The blockade of cardiac ß-adrenoceptors prevents or reduces the usual positive inotropic and chronotropic actions of catecholamine administration on cardiac sympathetic nerve stimulation. Blockade of p-receptors prolongs systolic ejection periods at rest and during exercise. Both alterations tend to increase myocardial oxygen consumption. However, these alterations are offset by factors that tend to reduce oxygen consumption, such as decreased heart rate and decreased force of contraction. The decrease in oxygen demand produced by a decrease in heart rate and a decrease in force of contraction is usually greater than the increase in oxygen demand that results from increased heart size and increased ejection time. The net result is that oxygen demand is decreased.
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