25, 9, 6
On venovenous ECMO, arterial saturation is a cardiac output monitor. A falling arterial saturation at unchanged circuit flow, settings and recirculation most often means a rising cardiac output, and increasing the sweep gas will not fix it because the membrane already fully saturates what reaches it. Here a 15-point rise in arterial saturation was achieved by cooling the patient - lowering cardiac output - with no change to the machine.
Echocardiographic cardiac output, and the content-based shunt equation
DOI 10.14814/phy2.15602
Shunt-fraction calculation from saturations, with total cardiac output derived from the known circuit flow; then therapeutic cooling
A single case. The same table computes the shunt fraction two ways - by content and by saturation - giving 0.52 against 0.47 and 0.38 against 0.30, so the derived cardiac output differs by over a litre per minute depending on which is used; haemoglobin was 7.5 g/dL, exactly the term the saturation-only simplification discards. Indexed passage only. [VERIFICATION REQUIRED]
Day 11 15:00 - arterial saturation 74%, venous 45%, circuit flow 6 L/min, shunt fraction 0.47, total cardiac output 11.3 L/min with 5.3 L/min bypassing the circuit. After cooling from 36.8 to 35.9 C - arterial saturation 89%, venous 63%, flow unchanged at 6 L/min, shunt fraction 0.30, total cardiac output 8.6 L/min. Day 13 - arterial saturation 81%, venous 46%, flow 5.64 L/min, shunt fraction 0.35, output 8.7 L/min against an echocardiographic estimate of 9.07 L/min (aortic area 4.95 cm2, VTI 20.69 cm, heart rate 89).
One patient on venovenous ECMO with a non-functioning lung and a hyperdynamic septic circulation