Methodologic implications for rehabilitation research: Differences in heart rate variability introduced by respiration.

Everyone was 40 or younger and free of heart disease, high blood pressure, diabetes and heart-active medication, so nothing here tells you how the measurement behaves in the older or sicker patients on your caseload. The two recordings were always done in the same order, spontaneous breathing first and paced breathing second, so a change over the session cannot be ruled out. The authors say randomising the order would settle it. Breathing depth was not measured. Only breathing rate and exhaled carbon dioxide were captured, and depth is also known to affect heart rate variability, so some of the difference may come from that rather than from pacing alone. Fifty-nine people from one centre, all with spinal cord injury. The authors expect the underlying physiology to apply to other rehabilitation patients, but that expectation was not tested here. The authors themselves note that heart rate variability does not map onto autonomic activity in a simple or linear way, and that the low-frequency share and the low-to-high ratio have little grounding in actual physiology. That weakens the meaning of the very measures that shifted most.

The supplied text contains no funding or competing-interest statement. The article is indexed as receiving extramural support from the National Institutes of Health, and the recordings came from a registered exercise trial (NCT02139436).

Do not take the similar average high-frequency power in the two breathing conditions as proof that unpaced and paced recordings agree. In the same people the two disagreed widely in both directions, and the authors calculate that a high-frequency reading of 500 ms2 taken while breathing normally could correspond to a paced value anywhere between roughly 0 and 1,000 ms2.