●       Introduction:
Any methods that can extract contrast from a coronary sinus (CS) prior to reaching the general circulation may reduce CIN. Anatomically, major veins of the heart drain blood to the CS with minimal connection with systemic veins through Thebesian veins. Thus, it might be theoretically possible to be able to remove the injected contrast before it reaches the systemic circulation and negatively affect the kidneys.
●       Coronary sinus and venous anatomy:
The anatomy of cardiac veins is depicted in Figure 2 . The coronary venous system is anatomically divided into two groups, named greater and lesser cardiac venous system. The former contains three different groups including CS and tributary veins (CS, great and small cardiac veins, anterior and posterior interventricular veins, left marginal vein, left posterior vein, an oblique vein of Marshall, ventricular septal veins), veins draining to the atria and veins draining to the left ventricle (right marginal vein, anterior cardiac veins, infundibular veins). The lesser cardiac venous system consists of Thebesian veins including venous sinusoids and small vascular channels (81–84).
The CS is located in the inferior portion of the left atrioventricular groove and drains to the posteromedial right atrium. CS is the largest structure in the cardiac venous system with a normal diameter and length of up to 12 mm and 30-63 mm, respectively. CS has two valves which include the valve of Vieussens and the Thebesian valve. The former valve is present at the junction between CS and the great cardiac vein. The latter is found at the junction of the right atrium and SC (81–83,85).
●      Animal studies regarding coronary sinus contrast removal
Animal studies have shown the clinical utility of this procedure. Movahed and colleagues performed first study of its kind and assessed the safety and feasibility of contrast removal from CS in pigs for the first time. They selected five swine.  Through the left carotid artery with an 8-F multipurpose catheter, the left main coronary artery was accessed, and 5 ml of non-ionic iodinated contrast was injected. Heartport catheter (Ethicon Inc., Cornelia, Georgia) was used for the engagement of CS through the external jugular vein (Figure 3 ). For three times, each time for 10 seconds, the tip of the catheter was inflated immediately after coronary injection and the CS blood was collected. The Catharos SENTINEL system used for contrast capture contains aspiration and 0.035 inches wire lumen as well as optical fibers for contrast detection. The catheter would be connected to the aspiration system and primed with normal saline. The catheter tip is positioned in a desirable place in the CS and holding and slowly pulling back the handle would result in deployment of the basket. They quantified the contrast amount in aspirated blood with high-performance liquid chromatography as well as a dual-energy technique. The latter technique was used to assess iodine mass. Recovered iodine was calculated from the collected iodine mass ratio which was assessed from dual-energy logarithmic subtraction per injected iodine. The procedure was tolerated well in all recruited samples. They finally reported that 50.6 ± 12% of injected iodine contrast was successfully aspirated from CS. It must be taken into account that although a pig’s heart closely resembles a human heart, a large left azygos vein drains into the CS and this point might be a potential explanation for lower than expected contrast removal (86).
Michishita et al. selected eight swine and sedated them with ketamine and xylazine with subsequent intubation using isoflurane. Five swine were treated with an extracorporeal system with adsorbing columns and three others were treated as controls. The right femoral vein was accessed and an 8-F blood suction catheter with multiple holes at the distal end was inserted into the CS. They also used a Fogarty catheter on the ostium of CS through the left jugular vein to protect any blood leakage from the CS to the right atrium. Due to the presence of the azygos vein receiving blood from CS, another balloon was also utilized to block this blood flow through the right jugular vein. The total contrast volume was 155 ± 14 ml and the swine were treated for 90 minutes with an extracorporeal system. All subjects tolerated the experiment with no adverse events. They found the mean contrast removal rate was 49.4% and iodine concentration was significantly lower compared with controls (P= 0.0003) (87).