We read with great interest Staudacher et al.’s illustra- tive cases of beta-blocker therapy on V-V ECMO [1].

The authors’ excellent work reminds us not to focus on the easily measured arterial oxygen saturation ( S a O2) but rather on the much more physiologically important vari- able of delivered oxygen ( DO2). Herein, we demonstrate physiologically and mathematically that beta-blockade for a patient completely dependent on V-V ECMO will always decrease DO2 irrespective of its effect on S a O2.

To illustrate this concept mathematically, there are some reasonable assumptions that must be made.

The first is that ECMO effective blood flow rate (EF) are within normal operational parameters of the membrane lung and remain constant during beta-blockade.

Second, that the membrane lung is well functioning such that the post-membrane lung blood oxygen saturation ( S mO2) is 100%.

Third, that the patient’s lungs are non-functional and contribute no oxygenation to the blood. Finally, given the relatively small contribution of dissolved oxygen to total oxygen content, we ignore 0.03 × Pm O2 in the calculation to simplify the math. With these assumptions in place, the arterial saturation on V-V ECMO equation simplifies to Eq.