Extracorporeal Membrane Oxygenation (ECMO) is an advanced form of life support used when a patient’s heart or lungs are unable to function adequately. This technology circulates the patient’s blood outside the body to add oxygen and remove carbon dioxide. The interface between the patient and the ECMO circuit is a specialized tube, known as a cannula. These devices are placed in major blood vessels to draw deoxygenated blood out and return oxygenated blood back into the body. The design and placement of the cannula are fundamental to the success of the therapy.
Cannula Design and Lumen Configuration
The Crescent and Avalon cannulas are dual-lumen, single-site catheters engineered for veno-venous (VV) ECMO support. “Dual-lumen” means a single catheter houses two distinct internal channels: one for draining deoxygenated blood and one for returning oxygenated blood. This design consolidates access and return functions into one device, departing from traditional ECMO cannulation that requires two separate insertion sites.
The Avalon cannula is a bicaval dual-lumen catheter constructed with two primary drainage ports. These ports draw blood simultaneously from the superior vena cava (SVC) and the inferior vena cava (IVC). The single return port is positioned in the right atrium between the two drainage ports, delivering the oxygenated blood.
The Crescent cannula shares the bicaval dual-lumen principle but incorporates a unique crescent-shaped internal geometry. This specialized lumen design optimizes flow dynamics within the catheter. This configuration may enable the Crescent to achieve high flow rates at lower pressures compared to the Avalon. Both cannulas are available in various sizes, measured in French units.
Anatomical Placement and Access
Both the Avalon and Crescent cannulas are designed for single-site cannulation, using the right internal jugular vein (IJV) as the preferred access point. This vein provides the central pathway into the heart’s right side required for VV ECMO support. Using a single puncture site, the cannula is advanced through the superior vena cava and into the right atrium.
Final positioning is precise and requires guidance using imaging modalities like transesophageal echocardiography or fluoroscopy. The goal is to situate the cannula tip in the inferior vena cava while ensuring the proximal drainage port rests in the superior vena cava. This bicaval arrangement maximizes the capture of deoxygenated blood from the major systemic veins.
This single-site approach contrasts with conventional VV ECMO, which often requires two separate cannulas in different locations, such as the femoral vein for drainage and the internal jugular vein for return. The consolidated jugular access minimizes the number of required incisions and vascular punctures.
Functional Support and Recirculation Dynamics
The primary function of both the Avalon and Crescent cannulas is to provide Veno-Venous (VV) support for patients experiencing severe respiratory failure. This system oxygenates the blood and removes carbon dioxide, allowing injured lungs to rest without providing direct circulatory assistance. The unique geometry of these dual-lumen cannulas maximizes the efficiency of gas exchange.
A major challenge in VV ECMO is recirculation, which occurs when freshly oxygenated blood returned to the body is immediately drawn back into the ECMO circuit drainage ports. This mixing reduces the net oxygen delivery to the patient. Both the Avalon and Crescent designs minimize this effect through specific port placement and orientation.
By draining from the SVC and IVC and returning blood directly into the right atrium, the oxygenated blood is directed across the tricuspid valve into the right ventricle. This strategic positioning creates distance between the return and drainage flows, reducing the likelihood of immediate re-circulation. The Crescent’s crescent-shaped lumen is an additional feature aimed at optimizing the flow path and maintaining low recirculation rates, even at high flow settings.
Impact on Patient Care and Mobility
The single-site access via the internal jugular vein is the most significant factor impacting patient care and mobility for both cannulas. Unlike the traditional approach involving femoral veins, neck access leaves the lower extremities free. This freedom allows for a substantial increase in patient mobility.
Patients cannulated with either a Crescent or Avalon can often participate in physical rehabilitation earlier, sit up, and even ambulate while on ECMO support. Early mobilization helps prevent muscle atrophy and reduces the risk of long-term weakness. Furthermore, single-site cannulation facilitates complex nursing procedures, such as placing the patient in the prone position to improve oxygenation in severe lung injury.
The large size of these dual-lumen cannulas and the need for precise placement introduce specific logistical considerations. Insertion requires continuous real-time imaging guidance, such as transesophageal echocardiography, to ensure the return port is properly oriented toward the tricuspid valve. Despite the benefits of mobility, the risk of cannula malposition or displacement is a concern, necessitating frequent monitoring and sometimes requiring repositioning to maintain optimal flow.