DISCUSSION
orthostatic, TBI
This study investigated the autonomic response to tilting in the acute phase of TBI along with potential changes induced by early tilt training. During head‐up tilt in patients, the total power of the frequency domain, SDNN, and RMSSD all decreased, whereas the remaining HRV measures remained unchanged. This was contrary to the reference group of healthy volunteers as HF and sample entropy were lower during head‐up tilt and LF, LF/HF, and DFA1 increased from supine to head‐up tilt. Further, the comparison at baseline revealed that the patients in the supine position had lower values of HF, total power, SDNN, and RMSSD while LF, LFHF, and DFA1 slopes were higher when compared to the healthy volunteers. As an additional finding, no change in the long‐term HRV analyses was found between patients receiving early tilt training compared to standard care within the first 3 days of intervention.The primary aim of early enrolment and tilting of TBI patients was achieved with a mean inclusion point of 12.8 days post‐event and a tilt to 70° of all patients with short‐term recordings. However, the long‐term recordings proved to be challenging within the context of intensive care. In order not to diminish sample size further, analyses of long‐term recordings were restricted to the first 3 days of recording. This meant a very short period to observe possible effects of the intervention. It would have been interesting to do a follow‐up of heart rate from the rehabilitation phase, however, this was not the primary focus of the study.To the best of our knowledge, this is the first study to report on the autonomic response to an orthostatic challenge in the acute phase after TBI. The observed tilt‐induced reductions in SDNN, RMSDD, and total power in short‐term recordings corresponded qualitatively to those of the included group of healthy volunteers and those which have been previously reported in healthy individuals (Montano et al., The primary strength of this study is that the design included an autonomic challenge in the acute phase; this has not previously been attempted so early after TBI. The major limitations were the small sample size and the large amount of missing data. In both the long‐term and short‐term analysis some extreme outliers were present (Figures 