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The effects of alterations in the frequency of contraction over the range from 94 to 220/min on left ventricular pressure, diameter, and dP/dt were studied in 10 dogs instrumented with ultrasonic diameter ganges and miniature pressure gauges. The same dogs were studied on separate days in the conscious state, after general anesthesia with pentobarbital Na, 30 mg/kg, and in the conscious state after pretreatment with propranolol, 3 mg/kg. End diastolic diameter was maintained constant during alterations in frequency by infusing saline intravenously. The maximum increases in peak dP/dt and dP/dt/P in the conscious state were 14 and 10%, respectively. After anesthesia, raising the frequency of contraction from 122 to 220/min caused maximum increases in peak dP/dt and dP/dt/P of 36 and 30%, respectively. In the conscious state after cardiac depression by propranolol, the maximum increases in peak dP/dt and dP/dt/P were 23 and 23%, respectively. Thus, increasing the frequency of contraction of the normal heart of the conscious dog causes only a slight inotropic effect, but this effect is significantly greater in the presence of myocardial depression produced by anesthesia with pentobarbital Na or in the conscious animal after a myocardial-depressing dose of propranolol.

Extent of Regulation of the Heart's Contractile State in the Conscious Dog by Alteration in the Frequency of Contraction