To the Editor:—
The elegant studies by Hanss et al. 1,2demonstrate that alterations in heart rate variability (HRV) may predict hypotension after spinal anesthesia. The authors found a significant association between an increased ratio of low-frequency HRV (LF) to high-frequency HRV (HF) (i.e. , LF/HF) and subsequent hypotension. A less predictive but still significant relation was found between decreased HF and subsequent hypotension.1
In their discussion, the authors briefly compare their results with our previous results3investigating altered autonomic function and hypotension after induction of general anesthesia. Both of these studies demonstrated an association between decreased HF and subsequent hypotension. Although the stronger relation that we found with decreased HF and subsequent hypotension may simply reflect obvious differences in the study conditions (e.g. , general anesthesia vs. spinal anesthesia, different patient populations), it might also relate to the different methods used to quantitate HF.
Heart rate variability measurements in the study by Hanss et al. 1are presented using normalized units, where the power in a specific frequency band (e.g. , HF) is normalized by the total power in the HRV spectrum (total HRV) for that patient (i.e. , HF in normalized units =[HF in absolute units]/[total HRV in absolute units]). A patient with decreased HF expressed in absolute units might have a normal value of HF when expressed in normalized units if the total HRV in his or her HRV power spectrum is also reduced (which is often the case in patients with autonomic dysfunction from multiple causes). For this reason, we used absolute measurements of HRV in our study. We found both a significant predictive value of decreased HF for postinduction hypotension and a strong correlation between HF and other measures of autonomic dysfunction. Historically, both absolute and normalized measurements of HRV have been used by different investigators.4Normalized units have advantages when HRV is used to examine “sympathovagal balance” (a major focus of the study of Hanss et al. 1), whereas absolute units have advantages when reflex integrity or “gain” (a major focus of our study) is examined.
Did the authors of the current study1examine any possible relation between absolute measurements of HF and subsequent hypotension (or is such an analysis possible with their data)? The relation that they observed with postspinal hypotension and normalized measurements of HF might be even stronger using absolute measurements of HF. A stronger relation, if it exists, would argue that preexisting alterations in reflex integrity or gain might also play a role in hypotension after spinal anesthesia (similar to previous findings with general anesthesia3,5). Furthermore, if a significant predictive relation between absolute measurements of HF and postspinal hypotension does exist, fairly simple clinical tests of autonomic function (e.g. , the change in heart rate with six vital capacity breaths, which is highly correlated with absolute measures of HF3) might also have significant predictive value.
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