Left Ventricular Response to Severe Exertion in Untethered Dogs

The left ventricular response to severe exercise was studied by telemetering direct measurements of left ventricular diameter (D) and pressure (P) and aortic blood flow from healthy dogs running at speeds up to 30 mph in the field. Severe exercise increased cardiac output from 101 to 478 ml/kg per min, heart rate from 95 to 297 beats/min, stroke volume from 31 to 44 ml, left ventricular isolength (iso) systolic pressure from 120 to 186 mm Hg, left ventricular end diastolic pressure from 6 to 18 mm Hg, and left ventricular end diastolic diameter from 58.9 to 60.1 mm, while end systolic diameter decreased from 53.0 to 52.2 mm. Two indices of myocardial contractility, (dP/dt)/P increased from 37 to 92 sec(-1), while dD/dt, the velocity of myocardial fiber shortening at isolength, rose from 54 to 119 mm/sec. All of these changes were statistically significant. When, in resting dogs, heart rate was first raised to exercise levels by electrical stimulation, severe exercise subsequently increased left ventricular end diastolic diameter more profoundly, from 55.7 to 59.7 mm, while end systolic diameter remained constant and the increases in left ventricular pressure, (dP/dt)/P and velocity(iso) were roughly comparable to those occurring during exercise in spontaneous rhythm. After propranolol, 1.0 mg/kg, severe exercise resulted in significantly smaller increases in cardiac output (from 82 to 240 ml/kg), in heart rate (from 87 to 186 beats/min), in left ventricular pressure(iso) (from 122 to 150 mm Hg), in (dP/dt)/P (from 32 to 44 sec(-1)), in velocity(iso) (from 47 to 59 mm/sec), and in slightly greater increases in end diastolic diameter, from 59.8 to 62.0 mm and pressure from 8 to 22 mm Hg, while end systolic diameter did not change significantly.Thus, the left ventricle responds to severe exercise with near maximal increases in heart rate and contractility, while significant increases in end diastolic diameter (Frank-Starling mechanism) and stroke volume occur as well. When heart rate was held constant severe exercise produced similar increases in contractility but end systolic size failed to diminish and the increases in end diastolic size were greater. Beta adrenergic receptor blockade interfered with the chronotropic and particularly the inotropic response to severe exercise and while the participation of the Frank-Starling mechanism was somewhat greater, the latter was not sufficient to increase cardiac output normally.