Myocardium

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The myocardium defines the middle layer of the cardiac wall between the endocardium and the pericardium and forms the muscular part of the heart.

Gross anatomy

The myocardium represents the middle layer of the cardiac wall. It is located between the endocardium and the epicardial layer of the pericardium within the walls of the cardiac chambers arranged in different sheets wrapped around in different orientations.

The left ventricular myocardium can be subdivided into the following layers or zones from inside to outside:

  • subendocardial
  • midmyocardial
  • subepicardial

The exact myocardial architecture has been somewhat controversial. It has been compared to a contractile complex three-dimensional mesh, made up of myocytes merging with their neighboring cells in a nonuniform anisotropic manner supported by an extracellular collagenous matrix-forming bundle of myofibres, which are configured in a helical or spiral pattern within the two ventricles .

The ventricular myocardium is thicker than the atrial myocardium, in particular, the myocardium of the left ventricle.

Boundaries

The inner border merges with the subendocardial tissue layer, which contains collagen, elastic fibers small blood vessels and nerves as well as the Purkinje fibers and connects the extracellular matrix of the myocardium to the endocardium .

The outer border is formed by the subepicardial layer, which is interconnected to the epicardium, the latter consisting of a lining of mesothelial cells, a subserosal layer of connective tissue and a variable amount of epicardial fatty tissue.

Arterial supply

The arterial supply of the myocardium is provided from the coronary arteries.

The vascular territories of the left ventricular myocardium are divided into 17 segments and illustrated by cardiac segmentation model of the American Heart Association (AHA) .

Venous drainage

Venous drainage of the myocardium is provided via the cardiac veins and the coronary sinus .

Lymphatic drainage

Cardiac lymphatic flow passes from endocardium through the myocardium to the epicardium where small lymphatic vessels drain into larger collecting vessels. In this process, myocardial contractions help to advance lymph flow. The lymphatic function is critical to maintaining the myocardial interstitial fluid equilibrium and cardiac function and it has been advocated that it aids tissue repair and prevents in adverse remodeling in case of myocardial injury .

Innervation

The myocardium is innervated by the cardiac conduction system. Myocardial conduction happens from cardiomyocyte to cardiomyocyte via the intercalated disks, which form the mechanical and electrical contacts between the myocardial cells .

Histology

The myocardium consists of cardiomyocytes grouped in strands also known as myofibres and the surrounding extracellular matrix with endomysial and perimysial components. The cardiomyocytes are linked with each other through distinctive junctional compounds, the intercalated discs, which facilitate intercellular electrical impulse conduction.  The myocyte strands diverge in different angles from the lining of the epicardial surface .

Radiographic features

The myocardium can be depicted and evaluated with ultrasound e.g. echocardiography, cardiac CT and cardiac MRI.

In most imaging techniques the myocardium displays a muscle-like appearance.

Echocardiography

Echocardiography has been traditionally used as a first-line imaging technique in the evaluation of cardiac morphology and function as well as in the assessment of myocardial contractility and the contractile reserve .

More recently it has been also used for the evaluation of myocardial strain or simplified for the assessment of regional deformations of the myocardium such as thickening, shortening or lengthening. Echocardiographic strain imaging techniques include tissue Doppler imaging speckle tracking .

Cardiac CT

Cardiac CT can be used for the assessment of myocardial morphology and myocardial perfusion. In addition, research has conducted with respect to the use as an alternative for myocardial extracellular volume quantification .

MRI

The myocardium can be visualized and is routinely evaluated with a whole series of sequences and imaging techniques assessing its function and inherent tissue properties, such as :

Specific less often used cardiac imaging techniques for the evaluation of the myocardium include:

In addition, the myocardial architecture and myofibril ultrastructure has been investigated with diffusion tensor imaging. Due to long acquisition times and this imaging technique has been mainly used in research .

Nuclear medicine
SPECT/PET

SPECT and PET imaging and is used in myocardial perfusion imaging and in the assessment of myocardial viability. PET permits measurement of absolute myocardial blood flow. Furthermore, it can be used in the evaluation of myocardial inflammation .

History and etymology

The word myocardium is derived from the Greek words 'myo-' muscle and 'kardia' heart.

Related pathology

The following pathologies and diseases are related to the myocardium:

and many more...

See also