Respiratory sinus arrhythmia as an index of parasympathetic cardiac control during the cardiac defense response.

The respiratory sinus arrhythmia (RSA) is being used by psychophysiologists as an index of parasympathetic cardiac control mainly in tasks within a tonic response paradigm. In procedures which engender phasic responses the belief exists that the RSA could be contaminated by slower nonrhythmic trends in the data. In the present paper two experiments are reported. The first experiment valuates, through beta-adrenergic blocking, the validity of the RSA as an index of phasic changes in parasympathetic cardiac control during phasic changes in sympathetic activation: the cardiac defense response (CDR) to intense auditory stimulation. The second experiment examines the RSA response pattern associated with the CDR. The results of the first experiment, that the RSA response pattern is not significantly influenced by the beta-adrenergic block, suggest that RSA may index phasic changes in parasympathetic cardiac control during phasic response procedures such as those which elicit the CDR. The results of the second study indicate that the CDR is associated with a pattern of changes in RSA made up of four components--reduction, increase, reduction and increase--which run parallel, but in opposite direction, to the heart rate changes. The results of both studies are consistent with a parasympathetic mediation of the first two components of the CDR and a sympathetic-parasympathetic interactive mediation of the last two components.