Heartbeat Synchronizes With Respiratory Rhythm Only Under Specific Circumstances

Abstract

Yasuma and Hayano (PMID.14769752) have theorized that respiratory sinus arrhythmia improves respiratory efficiency by the pairing of increases in heart rate with inhalation, when the concentration of oxygen in the alveoli is maximal. However, this phase relationship only occurs under specific circumstances. Saul et al2 applied vagal and sympathetic blocking agents, and found that the phase lag from breathing to heart rate is near 0°, but only under pure vagal conditions. Under pure sympathetic conditions, the phase relationship varies from 180° at low frequencies to approximately −180° at high frequencies. We asked eight healthy subjects to breathe at seven frequencies between 0.04 and 0.5 Hz for 2 min each, matching their strain-gauge respiration record to a computer-generated sine curve3 to ensure a constant respiratory depth and a sinusoidal shape for respiratory curves. Using Fourier filtration,3 we determined that the phase relationship between heart rate and respiration was 0° only at a respiratory frequency of approximately 0.1 Hz, in which the target frequency heart rate variability also was highest (Fig 1 ). When healthy people breathe regularly at this resonant frequency for the cardiovascular system, we also found that the baroreflexes are systematically stimulated and baroreflex gain increases.4 In addition, peak expiratory flow improves.4There also is preliminary evidence for an improvement in clinical asthma,5and for improvement in respiratory gas exchange efficiency and clinical function in COPD patients.6 Thus, the hypothesis of Yasuma and Hayani1 would be specifically relevant for sympathetically medicated heart rate variability, or for respiratory sinus arrhythmia associated with slow breathing at approximately 0.1 Hz... [Author reply: Although the phase of the maximum heart rate lags behind the phase of the maximum lung volume as respiratory frequency increases, the amplitude of RSA decreases progressively.7 These facts seem to be consistent with the hypothesis that RSA is a function of physiologic respite for the cardiovascular and respiratory systems in resting animals and humans, because the phase relationship at a reduced respiratory frequency is optimal to cardiac and respiratory energy savings by reducing unnecessary heartbeats during expiration and unnecessary ventilation during the waning phase of the heart beat].