Ventricular Hypertrophy

· Whenever cardiac muscle is forced to work harder to maintain output, the response of the myocardial muscle fibres is to hypertrophy, or increase in diameter. This change is accompanied by a thickening of the cardiac walls and an increase in the size & weight of the heart

· Forceful muscular activity causes the muscle size to increase [hypertrophy]. The diameter of the individual fibres increases due to increase in enzymes, additional mitochondria, increased size and perhaps number of myofibrils

· Approximate amount of hypertrophy that will occur in each chamber = ventricular output ´ pressure against ventricle must pump

· Chronic pressure loads characteristically result in concentric ventricular hypertrophy, with an increase in ventricular wall thickness that allows the heart to maintain its normal concentric position within the chest cavity

Aetiologies of ventricular hypertrophy

Causative factor	Pathological mechanisms
Aortic stenosis or regurgitation	Left ventricular musculature hypertrophies tremendously, sometimes as much as 4-5 times normal. May have a 1Kg heart instead of normal 300 g
Mitral stenosis	Left atrium dilates & hypertrophies. Right ventricle hypertrophies slightly. No left ventricular hypertrophy occurs.
Mitral regurgitation	Moderate hypertrophy of left ventricle occurs and some hypertrophy of right ventricle
Patent ductus arteriosus	Left ventricle hypertrophies as has to pump twice normal volume. Right ventricle hypertrophies as has to pump against increased pressure in lungs
Tetralogy of Fallot	Right ventricle hypertrophies as must pump against pressure in the aorta and it must pump an increased volume of blood. Left ventricle is normal as its work load is actually less than normal due to reduced volume of blood returning from lungs
Hypertension	Left ventricle hypertrophies due to increased arterial blood pressure poses resistance to the ejection of blood from the left ventricle
VSD	Right ventricle hypertrophies as must pump against increased pressure in the pulmonary artery and it must pump an increased volume of blood from the left ventricle. Left ventricle hypertrophies as has increased volume of blood to pump due increased blood returning from pulmonic circulation.
Hypervolaemia	Ventricular hypertrophy as excess extracellular fluid increases venous return to right side of heart —> increased volume of blood ejected
Pulmonary disease	Right ventricular hypertrophy as constriction of the blood vessels of the lungs (associated with lung disease) increases resistance to ejection of blood to right side of heart

Pathophysiology

· Hypertrophy facilitates an increase in myocardial pumping effectiveness

· Is one of the most important mechanisms by which the heart adapts to increased work loads whether these loads are caused by increased pressure the heart muscle must pump against or increased volume that must be pumped

· However, as the cardiac muscle enlarges, its needs for oxygen and nutrients increase accordingly. To the extent that myocardial blood supply cannot support escalating cellular needs, the force of cardiac contraction soon deteriorates

· Extreme degrees of hypertrophy lead to heart failure due to:

1. coronary blood flow does not always increase to the same extent as the mass of heart muscle.
2. fibrosis often develops in the heart muscle, the fibrous tissue often replacing degenerating muscle fibres.

· Because of the sometimes disproportionate increase in tissue mass relative to coronary flow, relative ischaemia sometimes develops as the muscle hypertrophies, and coronary insufficiency easily ensues. Therefore, anginal pain is a frequent accompaniment of many valvular and congenital heart disease