Differential hypoxia

Femoral VA ECMO with preserved native cardiac function results in “two arterial circulations” with distinct arterial gas tensions: A “native” cardiac circulation in which the gas tensions are determined by the ventilation and perfusion of the lung, and the returning ECMO circulation determined by the ventilation and perfusion of the membrane lung. In the presence of a severe lung shunt, systemic arterial and venous gas tensions vary greatly according to sampling site.

In patients supported with femoral VA ECMO, recovery of myocardial function shifts the mixing point between the native (anterograde) cardiac ejection and the ECMO (retrograde) blood flow to occur more distally along the aorta. The coronaries and cerebral circulations are preferentially perfused by native cardiac output, which has traversed the pulmonary capillaries. If the pulmonary oxygen transfer is poor, this results in poor oxygenation of the myocardium and brain and yet oxygenation of the lower body purely by the ECMO circuit blood flow. In addition, relatively higher oxygen extraction occurs from the upper body circulation, with even lower oxygen saturation in the venous return.

In order to detect this differential hypoxia (also called Harlequin syndrome, North-South syndrome, dual-circulation syndrome, or upper body hypoxaemia), arterial lines are routinely sited in the right radial artery for blood gas monitoring, with a SpO2 probe on a right sided finger or right ear lobe. High circuit inlet (access) O2 venous saturation (>85%) may also occur and contribute to lower patient oxygen delivery.

Treatment options (in roughly increasing order of invasiveness) include:

  • Tolerate reduced arterial O2 saturation. Note differential hypoxia is often a sign of cardiac recovery (i.e. a good thing) and lung shunts are reversible. Patients with differential hypoxia that are improving do not require invasive interventions.
  • Improve pulmonary oxygen transfer (FIX THE LUNG): consider increasing FiO2 / PEEP, recruitment manoeuvres, NO, positioning and physiotherapy, bronchoscopy, diuresis, antibiotics and proning (involve ICU ECMO Consultant). Drainage of large compressing pleural collections may be required (should be discussed with the ICU ECMO Consultant).
  • Change access cannula to SVC/ RA drainage position. This is very effective, however involves change of the configuration.
  • Change return cannula position to improve the delivery of VA ECMO circuit blood to the heart and brain. This would require surgical reconfiguration to the subclavian artery. This option would generally be considered too aggressive for isolated differential hypoxaemia, but may be an option if considering transitioning from VA ECMO to longer term ECL Support. One should also consider that coronary blood flow will continue to be supplied from the preserved native cardiac ejection.
  • Increase VA ECMO flow and MAP, and reduce inotropy: this aims to shift the mixing point proximally along the aorta. This is often unsuccessful, and ablating native cardiac output carries risks of thrombosis, and pulmonary oedema. As such, it is not usually a viable solution.

Less favoured and less commonly required options

  • VAV ECMO: by splicing a venous return cannula (typically placed into the SVC via the right internal jugular vein) into the circuit, oxygenated blood is delivered from the ECMO circuit to the right atrium. This oxygenated blood is then provided via native cardiac output to the coronaries and brain. As the venous return is against lower pressure, by default ECMO flow distributes preferentially into the venous return cannula (i.e. more VV than VA support). If substantial VA support is still required, this will be undesirable and a Hoffman tubing clamp (“gate clamp”) is applied to the venous return limb to divert the flow preferentially towards the arterial return. The resistance offered by the gate clamp is changed according to the patient’s need for VV vs VA support. To avoid thrombus formation, flow of at least 1 litre/min is maintained in each return limb, as monitored by an ultrasonic flow sensor. If cardiac or lung dysfunction resolves, the circuit may be simplified into pure VA or VV support, via removal of the venous or arterial return cannula respectively.
  • VA and VV ECMO: here two separate sets of cannulae, circuits and consoles simultaneously supply VA and VV ECMO support. The rationale is as for VAV ECMO, but the additional venous access cannula may permit higher flows.
  • Change VA ECMO return site to right subclavian artery (via surgical graft). In this case supracoronary vessels are supplied by ECMO blood flow.
  • Change to central ECMO

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