Spectral analysis of heart rate variability in the ICU: a measure of autonomic function.

Beat-to-beat heart rate variability (HRV) is a measure of autonomic nervous system activity, which can be quantified using frequency domain analysis. Despite its potential utility, routine serial analysis of HRV in an ICU setting has rarely been attempted. We have developed an automated system for real-time spectral analysis of HRV and have utilized this system to study the effect of alterations in HRV on mortality in a surgical ICU population. HRV measurements were performed every 6 hr on all patients in the ICU. Total spectral power in the variability signal (TP, a measure of overall autonomic activity) and the ratio of high frequency to low frequency components (HF/LF ratio, a measure of parasympathetic/sympathetic balance) were calculated. Over a 6-month period 7994 automated HRV measurements were made in 742 patients. Both low TP (low autonomic tone) and high HF/LF ratio (relative lack of sympathetic tone) were associated with increased mortality. A low HF/LF ratio (relatively high sympathetic tone) was associated with increased survival, especially in patients with low autonomic tone. We conclude that serial spectral analysis of HRV is practical in an ICU setting and that HRV parameters appear to be a clinically relevant indication of autonomic activity. Low sympathetic tone and vagal predominance are associated with increased mortality, while sympathetic predominance favors survival. Monitoring of HRV parameters has the potential to detect physiologic deterioration or response to therapy.