HYPOTENSION during spinal anesthesia for cesarean delivery has been of concern since the 1960s.1Along with multiple strategies aimed at treating or preventing hypotension, some investigators have attempted to identify patients more likely to have hypotension in the hopes of targeting treatment. In this issue of Anesthesiology, Hanss et al. 2report that measurement of heart rate variability (HRV), an assessment of sympathetic and parasympathetic balance, can identify those women at risk for spinal-induced hypotension.
Heart rate variability investigations started in obstetrics, with the observation that changes in fetal HRV precede changes in actual heart rate in cases of intrauterine asphyxia.3Power spectral analysis of HRV uses fast Fourier transforms to display power (variance) by frequency and reflects autonomic control of the cardiovascular system.4Standards for performance and analysis of HRV have been published.5Using spectral analysis of 2- to 10-min electrocardiographic recordings, two main power “components” of variability can be identified: low frequency (LF; 0.04–0.15 Hz) and high frequency (HF; 0.15–0.4 Hz). The major contributor to HF variability is vagal efferent activity. LF variability is a result of parasympathetic and sympathetic outflow. The ratio of LF to HF power is an indication of the balance of sympathetic to parasympathetic influences. A larger LF/HF ratio is interpreted as reflecting higher sympathetic versus parasympathetic activity. HRV has clinical relevance: Decreased variability is associated with heart failure, mortality after myocardial infarction, and is an early sign of diabetic neuropathy.5HRV is altered by pregnancy,6preclampsia,7,8and regional anesthesia or analgesia.9
The current report from Hanss et al. 2consists of two separate, closely related studies: The investigators first retrospectively determined a threshold LF/HF ratio related to the risk of development of hypotension and then prospectively confirmed its validity. HRV was assessed in 41 women at three separate times before elective cesarean delivery: the day before surgery, and the day of surgery before and after intravenous hydration. Based on the systolic blood pressure response to spinal anesthesia, responses were classified as mild (no systolic blood pressure < 100 mmHg), moderate (lowest systolic blood pressure 80–100 mmHg), or severe (systolic blood pressure < 80 mmHg or requiring more than 1 ml of the vasopressor mixture) hypotension. LF/HF ratios on the day before surgery were not significantly different between groups. However, on the day of surgery, before hydration, the patients “destined” for development of moderate or severe hypotension had significantly higher LF/HF ratios (median 2.8 for moderate, 2.7 for severe) than those who went on to have mild hypotension (median 1.2). After hydration, moderate-hypotension patients decreased LF/HF ratios to mild levels, whereas severe patients were unchanged. Based on these results, the authors prospectively studied 19 patients to examine the hypothesis that a LF/HF ratio of 2.5 or greater on the day of surgery would predict hypotension. This hypothesis was confirmed; patients with high LF/HF ratio had significantly more hypotension than the patients with low LF/HF ratios.
Is it physiologically plausible that HRV parameters can predict hypotension during cesarean delivery? Are the criteria defined by Hanss et al. 2optimal? Can this type of technology be adapted to the routine clinical environment? Will it be? Should it be?
Heart rate variability as a predictor of hypotension seems physiologically plausible. A recent report by Chamchad et al. ,10using a retrospective protocol similar to the first part of the current report by Hanss et al. ,2used a nonlinear mathematical method of analysis but also suggested that HRV predicts hypotension during spinal anesthesia. The underlying assumption that preexisting higher sympathetic activity indicates a higher risk of hypotension during anesthesia is consistent with classic teaching in anesthesiology. Other measurements that may reflect sympathetic activity, including systemic vascular resistance index,7a “supine stress test,”11and baseline heart rate,12have been reported to correlate with the risk of hypotension. However, recent studies suggest that hypotension during spinal anesthesia is significantly less likely in preeclamptic patients than in healthy pregnant women,13despite higher sympathetic tone and LF/HF ratios in preeclampsia.14
Have Hanss et al. 2determined and defined the correct criteria and threshold for prediction of hypotension? This seems unlikely to have resulted from this one study. Further work replicating, refining, and/or refuting these qualitative and quantitative findings will no doubt be necessary. It should also be noted that the HRV criteria from the day before surgery did not correlate with hypotension risk. Only the LF/HF prehydration and perhaps the response to hydration seemed to be determinative, suggesting that it was the acute preoperative condition rather than the chronic stable physiologic state that influenced the occurrence of hypotension.
Heart rate variability technology seems to be reasonably well suited to the anesthesiology environment (although as with most new technology, it would probably reach the labor and delivery suite last of all the possible anesthetizing locations). In principle, what is needed is a good-quality electrocardiographic signal and the appropriate software. Transferring what is still predominantly a research tool to the clinic always involves problems and compromises, but HRV seems to be as validated and interpretable as processed electroencephalographic and “cerebral function/anesthesia depth monitors” were when they made the transition to the operating room. One can imagine HRV equipment configured with a single number readout, with all the advantages and disadvantages that sort of output implies. Will a manufacturer start to offer this option on operating room patient monitors? If so, will anyone opt to buy it? The answer to the both questions is probably no, at least not without additional studies suggesting a predictive or therapeutically directive role for HRV. Can HRV do a better job than blood pressure, heart rate, and central pressure measurements in determining whether trauma patients need fluid or vasopressors or when intensive care unit patients are becoming septic? Can it do a better job than the electrocardiogram alone at detecting when coronary perfusion pressure is insufficient?
Finally, there is the question of whether this kind of monitoring is necessary, even if available. In reality, what should one do with the information that a patient is likely to become hypotensive after spinal anesthesia? Without HRV or some other predictor, clinicians assume that 50% or more of cesarean delivery patients will develop hypotension; how much would or should clinical practice change if we could identify which 50 or 70% were more likely to? Several editorials over the past decade have suggested that the search for the right drug, the correct amount of fluid, or some other formula to avoid hypotension may not be worth the effort, given how relatively easy and effective it is to treat hypotension when it does occur.15,16
It is a natural human tendency to want to predict the future, whether of the stock market, the outcome of a sporting event, events in our own lives, or the response of our patients to drugs and other interventions, so this sort of work is undeniably attractive. The quest to predict the future in the clinical environment is of value, both for the potential to improve clinical care and because measurements that can predict physiologic responses usually reflect something inherent in the mechanism underlying that particular physiology, thereby improving both medical care and understanding. The quest for accurate and available predictive tools in this and other clinical arenas will no doubt continue, and HRV may well belong in the toolbox of the would-be prophets.
Columbia University College of Physicians and Surgeons, New York, New York. email@example.com