Chambers, Walls, Myocardial Layers & the 17-Segment Model

The Four Cardiac Chambers

The heart has two thin-walled, low-pressure receiving chambers (atria) and two working pump chambers (ventricles), separated into right and left circuits by the interatrial and interventricular septa.

Right atrium (RA) receives systemic venous return from the superior vena cava (SVC), inferior vena cava (IVC), and coronary sinus. Internally it is divided by the crista terminalis into a smooth-walled posterior sinus venarum and a trabeculated anterior appendage lined by pectinate muscles. The fossa ovalis, a thinned depression on the interatrial septum, marks the remnant of the fetal foramen ovale and is the standard puncture site for transseptal catheterization procedures.

Right ventricle (RV) is the most anterior cardiac chamber, lying directly beneath the sternum. It is crescent-shaped in short axis and triangular in long axis, with a heavily trabeculated body and a smooth-walled outflow tract (infundibulum) leading to the pulmonic valve. The moderator band, a muscular trabeculation that carries part of the right bundle branch, crosses the RV cavity near the apex and is a reliable identifying landmark distinguishing the RV from the LV on nonstandard views. The RV wall is normally thin relative to the LV because it ejects against low pulmonary vascular resistance rather than systemic afterload.

Left atrium (LA) sits posteriorly, receiving oxygenated blood from four pulmonary veins (two right, two left). It is the most posterior cardiac chamber and lies immediately anterior to the esophagus, which is why transesophageal images of the LA appendage and mitral valve are so clear compared with transthoracic imaging.

Left ventricle (LV) is the dominant pumping chamber, ellipsoid ("bullet-shaped") in three dimensions, with a thick, densely trabeculated wall built to generate systemic arterial pressure. The LV apex is normally the thinnest myocardial region and is typically the last segment to complete contraction in the cardiac cycle.

Myocardial Wall Layers

Every cardiac chamber wall is built from three concentric tissue layers, from innermost to outermost:

• Endocardium — the innermost layer, a single sheet of endothelium over a thin layer of connective tissue; it is continuous with the valve leaflets and chordae tendineae. Because it is farthest from the epicardial coronary vessels and experiences the highest wall tension during systole, the subendocardium is the first region to become ischemic when coronary flow is reduced.
• Myocardium — the thick, contractile middle layer, and the layer responsible for generating force. Muscle fiber orientation changes helically across its thickness: obliquely oriented in the subendocardium, circumferentially oriented at midwall, and obliquely oriented in the opposite pitch in the subepicardium. This helical architecture produces the LV's characteristic wringing/twisting motion (apex rotates counterclockwise, base rotates clockwise, when viewed from the apex) and is the anatomic basis for longitudinal, circumferential, and radial strain measurement.
• Epicardium — the outer layer, histologically identical to the visceral layer of the serous pericardium; it carries the epicardial coronary arteries and veins along with a variable layer of epicardial fat.

The 17-Segment Model

Regional LV wall motion — normal, hypokinetic, akinetic, dyskinetic, or aneurysmal — is described using the standardized 17-segment model, adopted by the American Heart Association and used throughout echocardiography, cardiac MRI, nuclear perfusion imaging, and cardiac CT so that regional findings can be compared directly across imaging modalities.

The LV is divided into three short-axis circumferential "rings" plus a true apex:

The basal and mid rings are each divided circumferentially into six 60° segments, generated from the parasternal short-axis views at the mitral-valve level (basal) and papillary-muscle level (mid-cavity). The apical ring is divided into only four 90° segments because the cavity has already tapered substantially; these segments are visualized from the apical short-axis view near the true apex plus the apical 4-chamber, 2-chamber, and long-axis (3-chamber) views, which display the anterior, septal, inferior, and lateral walls longitudinally.

Segment 17, the apical cap, is the true muscular tip of the LV beyond the point where any cavity is still visible on imaging. Because there is no adjacent cavity against which to judge endocardial motion, the apical cap is assessed only qualitatively (present, thinned, or aneurysmal) and is excluded from quantitative wall-motion scoring and from strain/bullseye polar-map averages. This exclusion is a frequently tested, easy-to-miss distinction.

Segment-level data from all four levels are conventionally displayed as a bullseye (polar) plot, with the basal ring forming the outer circle and the apex at the center, giving an at-a-glance regional map that is directly compared against the coronary-territory overlay described with coronary anatomy later in this chapter.

Why This Model Matters

The 17-segment framework underlies nearly everything downstream in the exam blueprint: regional wall-motion scoring in ischemic disease, myocardial viability assessment during stress echocardiography, global longitudinal strain (GLS) calculation, and correlating a wall-motion abnormality with a specific culprit coronary vessel. Knowing which standard views generate which segments — and knowing that segment 17 is the outlier excluded from quantitative scoring — is high-yield content tested directly within the Pathology and Measurement domains, not only the Anatomy domain.