Law of LaPlace Topic Review

The Law of LaPlace describes the factors that determine left ventricular wall stress, which is a major determinant of myocardial oxygen demand.

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Left ventricular wall stress is the force acting against the myocardial cells. This is directly proportional to the left ventricular pressure and radius. Wall stress is indirectly proportional to two times the wall thickness.

This equation is quite important in order to understand the various disease states that can alter oxygen demand, resulting in angina, and the therapies that can relieve angina.

• LV pressure increases with states that increase “afterload” of the heart including systemic hypertension and aortic valve stenosis.
• LV radius increases in valvular heart disease (especially aortic regurgitation) or cardiomyopathies causing systolic heart failure.
• LV wall thickness increases in chronic hypertension or aortic valve stenosis as a compensatory mechanism to decrease wall stress (inversely proportional to wall stress), thereby decreasing oxygen demand because the stress will be distributed over a larger mass. Hypertrophic obstructive cardiomyopathy, or HOCM, similarly increases wall thickness. After myocardial infarction, the wall thins during remodeling, which increases wall stress.

The treatment of stable angina is focused on reducing wall stress and decreasing myocardial oxygen demand.