that maximising the content difference that way may improve accuracy. Their method works with extracorporeal gas exchange continuing, raising the possibility of continuous monitoring. Crucially, both methods are valid only if inlet and outlet conditions of both the circuit and the native lung are perfectly mixed - that is, only if there is no differential hypoxia. Their data show venous differential hypoxia is as common and important as the arterial form though far less recognised, and that left atrial saturation cannot be assumed to be 100% even in healthy lungs, let alone in shunt states.
Patients on venoarterial ECMO; the underlying evidence comprises a pilot study, a bench study and 16 animals with over 1,500 blood gas analyses