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    The Circumflex (Cx) coronary artery is a branch of the left main coronary artery after the latter runs it's course in between the aorta and the main pulmonary artery,.
The Cx travels in the left atrioventricular groove that separates the left atrium from the left ventricle. The Cx moves away from the LAD and wraps around to the back of the heart. The major branches that it gives off in the proximal or initial portion are known as obtuse marginal (OMB) coronary arteries. As it makes its way to the posterior portion of the heart, it gives off one or more left posterolateral (PL) branches.

   In 85% of cases, the Cx terminates at this point and is known as a non-dominant left coronary artery system as shown on the right (below).

left Coronary Artery
The obtuse marginal or OM branch serves as the boundary between the proximal and distal portion of the Cx. Thus, the portion of the artery prior to the origin of the OM is known as the proximal Cx, while the segment just below the OM is the distal Cx. An example of a dominant circumflex is shown on the left. Note that it supplies the posterior descending coronary artery or PDA. When supplied by the Cx, it is known as left PDA while it is called right PDA when it is suppled by the right coronary artery.

 Thus in summary, the Cx travels in the left atrioventricular (AIV) groove between the left atrium and left ventricle, as it wraps around to the back or posterior portion of the heart.
 The OM or obtuse marginal coronary arteries are given off in the proximal or early course of the artery, while the postero-lateral coronary arteries arise from the distal portion of the Cx.
 In 85% of cases, the Cx is non-dominant, while the "dominant" RCA supplies the PDA branch. In the other 15% of cases, a dominant Cx supplies the PDA branch.

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