Aim: Cardiovascular oscillations are tightly coupled to respiration. Respiratory sinus arrhythmia (RSA) is an important part of heart rate variability with unknown function. Stroke volumes from the right (r-SV) and left (l-SV) side of the heart are assumed to vary differently with respiration, but have not previously been recorded non-invasively and simultaneously in humans. The present study introduces an improved technique for capturing respiratory variations in r-SV.

Methods: Six young volunteers were investigated during spontaneous and metronome-paced breathing in the left lateral decubitus position. Heart rate (HR, from ECG), l-SV (from finger blood arterial pressure curve) and r-SV (pulsed ultrasound Doppler) were recorded. Left and right cardiac outputs (l-CO and r-CO) were calculated beat by beat from HR, l-SV and r-SV. Respiratory variations in cardiovascular variables and phase angles were estimated by spectral analysis at respiratory frequency (0.15-0.40 Hz).

Results: The amplitude of respiratory variations in l-CO was 17% of r-CO (94% CI (6%, 35%), P=0.03). The amplitude of the respiratory variations in l-SV was not different from r-SV (74%, 94% CI (50%, 127%) non-significant). Increases in HR and r-SV were in phase with inspiration, while l-SV decreased during inspiration.

Conclusion: The amplitude of respiratory variations in l-CO is significantly smaller than in r-CO. Respiratory variations in HR and in l-SV are in inverse phase; thus, RSA buffers respiratory variations in l-SV. RSA plays an important role in reducing oscillations in systemic blood flow resulting from respiratory changes in venous return.