In order to assess the role of the autonomic system in the age differences in heart rate, propranolol 0.2 mg./kg. and atropine 30 micrograms/kg. were given to 103 infants and children during the course of heart catheterization. Mean control minute heart rate varied from 118 at age 1 year to 81 at age 16 years. Heart rate after drugs was termed intrinsic heart rate after Jose, and the mean varied from 139 at age 1 to 102 at age 16 years. After propranolol alone, mean heart rate was 120 at age 1, 71 at age 16, and after atropine corresponding rates were 162 and 128. The scatter in predicting heart rate from age was not lessened by autonomic blockade, suggesting that inter-individual variations in heart rate are not entirely dependent on different levels of autonomic function. The inverse relation between heart rate and age persisted after autonomic blockade, indicating that different levels of autonomic function are not responsible for the age differences in heart rate. The most important determinant of the heart rate after blockade was the initial control heart rate. Increases in cardiac index followed autonomic blockade in patients over 4 years of age and in patients without intracardiac shunts, while stroke index decreased, and femoral artery pressure increased 2/7 mm. Hg.
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