Can heart rate variability become a screening tool for anesthesia-related hypotension?

Abstract

H EART rate variability (HRV) analysis as a diagnostic tool has been available for decades (1), and many have also learned about modern HRV analysis as a predictive or prognostic instrument for cardiological patients with diagnoses such as congestive heart failure or post-myocardial infarction (2). Numerous recent publications in the field of anesthesiology have presented findings concerning HRV as a potential tool for predicting acute circulatory disturbances, and particularly hypotension (3–11). As all in the field of anesthesiology and critical care medicine traditionally have in mind a central clinical theme to ‘avoid hypotension and hypoxia’ in all perioperative patients, it is relevant for the specialty to carefully study and try to establish validity for any diagnostic instruments which show promise in predicting circulatory disturbances, including hypotension. In this (12) and the previous issue (13) of Acta Anaesthesiologica Scandinavica, two studies are presented where HRV analysis is examined as a possible screening tool for anesthesia-related hypotension (12, 13). Both studies present findings where a preoperative HRV finding was associated with more hypotension during spinal anesthesia. The study by Hanss et al. (12) is a continuation in a series of studies in obstetrical patients, where they have systematically tested HRV parameters in relation to different parts of the peri-operative procedure in a highly selected patient group which is at high risk of hypotension. The study by Fujuwara et al. (13) includes use of a HRV analysis method which has different characteristics from the more commonly used fast Fourier transformation (FFT) method. Both studies provide information and experience which contribute to the process of evaluation and validation of HRV as a clinical tool for assessment of perioperative patients. The few studies published so far where HRV has been applied to predict hypotension can be regarded as preliminary studies within a new research area. Can hypotension during spinal anesthesia or other types of anesthesia be predicted from patterns in HRV? Moreover, is increased low frequency (LF) to high frequency HF power spectrum density ratio (LF/HF) a marker of an increased risk of hypotension, which is suggested by the findings in both studies in this issue? Is increased LF/HF caused by increased sympathetic activity, or is it mainly caused by reduced vagal outflow? Fujiwara et al. (13) also found that subjects with a low degree of randomness in their HRV seemed to be more susceptible to decreased blood pressure after spinal anesthesia. Although not specifically discussed in the paper, these subjects also had relatively low values of both LF and HF power, which was presented as absolute values. The latter finding could also indicate reduced vagal outflow in this subgroup. HRV results in this clinical anesthesia context are difficult to interpret because of several problems, some new, and some not so new (14). First, it is not clear exactly which phenomena in the body that HRV is measuring. Second, so far there are no findings to help to understand whether HRV is more useful in predicting hypotension then other already readily available (and much simpler) parameters. Including, for example, resting heart rate before spinal anesthesia. Third, some of the methodology is difficult to examine and understand. The first and perhaps most important issue concerns exactly which physiological or pathophysiological process HRV is assessing. Regulation of heart