This may be either acute or chronic, active or passive.

The acute form occurs as a result of severe muscular exertion or from a sudden interference with the passage of the blood through the pulmonary circulation.

The chronic is generally associated with hypertrophy and is referred to also as active in that the dilatation is associated with a thickening of the muscle wall.

In passive dilatation the increase in size of the cavity is accompanied by a thinning of the wall. This may be due to valvular interference, to changes in the muscle tissues of the heart and to all conditions that interfere suddenly with the emptying of the heart.

Dilatation is due either to a weakening of the cardiac walls, following interferences with nutrition, or to an increase in the cardiac blood-pressure.

The failure of nutrition generally is due to interference with the coronary arteries. In this the heart walls become soft and flabby, and in consequence dilate.