Extracorporeal membrane oxygenation (ECMO) is a modified cardiopulmonary bypass to support life, permitting further treatment and recovery during severe cardiac or pulmonary failure. Several randomized studies have shown ECMO to be a life-saving technique in various clinical settings (1, 2), and its use has expanded dramatically over the past two decades. Furthermore, the applications for ECMO have broadened as techniques and survival rates have improved in pediatric and adult patients.

Computed tomography (CT) is widely applied in the assessment of critical conditions, such as clinical suspicion of complications or an unexplained delay in improvement (3). However, specific hemodynamic changes associated with patients on ECMO determine the contrast-enhanced patterns. It is therefore important that radiologists in ECMO centers are aware of the imaging pitfalls associated with the use of CT in ECMO cases.

We retrospectively evaluate the imaging profiles of patients on ECMO therapy for both thoracic and abdominal CT examinations. We attempt to familiarize radiologists with the hemodynamic changes associated with the various modes of ECMO and the imaging pitfalls of contrast-enhanced CT imaging are emphasized.

Extracorporeal membrane oxygenation is classified primarily into two major types: veno-arterial (VA) type and veno-venous (VV) type. The redistribution of blood flow is significantly different between these two ECMO models. In VA-ECMO, blood is withdrawn from venous circulation, usually via a central vein or the right atrium by gravity siphon. The deoxygenated blood is actively pumped through the oxygenator, and the resulting oxygenated blood is returned to the arterial side of the circulation (typically to the aorta). In VV-ECMO, blood is taken from and returned to the central venous circulation (4). The direction of the aortic bloodstream is reversed in patients on VA-ECMO setting, flowing from the caudal to the cephalad region. In contrast, the aortic bloodstream flows in the normal direction with patients on VV-ECMO. Familiarity of the radiologist with the various types of apparatus positions is essential (Fig. 1).

The hemodynamic changes in patients on VA-ECMO need to be addressed when administering contrast medium. There is retrograde aortic flow driven by the VA-ECMO, which results in inconsistent mixing of the flow from the native heart and the extracorporeal circuit. A variety of situations also cause heterogeneous enhancement after contrast medium injection, depending on the native ejection fraction, the proportion of contrast medium as native preload and inflow to the oxygenator, the inflow rate, the total amount of contrast medium, and the scan time delay for blood redistribution.

During bolus tracking, the region of interest is usually placed over the descending thoracic aorta. This area is a relatively distal part of circulation, located far from the femoral artery where the blood returns from the oxygenator. If scanning is performed prior to complete opacification of the heart chambers, there is a possibility of pseudo-filling defects or contrast-blood layering in the arterial system, including the left heart, aorta, or major arteries (Figs. 2, 3). The contrast-blood levels are suggestive of a weakly pumping heart and loss of pressure gradients between different vascular systems, including arteriovenous and venovenous systems. Hence, the distribution of the injected contrast medium at this time is determined partly by the manually pushed pressure and partly by the hydrostatic pressure of the contrast agent (5). In patients on VA-ECMO with severely impaired left heart function, effective retrograde perfusion requires full bypass with an empty left ventricle and a closed aortic valve (6). Under this condition, the left heart cannot be opacified by injected contrast medium, as shown in Figure 4.

The presence of a poorly opacified aorta or a crescent filling defect within the aorta after contrast enhancement in a patient with a compromised true lumen is considered diagnostic for aortic dissection. Similar findings may be observed in the earlier phase of the CT scan for patients on VA-ECMO. In this situation, the radiologist reporting the CT mistook the pseudo-lesion for an emergency surgical indication (Figs. 3, 4). Therefore, if a filling defect or contrast-blood layering in the arterial system is observed in the post-contrast images, it is necessary to repeat the scanning in an adequately delayed phase (Fig. 2) and correlate with another imaging modality, such as transthoracic or transesophageal echocardiography, to exclude a pseudo-lesion (Fig. 5).

To prevent dilution of contrast medium in the ECMO system, Lidegran et al. (7) proposed administering the contrast agent into the arterial ECMO tubing after the membrane oxygenator, or into the venous line distal to the membrane oxygenator. If possible, it is suggested to reduce pump flow after contrast medium is administrated. Peripheral contrast injection with use of the cannulated limb is not recommended in patients on VA-ECMO due to insufficient circulation and venous return in the distal limbs. The blood flow compromise also occurs in the distal limb of cannulation and may result in arterial ischemia or venous congestion of the distal limb, especially in the setting of prolonged ECMO support. Even in the non-cannulated limb, the venous return is insufficient for contrast injection and it is inadequate for the peripheral administration of contrast medium due to poor cardiac pumping and insufficient perfusion of distal extremities in patients on VA-ECMO (Fig. 6).

Some of the patients on VA-ECMO have mechanical circulatory support devices, such as left ventricular assist devices (LVAD) (Fig. 2) or intra-aortic balloon pumps (IABP) (Figs. 4, 5). The IABP is used to decrease myocardial oxygen consumption by reducing after-load as well as augmenting coronary perfusion during diastole. The LVAD drains the blood directly from the left ventricle and pumps it into the oxygenator or arterial circulation. The circuit is activated by a pump, independent of primary cardiac function, whether preload or afterload. It permits full and stable ventricular unloading (8). In Figure 2, aortic contrast layering was observed in the first scan, but the left ventricle was fully opacified at the same time. The contrast agent in the left heart, probably arising from the native pulmonary circulation and partially driven by LVAD, causes faster contrast opacification than retrograde arterial flow from ECMO does.

Veno-venous ECMO is used in patients with respiratory failure. The cardiocirculatory function in patients on VV-ECMO is normal with the exception of poor pulmonary oxygenation. Dynamic CT images are comparable to those of healthy individuals (Fig. 7).

In conclusion, radiologists should be familiar with hemodynamic changes associated with the various modes of ECMO and should design imaging protocols that optimize contrast enhancement and enhance imaging diagnostic quality.