The effect of atropine on parasympathetic control of respiratory sinus arrhythmia in two ethnic groups.

The effects of low and high doses of atropine on respiratory sinus arrhythmia were assessed to ascertain whether any differences exist in the degree of parasympathetic control on cardiac rhythm between two ethnic groups. The standard deviation of the R-R interval (the R peak-to-peak interval of two QRS complexes of an electrocardiogram) has been used as a noninvasive parameter to measure the degree of parasympathetic cardiac control. Nine black and nine white healthy male volunteers took part in study after approval by the Research and Ethics Committee of the Medical University of South Africa. Thirty consecutive electrocardiographic complex were recorded during each of the following: 3 deep inspirations and expirations, incremental cumulative atropine injections of 0.001 mg/kg until 0.005 mg/kg, followed by two injections of 0.01 mg/kg and 0.015 mg/kg of atropine each. After each of the last two injections the deep inspiration and expiration procedure was repeated. No significant differences could be found at any stage for any parameter between the groups. In both groups the R-R interval variation increased threefold during voluntary induced respiratory sinus arrhythmia. This effect was blocked after 0.01 mg/kg of atropine. The degree of parasympathetic control was not affected by any respiratory maneuver, but was affected by atropine. A significant inverse quadratic relationship was found between parasympathetic control and heart rate change (R = .984 for whites, and R = .905 for blacks). A poor correlation was found between the R-R interval variation and heart rate changes. In conclusion, ethnicity does not affect parasympathetic activity.(ABSTRACT TRUNCATED AT 250 WORDS)