The dose dependent responses to atropine of 11 indices of autonomic function were investigated in 10 healthy volunteers. Five subjects were given cumulative doses of atropine (0.1-2.0 mg/70 g, intravenously). The other five received equivalent volumes of water for injection. The ratio of the longest to the shortest R-R intervals in the electrocardiogram during forced expirations and deep inspirations (respiratory ratio), deviations in the periods of consecutive cardiac cycles, the ratio of the longest to the shortest R-R intervals during a Valsalva manoeuvre (Valsalva ratio), the ratio of the 30th to the 15th R-R intervals after standing up from a low sitting position (30:15 ratio), and the means of R-R intervals recorded in standing or lying positions were calculated with different computer algorithms. Sympathetic orthostatic effects on the above indices were measured by comparing values in supine and standing positions after atropinization. The recumbent respiratory ratio was the most sensitive test to atropinization. Its maximum response was a 97% decrease, indicating specificity for the vagal tone. The decrease in other indices in response to atropine ranged from 34% to 94%. The average orthostatic effect on the indices after atropinization was a 14.6% increase. The Valsalva ratio failed to respond significantly to any degree of muscarinic antagonism. In conclusion, the Valsalva ratio is unlikely to be closely related to the parasympathetic control of the heart. However, the resting vagal tone can be selectively measured in a live and conscious person by using the respiratory ratio.
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