Warth et al. (1) have contributed an important addition relating to the higher evidence of music therapy in palliative care and have shown the benefits of music therapy compared with a control arm in a randomized controlled trial using the intention-to-treat principle for its data analysis in 84 patients. However, in measuring and explaining the endpoint “Triggering of a physiological relaxation response” by means of heart rate variability (HRV) they made two crucial methodological errors. On the one hand, they used photoplethysmography to measure the intervals between successive heartbeats. In contrast to an ECG-based measurement of the RR intervals, the measurement of the pulse rate is affected by further factors—vascular stiffness, among others. A systematic review (2) found that pulse oximeters are suitable for measuring RR intervals to a very limited degree only. Formally, the correct term to be used should not be HRV but, in this case, pulse rate variability (PRV). Especially the high frequency band is subject to overestimates in PRV compared with HRV, as has also been shown elsewhere (3). For this reason, national guidelines on HRV do not recommend this measuring method (4). On the other hand, only the HF band was considered. This may wrongly be interpreted as raised if the analysis does not include the total power (TP) as the reference value. An increase in the HF in a setting of a simultaneous stronger increase in the other frequency bands (for example, LF) is imaginable, which would mean that the relative proportion of the purely parasympathetic HF band even decreases. Preferably the HF that uses the total power as the reference value could be used (4) or, additionally, the changes of other frequency bands, including the parameter LF/HF ratio, might also be considered.
References
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