To the Editor:—
We congratulate Hogue et al. 1for their excellent investigation of the autonomic effects of dexmedetomidine in humans. Although a good understanding of the hemodynamic effects of different anesthetic drugs in patients is the daily routine for most anesthesiologists, the origin and mechanisms of these drug effects are still confusing and complex. The author's use of heart rate variability (HRV) in addition to baroreflex responses and systemic catecholamine concentrations in these volunteers exemplifies the need to use multiple measurement techniques when examining the sympathetic and parasympathetic nervous system. We suggest that the addition of blood pressure variability (BPV) to this study would help strengthen the HRV data and expand their measurement techniques. As the authors stated, the low-frequency oscillatory component of the respiratory rate interval is considered a marker of sympathetic activity. Similarly, one can identify low-frequency oscillations that emanate from a different end organ, arterial smooth muscle, known as Mayer waves. 2The simultaneous measurement of HRV and systolic BPV has been described and used by Malliani et al. and showed increases in both low-frequency components with tilt, 3exercise, 4mental stress, 5physical exercise, 6moderate hypotension, 3and coronary artery occlusion. 7In conditions that directly affect one of the respective end organs, i.e. , heart (HRV) versus peripheral arteries (BPV), a unilateral response may be identified. 8Thus, the low-frequency component of systolic BPV, in addition to that generated by HRV, represents a good marker of sympathetic activity. 9Unfortunately, although the high-frequency component of HRV is considered a marker of parasympathetic activity, the high-frequency component of BPV is not. This is because the high-frequency oscillations in blood pressure are largely influenced by the mechanical changes induced by respiration. 9Therefore, we believe that analysis of the changes in low-frequency oscillations of the systolic blood pressure simultaneously with low-frequency and high-frequency oscillations measured in HRV will help investigators better explore and quantitate the effects of anesthetic drugs on neural regulatory mechanisms.