https://orcid.org/0000-0002-9352-1504
Dear Editor,
Recently we had the opportunity to read the article published by Vukomanovic et al. [Citation1] in your journal. This study aimed to determine the autonomic nervous system function, assessed by heart rate variability (HRV), oxygen uptake (VO2), and heart rate kinetics, as well as the interaction between various clinical and glucose regulation parameters in individuals recently diagnosed with uncomplicated type 2 diabetes (T2D). Among the new and relevant results present by this study, the authors found that the only long-term HRV measure independently associated with all indexes of the cardiopulmonary exercise test was the standard deviation of all normal heart cycles (SDNN). This result is essential as this was the first time the association between HRV measure and cardiorespiratory fitness was demonstrated in individuals with T2D.
Despite the clinical importance of the study findings, there is an issue that, in our opinion, are worthy of further discussion. Although the authors' SDNN definition is according to the HRV guidelines [Citation2] as well as the data collection and analysis methodology had been performed according to the standardisation checklist of procedures for HRV recently published [Citation3], the clinical significance attributed to this HRV measure can be questioned. According to the Task Force's publication [Citation2], the SDNN reflects all the cyclical components responsible for the heartbeat variability in the registration period, encompassing both high and low-frequency variations of the heartbeats. In addition, as the variance is mathematically equal to the total power of the spectral analysis, SDNN is consequently correlated to the total power of HRV frequency domain analysis [Citation2]. In this sense, it is established in the scientific literature that SDNN reflects both sympathetic and parasympathetic cardiac modulation [Citation2,Citation4]. Otherwise, in the discussion section, Vukomanovic et al. [Citation1] considered that this HRV measure reflects the sympathetic function. This misinterpretation may have its origin in studies that showed a significant correlation between SDNN and spectral power component of low frequency (LF) as part of their results [Citation5,Citation6]. Vukomanovic et al. [Citation1] cited a systematic review [Citation7] which in turn cited a study published by Otzenberger et al. [Citation5] which shown an association of SDNN not only with LF but also with spectral power component of high frequency (HF) during sleep in men, reinforcing our suspicious in respect to the cause of the misinterpretation.
In conclusion, the existence of an association between these two HRV measures (SDNN and LF) in some studies did not mean that SDNN can be interpreted as a sympathetic activity measure. As the clinical interpretation of HRV measures is crucial to analyse the cardiac autonomic modulation properly, we hope that this letter could help interpret the study findings, especially concerning SDNN be recognised as an estimate of overall HRV.
Disclosure statement
The authors report no conflict of interest.
References
- Vukomanovic V, Suzic-Lazic J, Celic V, et al. Association between functional capacity and heart rate variability in patients with uncomplicated type 2 diabetes. Blood Press. 2019;28(3):184–190.
- Task Force of the European Society of Cardiology and the North American Society of Pacing and Electrophysiology. Heart rate variability: standards of measurement, physiological interpretation and clinical use. Eur Hear J. 1996;17(3):354–381.
- Catai AM, Pastre CM, Godoy MF, et al. Heart rate variability: are you using it properly? Standardisation checklist of procedures. Braz J Phys Ther. 2020;24(2):91–102.
- de Andrade PE, do Amaral JAT, Paiva L da S, et al. Reduction of heart rate variability in hypertensive elderly. Blood Press. 2017;26(6):350–358.
- Otzenberger H, Gronfier C, Simon C, et al. Dynamic heart rate variability: a tool for exploring sympathovagal balance continuously during sleep in men. Am J Physiol. 1998;275(3):H946–H950.
- Wang HM, Huang SC. SDNN/RMSSD as a surrogate for LF/HF: a revised investigation. Model Simul Eng. 2012;2012:1–8.
- Benichou T, Pereira B, Mermillod M, et al. Heart rate variability in type 2 diabetes mellitus: a systematic review and meta-analysis. PLoS One. 2018;13(4):e0195166.