The effects of atropine on respiratory sinus arrhythmia in asthma.

Heart rate is rhythmically related to respiratory rate, in a phenomenon known as respiratory sinus arrhythmia (RSA). Amplitude of RSA has been found to be an accurate noninvasive measure of vagal tone in the normal population. The purpose of this study was to examine the relationship between RSA and cardiac vagal tone among asthmatics, a group which has been described as having elevated RSA. This was accomplished by examining the effects of atropine on RSA in asthmatics, and comparing the results to those obtained elsewhere among normal subjects. Atropine at a dose of 1 mg/75 kg body weight was administered to 18 asthmatics, and produced decreases in RSA, equivalent to those previously found among normal subjects. The results suggest that the relationship between cardiac vagal tone and RSA is unaffected by the presence of asthma.