Cardiovascular response to sudden strenuous exercise.

Cardiac responses to sudden strenuous exercise were studied in seven dogs instrumented to measure cardiac output (Q), stroke volume (SV), left ventricular (LV) pressure, aortic pressure and circumflex coronary blood flow (CBF). The dogs were run on a treadmill at 12.9 km/h, 20% grade for 15s without prior warm-up. During the first 10s of the run, HR and Q increased rapidly. By 2s, myocardial O2 requirements, as estimated from either the tension time index (TTI) or calculated wall tension, had increased significantly above resting values. Simultaneously, mean CBF fell 13% and stroke CBF fell 49% below resting values at the onset of sudden exercise (2s) and was associated with 17% and 21% decreases in positive and negative LV dP/dt, a 9% decrease in (dP/dt)/P and an 11% decrease in SV. After the initial decrease at 2s, these variables then increased rapidly as the run continued except for stroke CBF which never reached resting levels. HR, Q, and LV dP/dt reached near maximal values for this level of exercise by 10s after the start of exercise. In contrast, mean CBF was still increasing when the run was terminated. These data show that sudden strenuous exercise results in a transient decrease in myocardial O2 delivery at a time when myocardial O2 requirements are rapidly increasing. This imbalance between myocardial O2 supply and demand is due to a transient decrease in coronary blood flow followed by a delay in metabolic regulation of CBF.