To determine the hemodynamic adaptations during the earliest phase of exercise without warm-up, we performed first-pass radionuclide angiocardiography on 19 well-trained college athletes (mean age 19.7 ± 1.1 years,mean body surface area 1.96 ± 0.12 m^2) during sudden strenuous exercise. The subjects were studied at rest and at 20 and 45 s during exercise on a bicycle ergometer. Each attained the predetermined workload (2,700 kpm at 90 rpm)in 5-10 s and maintained that workload for the duration of the study. Maximal cardiac index occurred at 20 s in 5 subjects and at 45 s in 14 subjects. Measurements of change in heart rate (HR), systolic blood pressure, left ventricular ejection fraction (EF) and end-diastolic volume were more homogeneous when analyzed by the magnitude of cardiac output attained than by the time of measurement during exercise. The cardiac index (Cl) increased from 3.5±0.8 liters/min/m^2 at rest to 9.1 ± 2.3 liters/min/m^2 at the intermediate study level by an increase in HR (71 ± 12 to 156 ± 14 beats/min, p < 0.0001) and EF (0.59 ± 0.07 to 0.65 ± 0.07, p < 0.003). The transition to maximal Cl resulted from an increase in end-diastolic volume index (EDVI) (90 ± 22 to 107 ± 21 ml/m^2, p < 0.0007) through the Frank-Starling mechanism. Thus, during sudden strenuous exercise, Cl is raised to an intermediate level by an increase in HR and by enhancement of the contractile state of the myocardium leading to increased stroke volume index. Increase to maximal Cl occurs through an increase in EDVI and stroke volume index.

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