Interactive effects of simultaneously varying respiratory frequency and tidal volume on respiratory sinus arrhythmia

The effects of respiratory frequency (RF) and tidal volume (TV) on respiratory sinus arrhythmia (RSA) have been established separately. We assessed the interactive effects of simultaneously varying RF and TV on high-frequency power of RR intervals (HF_RR). 25 subjects performed three 30-s breathing protocols: linearly increasing RF (RF_LI) at fixed TV; linearly increasing TV (TV_LI) and decreasing TV (TV_LD) at fixed RF, and RF_LI with TV_LI-TV_LD. From time-frequency spectra, HF_RR and instantaneous RF were computed. RF_LI-HF_RR correlations were strong in separated and combined conditions, with greater regression slope in combined (p<;0.001). Three types of TV_LI-HF_RR relations were found: negative linear, no change and positive linear. Combined protocol transformed this inconsistent response into a systematic negative linear relation with greater slope (p<;0.001). TV_LD-HF_RR correlations were strong in separated and combined conditions, with steeper slope in combined (p<;0.001). Our fast, non-fatiguing and linearly varying protocols document that: 1) RF_LI consistently attenuates RSA, 2) the separate effects of TV_LI-TV_LD, especially TV_LI, on RSA, are ambiguous and present hysteresis, and 3) RF_LI combined with TV_LI-TV_LD induce greater attenuating effects on RSA. The nonlinear effect of TV on RSA questions the usual practice of normalizing RSA for TV.