This study tested various sources of changes in respiratory
sinus arrhythmia (RSA). Twenty-two healthy participants
participated in three experimental conditions (mental stress,
relaxation, and mild physical exercise) that each consisted
of three breathing parts (normal breathing, breathing compressed
room air, and breathing compressed 5% CO2-enriched
air). Independent contributions to changes in RSA were found
for changes in tonic vagal modulation of heart rate, central
respiratory drive (i.e., PaCO2), respiratory depth,
and respiratory frequency. The relative contributions to changes
in RSA differed for mental stress and physical exercise. It
is concluded that uncorrected RSA will suffice to index
within-subject changes in tonic vagal modulation of heart rate
in most situations. However, if the central respiratory drive
is expected to change, RSA should ideally be corrected for changes
in PaCO2, respiratory depth, and respiratory frequency.

The extent to which variation in posture and respiration can confound pre-ejection period and respiratory sinus arrhythmia (RSA) as indices of cardiac sympatho-vagal activity was examined. Within-subjects changes in these measures were assessed in 36 subjects during different postures and (paced) respiratory frequencies. Changes from supine to sitting to standing led to reduced RSA values and longer pre-ejection periods, reflecting the known decrease in vagal but not the increase of sympathetic activity. Multilevel path analysis showed that within-subjects changes in sympatho-vagal balance were faithfully reflected by changes in interbeat interval, but imperfectly by changes in RSA and pre-ejection period. It was concluded that pre-ejection period should be stratified for posture and RSA for respiratory frequency to reliably index changes in sympatho-vagal balance when these factors are prone to change (e.g., during 24-h ambulatory recording)