Basic Cardiac Anatomy

Anatomical Structures of the Heart

The heart is a 4 chambered organ that pumps blood throughout the body in order to meet its metabolic needs. Structures are typically divided into a right and left side.

Right:

Flow on the right side of the heart begins as venous blood drains from the superior vena cava (SVC) and the inferior vena cava (IVC) into the right atrium (RA). The RA then pushes blood through the tricuspid valve (TV) and into the right ventricle (RV). The RV subsequently pushes blood through the right ventricular outflow outflow tract, past the pulmonic valve (PV), and into the pulmonary arteries, where it is oxygenated by the lungs. 

Left:

Oxygenated from the lungs then returns via the pulmonary veins to the left side of the heat and drains into the left atrium (LA). The LA then pushes blood through the Mitral valve (MV) and into the left ventricle (LV). The LV subsequently pushes blood through the left ventricular outflow outflow tract, past the the aortic valve (AV), and into the aorta, where blood goes to perfuse the rest of the body.

Orientation of the Heart in the Chest

Understanding how the heart sits in the chest informs a lot about ECG interpretation.

There are three primary planes of the heart: four-chamber (frontal), short axis (transverse), and long-axis (sagittal). These planes do not correspond to the standard anatomic planes of the body, which are aligned with vertical midline structures, such as the esophagus. Conversely, the major axis of the heart is oriented obliquely; thus, the heart's long and short axes do not correspond to those of the body. The heart’s primary planes cut the body obliquely. The apex of the heart is directed right to left, instead of vertically. 

Frontal View

Notice that the apex is not pointed directly downwards, but rather downward and leftward. This is also known as the four-chamber view. 

Transverse View

Notice that the left heart is oriented more posteriorly. This is especially true of the left atrium. This is also known as the short axis view.

Saggital View

Notice that the RV sits closest to the chest. This is also known as the long-axis view.

https://www.techmed.sk/echosoft/echoviews/views-atlas/01-PLAX-Parasternal-long-axis.png

Layers of The Heart

The ventricular wall is composed of three layers, from most superficial to deep: the epicardium, the myocardium, and the endocardium. Cells at these different layers have distinct electrophysiologic properties. The thickness of these layers also varies throughout the heart. At the obtuse margin, left ventricular wall is three times thicker than the right, with a typical thickness of 12-15 mm. At the apex of the left ventricle, the muscle is only 1-2 mm. 

The heart muscle is encapsulated by various layers of pericardium as depicted below. Coronary arteries and veins are situated on the epicardial surface of the heart, in the pericardial cavity. 

Cardiac Blood Supply

The heart supplies its own blood through the epicardial coronary vessels. Different vessels supply different regions of the heart and different elements of the conduction system. Understanding coronary anatomy will be helpful in localizing lesions using an ECG when there is a clinical syndrome of coronary vascular compromise, based on the electrical disturbances caused by downstream myocardial damage. The following picture and the corresponding coronary map under it should help you understand the essentials to coronary anatomy:

Coronary Map

The above map of coronaries are by no means exhaustive, but the goal of understanding coronary anatomy is to learn how to explain ECG findings based on different clinical presentations, mainly in the setting of acute myocardial infarctions (MI), which will be touched on later when ST elevations are discussed. For example, if the AV nodal coronary artery branch is occluded such as in an inferior ST-elevation MI (STEMI), this may precipitate an AV nodal block.

Variations In Coronary Supply to Cardiac Structures

Different coronaries are not always supplied by the same side of the heart and can vary person to person. Below are some of the clinically relevant variations you might encounter (Wellens Textbook):

Coronary Dominance

In approximately 70% of individuals, the RCA is the dominant coronary artery that supplies most of the myocardium (“right-dominant circulation"). In this case, the posterior part of the interventricular septum is supplied by the RCA and the posterior descending artery branches off. Our map on this page assumes right dominance since it is the most common. 

In contrast, 30% of individuals are “left-dominant”, where the LCx supplies the posterior part of the interventricular septum. In rare circumstances, there can be “codominance”, where both the LCx and RCA share supply to the posterior interventricular septum. Both left-dominance and codominance are normal variants. (Wellens Textbook -https://www.ahajournals.org/doi/10.1161/CIRCOUTCOMES.111.964593#:~:text=In%20a%20right%2Ddominant%20circulation,LCX)%20artery%20supplies%20this%20territory.)

The Cardiac Conduction System

A network of cells that initiates and distributes electrical impulses throughout the heart muscles in an organized way to enable synchronized contraction of the various chambers of the heart. The "How the heart works" section covers this. 

The sinoatrial node (SAN), located at the junction of the superior vena cava and the right atrium, is where the heartbeat is initiated. The SAN is a group of small, elongated muscle cells with a rich capillary supply and sympathetic and parasympathetic nerve endings. Typically, it generates an action potential about once a second. 

The impulse spreads from the SAN across the atria to the atrioventricular node (AVN) through three internodal tracts of Bachmann, Wenckebach, and Thorel. The AVN sits near the interatrial septum and is innervated by sympathetic and left vagal nerves. The arrangement of the small cells of the AVN and the slow actional potential upstroke (explained in further detail in the next section) causes a significant delay in conduction between the atria and the ventricles. Sympathetic stimulation reduces the delay by increasing conduction velocity and vagal stimulation increases the delay by slowing conduction velocity. 


From the AVN, the impulse is conducted to the top of the interventricular septum by the bundle of His. It branches into the left and right bundle branches near the attachment of the tricuspid septal cusp. The left bundle also divides into the posterior and anterior fascicles. These bundles travel below the endocardium down the walls of the interventricular septum. They divide into the fibres of the Purkinje system which then conduct the impulse through the endocardium towards the epicardium, which initiates ventricular contraction. 

Left Anterior Versus Posterior Fasicles

The left posterior fascicle (LPF) is much less likely to be blocked than the left anterior fascicle (LAF). While the septal branches of the LAD perfuse the LAF and LPF, the LPF has an additional blood supply from the RCA.  


(Page 45 of The ECG in Acute Myocardial Infarction and Unstable Angina book - Developments in Cardiovascular Medicine)

Coronary Territories on ECG

The various leads in the ECG can correspond to specific coronary arteries (https://derangedphysiology.com/main/required-reading/cardiology/Chapter%201.1.8/ecg-localisation-coronary-artery-territories). The picture below depcits this distrubution:

Putting It All Together

See the video below to see the components discussed above put together!