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Cardiac Anatomy - The Left Ventricle

Image Courtesy of St. Jude Medical

Left Ventricle

          The left ventricle is the largest and strongest chamber of the heart. This big muscle is responsible for most of the pumping action that delivers the blood throughout the body. As the left ventricle contracts, blood passes through the left ventricular outflow track, across the aortic valve and into the aorta where it is routed to smaller arteries throughout the body.

          The upper aspect of the ventricle, the area where the mitral and aortic valves are located, is referred to as the base of the chamber. At the far end of the chamber is the ventricular apex. Contraction of the left ventricle occurs apex to base forcing as much blood as possible out into the aorta. This apex to base contraction occurs due to the depolarization from the atria being delivered to the ventricular myocardium by way of the protected conduction pathways of the bundles.

          After leaving the AV node, the electrical wave front passes into the His-Purkinje system and into the right and left bundle branches. The left bundle splits into anterior and posterior branches.(2) The bundles deliver the signal to papillary muscles near the apex of the ventricles where the purkinje network delivers the electrical conduction into the myocardial tissue cause initiation of local myocardium at or near the apex. The depolarization then moves towards the base. This creates the highest level of cardiac output for the ventricles.

          The apex of the left ventricle is the most anterior and inferior structure of the heart. When viewed in the AP (anterior - posterior) view, the apex of the left ventricle is easily noted at the lower right hand aspect of the cardiac silhouette.

Left Ventricular Structures

• Mitral Valve (AV Valve)

          Blood passes from the left atrium to the left ventricle through the mitral valve. As with the tricuspid valve, the mitral valve appears best when observed from an LAO projection. Viewed from the angle, the entirety of the valve opening is presented to the observer. This provides the characteristic "clock" view with twelve o'clock appear at the top of the valve, 6 o'clock at the bottom while 3 and 9 appear at the lateral and septal aspects respectively.  There are two cusps to the mitral valve, the posterior and anterior cusps.(1) Because the mitral valve only has two cusps, it is also referred to as the bicuspid valve.

• Left Bundle Branches; Left Anterior / Left Posterior

          Electrical activation normally enters the left ventricle by way of the left bundles. Below the His bundle, the main bundle splits into the right and left branches. The left further subdivides into the anterior and posterior bundles. Conduction through the bundles is protected meaning that the electrical wave front does not have access to the surrounding myocardium. If the depolarization entered the ventricular myocardium up near the base, the ventricular myocardium would contract base to apex, thus preventing the majority of blood from exiting the chamber. Note that this scenario is functionally the same thing that happens in Idiopathic Hypertrophic Sub-Aortic Stenosis, also referred to as subvalvular stenosis. (3)

• Left Ventricular Papillary Muscles; Anterior / Posterior / Lateral

          The electrical activity exits the bundles at the anterior and posterior papillary muscles located down near the ventricular apex. From here, the depolarization wave front travels through the ventricular myocardium back up to the base. Note that in some hearts there is a smaller lateral papillary muscle.(1)

• Left Ventricular Outflow Track

          Immediately below the aortic valve is the left ventricular outflow track. This area is of importance due to its positioning behind the right ventricular outflow track and how this affects identification of ectopics that originate from these two zones. Because the left outflow track is more posterior, an R wave will be visible in leads V1 or V2 when they originate from the LVOT. RVOT ectopics will not show a significant R wave until lead V3.

• Membranous Septum / Limbus Marginalis

          The membranous septum can be located in the ventricle just below the right and posterior aortic valve cusps.(1) This thin tissue separates the left ventricle from the right atrium. Separating the membranous septum from the muscular portions of the left ventricle is the limbus marginalis.(1)

• Aortic Valve; Left Semilunar Cusp / Right Semilunar Cusp / Posterior Semilunar Cusp

          The aortic valve is one of the semilunar valves. It contains three cusps, the left cusp, the right cusp and the non-coronary or posterior cusp.(1) The base of the aortic cusps is often the site of successful ablation of outflow track PVC's that, while appearing from the surface ECG to originate from the RVOT, actually emanate from the LVOT. Unipolar mapping may be used to help differentiate this when looking for early activation in the RVOT.

(1) Information found in "The Netter Collection of Medical Illustrations, Volume 5; The Heart" by Netter
(2) Information found in "Rapid interpretation of EKG's" by Dubin
(3) Information found in "Netter's Cardiology" by Runge & Ohman
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