EXPLANATION

Extracorporeal membrane oxygenation (ECMO) is a form of temporary mechanical support for patients with severe, life-threatening cardiac or respiratory failure. In neonates and infants with congenital heart disease, particularly following complex surgical interventions such as the Norwood procedure for hypoplastic left heart syndrome (HLHS), ECMO provides a vital bridge to recovery or further surgical intervention.^¹ In veno-arterial (VA) ECMO, deoxygenated blood is drained from the venous system—typically via the right atrium or internal jugular vein—and pumped through a membrane oxygenator where gas exchange occurs. The oxygenated blood is then returned to the arterial system, usually via the aorta or carotid artery, to support systemic circulation and oxygen delivery.^²,³

VA ECMO differs from veno-venous (VV) ECMO in that it supports both the heart and lungs. This makes it the preferred mode in patients with significant myocardial dysfunction, such as those who have undergone a Norwood procedure. Typical VA ECMO parameters include flow rates of 100–150 mL/kg/min, careful monitoring of circuit pressures, and regular assessment of gas exchange and end-organ perfusion.^¹,³

Key circuit components include a centrifugal pump, a membrane oxygenator, a sweep gas system, and a heat exchanger. The sweep gas, typically 100% oxygen or a blended gas, drives the removal of carbon dioxide and supports oxygenation across the membrane. Without active sweep flow, the membrane oxygenator cannot perform gas exchange, rendering the circuit ineffective for oxygen delivery.

When there is evidence of cardiac and pulmonary recovery, clinicians may attempt aclamp trial.^⁴ Prior to clamping off, the ECMO flows are gradually weaned and the patient is transitioned to full ventilator and inotropic support. Echocardiography is performed during weaning to assess myocardial recovery, assessing LV/RV contractility, ventricular unloading, atrioventricular valve competency, and neo/aortic valve opening. If everything looks favorable, the clamp trial begins as ECMO flow is stopped by clamping both the venous and arterial cannulae.

It is standard practice to discontinue the sweep gas to the oxygenator during the clamp trial.^⁴ Although ECMO flow to the patient is halted, residual blood remains in the ECMO circuit, and low-flow recirculation through a bridge between the venous and arterial limbs is typically maintained to prevent blood stasis and reduce the risk of thrombosis. Leaving the sweep gas on allows continued gas exchange in the oxygenator, which can strip CO_₂ from the blood. As a result, the blood in the circuit becomes progressively hypocarbic. When the trial ends and full ECMO support is reinitiated, this hypocarbic blood is reinfused into the patient, potentially leading to abrupt systemic hypocarbia. Such sudden changes in PaCO_₂ can cause cerebral vasoconstriction, reducing cerebral blood flow and increasing the risk of neurologic complications, especially in neonates who have immature autoregulatory mechanisms. To avoid this, the sweep gas will be turned off during a clamp trial to prevent alterations in blood gas composition within the idle circuit.

If the patient fails to tolerate the clamp trial, as demonstrated by persistently low mixed venous oxygen saturations, rising lactate, worsening acidosis, or hypotension, ECMO must be urgently reinitiated.^²,⁴ As part of this reinitiation, it is essential to ensure that sweep gas is turned back on and adjusted appropriately. Failure to do so can result in continued inadequate oxygenation and CO_₂ clearance, further worsening the patient’s condition. Additionally, ventilator and inotropic support must be carefully titrated before, during, and after the trial to optimize conditions for both native and mechanical support.^⁴

In neonates with single-ventricle physiology, particularly after the Norwood procedure, clamp trials can fail for several reasons. These patients rely on a delicate balance between systemic and pulmonary blood flow (Qp:Qs), and myocardial recovery may be incomplete following a prolonged cardiopulmonary bypass run. Additionally, pulmonary vascular resistance may still be elevated, or the lungs may have residual injury or edema.^³ Even with restored ventilation and inotropes, the circulation may not be capable of supporting end-organ perfusion without extracorporeal assistance. Finally, given the technical nature of the Norwood procedure, the patient may need to return to surgery to revise the shunt or Damus–Kaye–Stansel (DKS) anastomosis.

An alternative to the traditional clamp-off trial is the pump-controlled retrograde trial off (PCRTO) protocol, which allows for a more gradual assessment of the patient's readiness for decannulation.^⁵ In this method, the centrifugal pump speed is reduced to allow retrograde flow through the arterial cannula. This setup minimizes ECMO support while preserving circuit patency, enabling continued low-flow circulation and reducing the risk of thrombus formation. Unlike the abrupt cessation seen in clamp trials, PCRTO enables a more physiologic transition to native circulation, mitigating hemodynamic instability.^⁵ This method is especially valuable in neonates and critically ill patients with marginal myocardial recovery, as it provides a safety mechanism for immediate reinitiation of full support if the patient fails the trial. Additionally, it allows for longer evaluation periods than clamp-off trials, facilitating a more accurate assessment of cardiopulmonary readiness for decannulation.^⁵

Other answer choices are less consistent with the clinical presentation. Air entrainment (A) into the arterial cannula would result in immediate hemodynamic collapse or neurologic injury due to arterial embolism. Oxygenator failure (C) usually presents gradually, with rising transmembrane pressure gradients, worsening oxygenation over hours, and sometimes hemolysis—not an abrupt desaturation event after reinitiating ECMO.

REFERENCES

1. Valencia E, Nasr VG. Updates in Pediatric Extracorporeal Membrane Oxygenation. J Cardiothorac Vasc Anesth. 2020;34(5):1309–1323. doi:10.1053/j.jvca.2019.09.006.

2. Peek GJ, Harvey C. Weaning and decannulation in neonatal respiratory failure. In: MacLaren G, Brodie D, Lorusso R, Peek G, Thiagarajan R, Vercaemst L. Extracorporeal Life Support: The ELSO Red Book. 6th ed. Extracorporeal Life Support Organization; 157-164.

3. Nadkarni AS, Delany DR, Schramm J, Shin YR, Hoskote A, Bembea MM. ECMO considerations in the pediatric cardiac population. Curr Pediatr Rep. 2023;11(2):86–95. doi:10.1007/s40124-023-00292-5. E McCartney SL, Krishnan S.

4. Krishnan S, Schmidt GA. ECMO Weaning and Decannulation. In: Schmidt GA, ed. Extracorporeal Membrane Oxygenation for Adults. 3rd ed. Springer; 2022:265–275.

5. Stukov Y, Dibert TT, Narasimhulu SS, et al. Pump-controlled retrograde trial off extracorporeal membrane oxygenation. Multimedia Manual of Cardiothoracic Surgery. 2025. Available from: https://mmcts.org/tutorial/1972