Patients with severe ARDS have significant impairment in their pulmonary gas exchange which impairs the ability to oxygenate and decarboxylate the blood. Lung compliance is very low which forces limits on the minute ventilation used to prevent barotrauma to the lungs (referred to as low tidal volume ventilation or lung protective ventilation). Without adequate minute ventilation the patient develops physiologically significant hypoxia and hypercapnia.

Using the Murray score as a guide we can assess whether a patient with ARDS would potentially benefit from venovenous extracorporeal oxygenation (vv-ECMO).

Once on vv-ECMO however, there are two factors that are most crucial in determining adequate blood gas exchange.

Percent ECMO blood flow / cardiac output

EBF/CO is the ratio of ECMO circuit blood flow rate as percent of cardiac output. Both are measured in L/min, and therefore you end up with a dimensionless percentage. Since the membrane lung is providing the oxygenation and CO2 removal, the larger portion of the total cardiac output that is passed through the oxygenator the more gas exchange that can occur.

If EBF/CO can be consistently maintained >60%, then oxygenation saturation (SaO2) is almost always >90%.

vv-ECMO circuits can usually support blood flows up to about 5 Lpm before cavitation/chatter limit further increases. So for typical patients, EBF/CO is not usually a problem unless the patient has a very high cardiac output as seen in liver failure, septic shock, hyperthyroidism, or severe anemia.

Maintaining a patient’s hemoglobin level above 10 g/dL with red blood cell transfusions can improve O2 delivery and achieve adequate SaO2, while allowing for lower EBF/CO ratios and lower total circuit blood flow rates.

Recirculation Blood Flow

Recirculation in ECMO circuits occur when oxygenated blood is returned to the ECMO circuit without passing through the peripheral vasculature. This represents wasted ECMO blood flow, and in can be thought of as reducing the effective EBF/CO.

Recirculation blood flow (RBF measured in Lpm) can be assessed by comparing pre-oxygenator pO2 with a peripheral pO2 (not ScvO2 from the pulmonary artery catheter which should be the same as post-pump pO2). Measuring a true mixed peripheral pO2 is difficult in vv-ECMO because pulmonary artery catheter has received oxygenated bl