**1. Introduction**

Experimental and clinical studies have demonstrated that atrial myocardial tissue surrounding the vein of Marshall (VOM) can support electrical focal activities [1] or stable reentries [2] priming atrial fibrillation or synchronized atrial arrhythmias. Moreover, the epicardial region along the path of VOM contains autonomic parasympathetic [3, 4] and sympathetic [5] innervation that have been implicated in triggering AF [6] unveiling important technical issues in the treatment of this arrhythmia and in the maintenance of sinus rhythm after ablation procedures. Thus, VOM is a promising therapeutic target because it fits perfectly with Coumel's triangle components (trigger, substrate and autonomic tone). Since it is insulated by epicardial fat, physical ablation of the VOM bundle by radiofrequency has been highly challenging and potentially harmful. Chemical ablation by retrograde ethanol

infusion in the Marshall vein (VOM-ETHO) has provided a new attractive approach for an efficient elimination of triggered activity originating from this region. As the atrial tissue surrounding the VOM connects the mitral annulus (coronary sinus) to the posterior left atrium (as well as the lateral ridge), this technique has proved to be highly effective in determining a complete mitral isthmus block both in terms of acute success and lesion durability [7].

## **2. Anatomical considerations**

Embryologically, the VOM is a remnant of the left superior vena cava, which, as it becomes atretic during fetal growth, may remain open in form of small vein diramation draining into the coronary sinus [5]. In 1850 Marshall first described this venous structure draining into the coronary sinus with trajectory directed toward the lateral and posterior wall of left atrium and directed up to the left pulmonary veins [8]. The VOM descends obliquely, posterior to the left atrial (appendage (LAA) on the epicardial aspect of the LA lateral ridge, running along the postero-lateral LA toward the CS. A comprehensive study of the atrial venous anatomy is provided by Valderrabano and colleagues based on analysis of a series of VOM-ETHO procedures performed on a large population of 218 patients scheduled for atrial fibrillation ablation interventions [9]. In this research, beyond the VOM, that was the most commonly cannulated vein, other atrial veins were variably opacified by dye infusion through collateral flow. A consistent pattern of atrial branches arising from coronary ostium were observed (as depicted in **Figure 1**): septal vein, a second inferior vein, the VOM, LAA veins, anterior roof veins. Other veins not connected to the CS were detected such as roof veins commonly connected with posterior veins and extracardiac collaterals.

#### **Figure 1.**

*Diagrammatic representation of atrial venous circulation from the posterior aspect of the left atrium. From: Valderrabano et al. [9].*

#### *Vein of Marshall Ethanol Infusion in Setting of Atrial Fibrillation Ablation DOI: http://dx.doi.org/10.5772/intechopen.105593*

VOM is typically localized at the ostial aspect of the valve of Vieussens. The incidence of VOM identification is about 75–92% according to data in the literature [9, 10]. Distance between CS ostium and VOM is 4.25 ± 2.57 cm, with substantial variability. VOM length before branching was 2.99 ± 1.82 cm. VOM is typically a true atrial vein, with branches and visible venules draining the neighboring atrial tissue. Variable branching was present in 78.2% of cases. According to relation to the left inferior pulmonary vein, VOM presents variable trajectory: smaller VOM which terminates before reaching the left inferior PV (17.6%), VOM visible up to the left inferior pulmonary vein (72.8%), VOM can reach the left superior pulmonary vein (9.6%). Communication between VOM and left pulmonary veins was demonstrated by contrast drainage in the left PVs during the VOM venogram, appearing to connect through the left pulmonary vein carina (it happens in 37.7% of cases).
