Seeking tau in head injury

The aim of this study was to determine if heart rate, blood pressure (diast olic, systolic, and mean), intracranial pressure, and cerebral perfusion pr essure of patients with head injury have circadian rhythm. We studied data of 13 patients with severe head injury using the Iterative Cosinor method. We detected a circadian rhythm for at least one of the parameters in 11 (85 %) of the 13 patients. The period of the rhythms (tau) was often not equal to 24 hours. Also, taus within and between patients were different. The res ults of our study showed that in these patients, when parameters are record ed hourly, tau can be estimated reasonably accurately (average error 0.7 ho urs). The distribution of taus suggests that the patients in our sample had free-running circadian rhythm. We used the surrogate data technique to val idate the results of our study. Possible reasons for uncertainty in tau-est imation and proposed changes in data collection protocols and inclusion cri teria are discussed.