What does the left ventricle have in common with the pharaohs?
Basic anatomy of the heart
First, let’s look at the terminology used to describe the four walls of the left ventricle. Unfortunately, this nomenclature is somewhat confusing, because different people tend to use different terms to describe the same region. You need to know about it, however, in order to understand the exact anatomical location of the problems that you detect with your ECG skills. Once you are familiar with the anatomy, the relationship between anatomical regions and ECG leads is relatively simple to understand.
For the sake of simplicity, think of the left ventricle as an upside down pyramid. The base of the inverted pyramid is the valve level, and the tip of the inverted pyramid corresponds to the apex.
The front of the pyramid is called the anterior wall. The back of the pyramid is called the inferior wall. The left ventricle’s free wall is called the lateral wall. The interventricular septum separates the left ventricle from the right ventricle and lies medially.
To describe the septum and lateral wall of the left ventricle more accurately, both are divided into two further subsections. The septum is divided into: the anterior septum – that is the part adjacent to the anterior wall; and the inferior septum – that is the part adjacent to the inferior wall. The lateral wall is divided into: the anterolateral wall (or simply the lateral wall, the part closer to the anterior wall); and the posterolateral wall (or simply the posterior wall, the part adjacent to the inferior wall).
All four walls are divided into a basal section, which is close to the valve level; a mid section, which is situated in the middle of the ventricle; and an apical section, which is situated at the apex of the heart. As with all pyramids, the left ventricle becomes more narrow as it gets closer to the tip or apex. For this reason, the separation into an anterior and inferior septum and an anterolateral and posterolateral wall is only done for the broad basal and mid wall sections, and not for the narrow apical section.
Therefore, when looking at the narrow apex, we refer to the septal and the lateral wall. The extreme tip of the pyramid is referred to as the supra-apical section.
So, when thinking of the left ventricle, just imagine a pyramid with the four sides "front", "back", "side", and "septum". You now understand what is being referred to, for example, when we talk about the basal inferior wall or the apex.
Blood supply of the heart
The heart tissue is perfused by two arteries. These are called the right coronary artery (RCA); and the left coronary artery, (LCA).
The left coronary artery is divided into the "left anterior descending artery" or LAD; and the circumflex artery, or LCX. Some people use the Latin terms for these instead, which are ramus interventricularis anterior and ramus circumflexus. But to keep things simple, we will stick to the English terms.
Let's go a step further now, and find out which arteries supply which wall sections of the left ventricle. If you are still having trouble with the names of the four walls of the left ventricle, go back over that information again first.
Coronary supply areas
Each coronary artery supplies a specific area of the myocardium. This shows some variation, but for most humans, the pattern is very similar, and is called the normal supply type. This is what is being referred to in the following description.
The right coronary artery supplies
- the inferior wall,
- the basal inferior septum,
- and the right ventricle.
The LAD supplies the anterior wall, the entire apex, and the anterior septum. Together, these regions make up a large proportion of the myocardium. You now understand the significance of an anterior myocardial infarction.
The circumflex artery supplies both the anterolateral and posterolateral sections of the lateral wall. Usually, the entire apex is supplied by the LAD. Because of this, the apical section of the inferior and the lateral wall is usually supplied by the LAD, and not by the RCA and LCX.
You now understand the nomenclature and the supply pattern of the coronary arteries. Later in this course, you will learn which ECG lead is responsible for which section of the cardiac pyramid.