The effects of aging on cardiac output in the intact, unanesthetized rat were determined in 50 12 mth old and 45 24 mth old male Wistar rats. Heart rate, phasic, and mean blood pressure were measured through an implanted intra arterial catheter. Cardiac output determinations were made by 86RbCl indicator dilution technique. In 30 young and 26 old rats studied at rest the heart rate was slightly higher (p < .05) and the mean blood pressure was slightly lower (p < .05) in the old than in the young rats. The cardiac index and the stroke index were significantly lower in the old than in the young rats (p < .005 and p < .001, respectively). In 20 young and 19 old rats studied after beta adrenergic blockade (propranolol 1 mg/kg), there were no age differences in heart rate or blood pressure. However, after beta adrenergic blockade, cardiac index and stroke index remained significantly lower in the old than in the young rats (p < .025 for indices). Thus, the decline in cardiac output and stroke volume with age was not related to differences in sympathetic nervous system tone.
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