Aortopulmonary collaterals (APCs) are common in patients with single ventricle physiology, occurring in up to two-thirds of patients after the bidirectional Glenn procedure and one-half of patients after the Fontan procedure. These collaterals provide a source of blood from the systemic arterial circulation to the pulmonary circulation resulting in recirculation of oxygenated blood through the pulmonary circulation and volume overload of the single ventricle. The mechanism as to how and the reason why collaterals develop is not understood completely. Some of the proposed triggers of APC formation include hypoxemia-induced angiogenesis, chronic chest wall inflammation, and small pulmonary artery (PA) size. The benefits provided by aortopulmonary collateral blood supply include improved systemic oxygenation and pulmonary artery development and growth. However, the negative sequelae of volume overload to the single ventricle includes single ventricle dilation and dysfunction, heart failure, and ineffective pulmonary blood flow outweigh the potential benefits. Early studies did not demonstrate a benefit of coiling APCs in patients prior to the Fontan nor a benefit in length of hospital stay following the Fontan palliation. However, more recent studies have demonstrated that significant collateral burden is associated with prolonged pleural effusions, longer intensive care unit stays, and overall longer hospital stays. Studies have also demonstrated that pre-Fontan coil embolization has been associated with improved preoperative hemodynamics.


Transcatheter thrombotic coil embolization has traditionally been utilized to occlude APCs as an alternative to surgical exposure and ligation. Often these collaterals are difficult to locate and surgically transected due to their location which could potentially prolong operative time and lead to increased blood loss. As such, transcatheter occlusion of collaterals in the cardiac catheterization lab has proven to be an advantageous alternative. Coils are often composed of steel or platinum and are sometimes embedded with fibers that promote thrombosis. They are available in many different diameters and lengths. They serve to provide mechanical occlusion and promote occlusion of collaterals through thrombosis. Coils are typically inserted into proximal normal-caliber systemic arteries that supply the collateral vessels, which serves to limit collateral blood flow, left to right shunting, and volume overload. However, coil embolization of the proximal systemic arterial supply does not occlude the collateral vessels themselves. Thus, the collateral vessels are left intact for possible future vascular supply from either the same or new “feeding” blood vessels. Additionally, placement of coils into proximal systemic arteries does prohibit further access to those particular vessels during future cardiac catherization procedures.


Particle embolization is a newer technique utilized for APC occlusion that may provide more efficient distal embolization by targeting smaller arteries and more distal arterioles. Thus larger “feeding” arteries are left intact, allowing future access with catheterization. The most common materials for particle embolization include polyvinyl alcohol (PVA) microparticles and tris-acryl gelatin microspheres (TAGM). APC occlusion is due to thrombus formation around the PVA particles. While PVA particles themselves are non-absorbable, surrounding clot may dissolve after a few weeks, and the vessels may recanalize. Due to the small size of both PVA and TAGM particles, there is a significant risk of systemic embolization, especially if deployed into larger caliber vessels. Systemic embolization into a vessel supplying the central nervous system can lead to stroke and spinal cord injury. Particular vessels to avoid include the vertebral and carotid arteries as well as the artery of Adamkiewicz. It is imperative to perform frequent neurovascular exams in the initial post-catheterization period.


Choice C is the correct answer; particle embolization occludes vessels more distally and does not occlude the proximal “feeding” vessel. Choice A is incorrect as particle embolization poses a higher risk of systemic embolization due to small particle size. Choice B and D are incorrect because coil embolization is more effective for occlusion of more proximal and larger diameter vessels.


References


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3. Latus H, Gummel K, Diederichs T, et al. Aortopulmonary Collateral Flow Is Related to Pulmonary Artery Size and Affects Ventricular Dimensions in Patients after the Fontan Procedure. PLoS One. 2013; 8(11): e81684.


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