Lack of circadian variation in the activity of the autonomic nervous system after major abdominal operations

Objective: Most sudden postoperative deaths occur during the night and we conjectured that this was associated with circadian variations in the autonomic nervous tone, reflected in heart rate variability. Design: Prospective clinical study. Settings: University hospital, Denmark. Subjects: 44 patients who had had major abdominal operations. Interventions: Patients were monitored with 24-hour Holier ECG on the second postoperative day-evening-night. We calculated heart rate variability from the standard deviation of all normal R-R intervals (excluding ectopics-NN intervals) around the mean NN interval for the period of measurement (SDNN), the root mean square of the standard deviation of the differences between NN intervals (RMSSD), the percentage of NN intervals differing by more than 50 msec from adjacent NN intervals (pNN50) and the coefficient of component variance (meanNN/SDNN). Main outcome measures: Heart rate and heart rate variability. Results: Circadian variation calculated from the SDNN (p = 0.43) the pNN50 (p = 0.11), the RMSSD (p = 0.47), and mean NN:SDNN ratio (p = 0.13) was absent postoperatively. Circadian variation in the heart rate was present but was set on a higher level compared with reference values. Conclusion: After major abdominal operations there was a lack of circadian variation in the autonomic nervous tone.