Heart rate variability following cardiac surgery fails to predict short-term cardiovascular instability.

The heart rate variability of 40 patients has been examined by spectral analysis following cardiac surgery. The heart rate variability was measured upon patient arrival in ICU in both a resting supine position, and following passive straight-leg raising. After 12 hours in ICU, the patients were classified as having been cardiovascularly stable or unstable according to a specially devised inventory. Their heart rate variability data was then examined to seek any predictor of instability. Passive straight-leg raising induced a decrease in spectral power across all of the component frequency bands. The LF/HF ratio rose with passive straight-leg raising, but failed to reach significance. None of these changes were sustained. There was no significant difference in heart rate variability patterns between the stable and unstable groups, and so no predictor was identified. Initial clinical assessment was also studied, and it too provided no reliable prediction of short-term cardiovascular instability.