Effects of Ischemia, Hypoxia, and Acidosis on Cardiac Systolic and Diastolic Function, and Glycolytic Metabolism in Normal and Hypertrophied Hearts

The normal heart, especially during states of increased cardiac work such as exercise, functions precariously close to the limits of the coronary circulation to supply oxygen. Oxygen extraction by the myocardium is greater than for any other tissue. Since there is little oxygen content or “oxygen reserve” in coronary venous blood under resting conditions, significant increases in myocardial oxygen demand must be met by decreasing coronary vascular resistance and increasing coronary blood flow. A normal heart can increase its coronary flow by five- to sixfold. However, such coronary “flow reserve” is reduced in common clinical conditions such as left ventricular hypertrophy and coronary atherosclerosis, and ischemia occurs when myocardial oxygen demand increases during exercise. With severe atherosclerosis, resting coronary flow may be reduced, resulting in ischemia in the absence of an increased oxygen demand. Myocardial tissue hypoxia can also occur in the presence of normal or increased coronary flow if arterial hypoxemia is present secondary to pulmonary disease, right-to-left cardiac shunts, or other rarer causes such as exposure to high altitude or carbon monoxide.