Basic anatomy of the heart
Basic anatomy of the heart
A good understanding of basic cardiac anatomy is essential for interpretation of the ECG. This section is dedicated to covering the basics and common anatomical terminology that you will need moving forward. Further anatomy will be covered as needed and as is relevant to future modules.
The heart is a four-chambered pump (Figure 1) that pumps blood throughout the body in order to meet its metabolic needs. It’s typically separated into a right and left side, with each side consisting of two chambers: an atrium (upper chamber) and a ventricle (lower chamber).
On the surface of the heart, as seen in Figure 2, the separation of the atria and the ventricles is delineated by the atrioventricular (AV) groove, whereas the separation of the two ventricles from each other is delineated by the interventricular (IV) groove.
These grooves also house the major coronary arteries and coronary veins (Figure 3), which enable blood circulation to and from the heart itself; the anatomy of the coronary vasculature will be covered in more detail in a later section. The crux is the intersection point of the interventricular groove and atrioventricular groove, on the posterior surface of the heart.
Each chamber has a valve at its exit, ensuring unidirectional flow and preventing backflow. The chambers of the heart are composed of layers of cardiac muscle tissue (a.k.a. myocardium) anchored to a fibrous skeleton (Figures 4a-c). The fibrous skeleton also provides structural support for the valves, and helps physically and electrically separate the atria from the ventricles.
The “top” of the heart, where the atria are located, is referred to as the base of the heart. The “bottom” of the heart, or the tips of the ventricles, is called the apex. This makes sense if you think of the shape of the heart as an upside-down cone (Figure 5).
Figure 6 depicts a cross-sectional view of the cardiac chambers. A thin wall of myocardium separates the right and left atria, called the interatrial septum. A thicker wall of myocardium separates the right and left ventricles, called the interventricular septum. The uppermost part of the interventricular septum, known as the membranous septum, consists of collagenous tissue and forms part of the heart’s fibrous skeleton, while the rest is called the muscular septum. For each ventricle, the interventricular septum forms the inner (medial) boundary, whereas the free wall forms the outer (lateral) boundary.
If we were to take a cross-section of the ventricular wall (Figure 7a), we would see that the ventricular myocardium consists of three main layers. From the innermost to the outermost, these are the endocardium, subendocardium, mid-myocardium, and epicardium. The heart is further encased in the pericardium (Figure 7b), which includes two inner serous layers — the parietal and visceral pericardium — along with an outer fibrous pericardium.
The larger coronary vessels mentioned in Figure 3 lie on the epicardium, within the serous pericardial layers. They are therefore referred to as epicardial vessels. Smaller vessels that branch off the epicardial vessels and permeate into the inner layers of myocardium are often referred to as intramuscular vessels, which further subdivide to form the coronary microcirculation. The subendocardium is also home to the Purkinje fibres, which are an important part of the cardiac conduction system. This is covered in the next module.
For thoughtful ECG interpretation, it’s very important to understand the typical orientation of the heart in the chest from different planes.
When looking at Figures 8a-c, a few salient observations can be made about the orientation of the heart:
Figure 9 depicts the normal flow of blood through the various structures of the heart.
In the right heart, blood first enters the heart from the venous system via the inferior vena cava and superior vena cava. These empty into the right atrium, which then pumps blood across the tricuspid valve into the right ventricle. The right ventricle then pumps blood through the pulmonic valve into the lungs, where the blood gets oxygenated. Notably, the entry point and exit point of the right ventricle are called the RV “inflow tract” and “outflow tract” respectively. In the left heart, blood that exits the lungs first collects into the left atrium, which pumps it across the mitral valve into the left ventricle. The left ventricle then pumps blood through the aortic valve into the systemic circulation. The left ventricle also has an inflow and outflow tract.