EXPLANATION


Extra-corporal membrane oxygenation (ECMO) has been used since the 1970s for children with cardiopulmonary failure, in particular, post cardiotomy cardiac failure and respiratory failure due to congenital diaphragmatic hernia. Over the years, perfusion techniques and anticoagulation strategies have improved, which has led to an increased number of indications for ECMO and a recommendation for earlier ECMO initiation. Veno-arterial ECMO (VA-ECMO) is typically used for acute cardiac failure with or without respiratory failure, whereas veno-venous ECMO (VV-ECMO) is used for hypoxic or hypercarbic respiratory failure with preserved cardiac function despite maximal mechanical ventilatory support.


There are several VA ECMO cannulation strategies in use. In adults, adolescents, and larger children, femoral artery and femoral vein cannulation is most often used in the non-surgical setting due to the ease of vessel accessibility. In children less than 30 to 40 kilograms, neck cannulation via the internal jugular vein and the carotid artery is generally preferred, as the femoral vessels are smaller and do not accommodate cannulas large enough to generate adequate flows to provide full circulatory support.


During VA-ECMO support, the patient’s cardiopulmonary and systemic circulation are in a parallel configuration with the ECMO circuit. The blood supplied to the patient from the ECMO circuit via the arterial cannula originates from the oxygenator and is full saturated with oxygen unless set otherwise. Depending on the proportion of total systemic venous return draining to the ECMO venous reservoir, the remaining venous blood will pass through the lungs, return to the left atrium and then be ejected from the left ventricle into the aorta to mix with blood flow from the ECMO arterial cannula. In the presence of significant pulmonary pathology, there is likely significant pulmonary venous desaturation resulting in a systemic oxygen saturation (SpO2) less than 100%. With VA ECMO via carotid arterial cannulation, such as the one described in the stem, mixing occurs at the level of the carotid artery and ascending aorta. Therefore, the SpO2 will be lower than 100% in all four limbs. Management of this problem includes the following: (1) adjusting the mechanical ventilator settings to improve pulmonary gas exchange if possible; (2) increasing the ECMO flow rate such that a greater percentage of the total blood volume passes through the oxygenator and, (3) increasing the inspired concentration of oxygen in the ECMO sweep gas.


North-South syndrome, or Harlequin phenomenon, is a condition in which the lower body systemic saturation is higher than the upper body. This differential hypoxemia occurs specifically in patients on VA ECMO with femoral cannulation and is caused by mixing of 100% oxygenated and relatively deoxygenated blood in the aorta. With femoral cannulation, 100% oxygenated blood from the femoral arterial cannula flows to the upper body via the thoracic aorta, aortic arch and ascending aorta in a retrograde fashion and mixes with the blood ejected from the left ventricle. In the setting of poor left ventricular (LV) function and low cardiac output, there is negligible mixing of retrograde blood from arterial cannula with antegrade blood via native cardiac output. However, in the setting of improving myocardial function/cardiac output and poor pulmonary function, an increasing amount of blood is ejected from the left ventricle, which is relatively desaturated in comparison to blood from the femoral arterial cannula. Thus, 100% oxygenated blood from the ECMO arterial cannula will combine with blood ejected from the LV at a mixing zone in the aorta, called “North-South” line. Blood from “South” region will have a higher oxygen saturation than blood from the “North” region. The anatomic location of the North-South line will depend on ECMO flow rate, native ventricular function/cardiac output, and systemic vascular resistance. Management of North-South syndrome usually involves adding an additional cannula to the ECMO circuit that delivers blood with 100% oxygen saturation to the superior vena cava/right atrium. North-South syndrome is unlikely in the patient described in stem because there are carotid arterial and jugular venous cannulas present rather than femoral arterial and femoral venous cannulas.


Recirculation is a clinical scenario that occurs during VV ECMO when a large proportion of the oxygenated blood from the outflow cannula is recycled through the inflow cannula and returns to the ECMO circuit without circulating through the patient. Most of the blood passing through the lungs and left ventricle will not have passed through the oxygenator and will be desaturated. However, the pre- and post-oxygenator blood will be bright red with a high oxygen saturation. The degree of recirculation depends on cannula type and position, ECMO pump flow, and cardiac output. Dual-lumen cannulas or long cannulas lying near each other increase the risk of recirculation. Right ventricular dysfunction/low cardiac output also puts the patient at risk, as there is less forward blood flow. Additionally high pump flows may cause the inflow cannula to draw blood in from the outflow cannula and lead to recirculation. This is not possible in the patient described in the stem as the patient is on VA ECMO.


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