In a recent study, Yoon et al1 investigated heart rate (HR) control before and after intracarotid amobarbital injection by means of spectral analysis. There are several methodological problems we would like to comment on.
As the authors stated, HR oscillations in the high-frequency (HF) band (0.15-0.4 Hz) are purely mediated by parasympathetic efferents at least for the supine position.2- 4 Low-frequency (LF) oscillations in HR (0.04-0.15 Hz) were thought to reflect "predominantly sympathetic" activity. Several studies using pharmacological autonomic blockades have shown, however, that there is little2,3 if any4 sympathetic contribution to the LF band of HR for the supine position. The assumption that the LF/HF ratio provides a measure of "sympathovagal balance" seems even more problematic, particularly for the supine position.4 This approach would have been more appropriate if LF and HF oscillations were indicators of a stable sympathetic and parasympathetic tone, respectively, only modulated by cerebral hemispheric control. This paradigm, however, neglects the real origin of the LF and HF bands. The LF oscillations in HR are the consequence of the corresponding oscillations in arterial blood pressure (ABP) mediated by the baroreflex.5 Therefore, a change in LF oscillations in HR may be either a consequence of an altered sympathetic (or parasympathetic!) outflow to the heart, or a change in vasomotor activity (LF band of ABP), or an altered baroreflex sensitivity (the ratio of HR/ABP oscillations in the LF band), or a combination of them. In addition, respiratory activity that is the causative input variable for the HF band of HR may additionally influence the amount of HR oscillations in the LF band, if the breathing frequency shifts into the LF band. In the study by Yoon et al,1 continuous measurements of ABP and respiration were not performed or the data were not provided. Therefore, it cannot be excluded that hemispheric inactivation could have altered not only the sympathetic or parasympathetic outflow to the heart but also the input variables ABP and respiration. Furthermore, the transferal characteristics from the input to the output variables (baroreflex function and cardiorespiratory coupling) are nonstationary and may also be under additional hemispheric control.
Linden D, Diehl RR. Spectral Analysis of Heart Rate: Interpretational Limitations. Arch Neurol. 1998;55(2):264-265. doi: