RESPIRATORY SINUS ARRHYTHMIA, CARDIAC VAGAL TONE, AND RESPIRATION - WITHIN-INDIVIDUAL AND BETWEEN-INDIVIDUAL RELATIONS

Respiratory sinus arrhythmia (RSA) is frequently employed as an intra- and interindividual index of cardiac parasympathetic tone, although t he relationship of RSA to interindividual differences in cardiac vagal tone remains questionable. Our study examined between- and within-sub ject relations among RSA, cardiac vagal tone, and respiratory paramete rs. Twenty-nine young adults performed two sessions of tasks under no medication and single and double autonomic blockade (intravenously adm inistered propranolol and atropine). Parasympathetic tone was determin ed from heart period responses to complete vagal blockade. Results ind icated the following. Resting RSA does not accurately predict individu al differences in cardiac vagal tone. However, RSA and heart period to gether do predict such individual differences reasonably well. The rel ationship between individual variations in RSA and vagal tone is not i mproved by controlling respiratory parameters. Substantial cardiac vag al activity occurs during inspiration, and intraindividual variations in respiratory measures confound the association between RSA and cardi ac vagal tone.