Impaired cerebral autoregulation during upright tilt in patients with severe brain injury

Objectives: Upright tilt is an important tool for early mobilization of patients after severe acquired brain injury (ABI). Early mobilization is considered to be of importance for increasing awareness, in the prevention of contractures and pulmonary infections and to improve long-term outcome. A challenge for early mobilization is orthostatic instability that is often observed during heap-up tilt, which could lead to decreased cerebral blood flow velocity (CBFV) and decreased long-term outcome. We examined cerebral blood flow autoregulation in patients with severe brain injury and impaired consciousness using a tilt table and compared the results to those of healthy volunteers. Methods: Fourteen patients (seven men, mean age = 56.6 ± 17.5 years) with severe ABI and 15 healthy volunteers (seven men, mean age = 36.1 ± 12.5 years) were included. The CBFV was measured by Transcranial Doppler in the middle cerebral artery and expressed as the flow velocity. Continuous arterial blood pressure (ABP) was recorded from the index finger using photoplethysmography. CBFV autoregulation was determined by the correlation between CBF and BP. An estimated cerebral perfusion pressure (CPP) was established through an equation, taking into account the hydrostatic pressure during head-up tilt. After 30 minutes of resting, baseline values were obtained as the mean of 5 minutes. The subjects were then tilted head-up to 30, 60 and 80° with a 1 minute interval and remained at 80° for 20 minutes or until the occurrence of significant haemodynamic changes, i.e. a decrease in systolic BP > 20 mmHg, diastolic BP > 10 mmHg or an increase in heart rate > 30 bpm. Measurements were continued in the subsequent supine position to ensure a total recording period of 30 minutes. Cerebral autoregulation was assessed by calculating the mean flow index (Mx) between CBFV and MAP (Mxa) or CPP (Mxc). Spectral analysis of the HR variability was used to investigate baroreflex activity. Results: In the supine position, MAP and HR were higher in the patients compared to the controls (p = 0.0004 and p < 0.0001, respectively) and the CBFV were lower in the patients (p < 0.001). The patients showed a significant increase in the Mxc index during head-up tilt pointing at impaired autoregulation (p = 0.001). Spectral analysis of HR variability in the frequency domain revealed significant lower magnitudes of ~ 0.1 Hz spectral power in patients compared to healthy control, indicating baroreflex dysfunction in patients. Conclusions: Patients with severe ABI and orthostatic instability during head-up tilt show unco-ordinated loss of autoregulation of cerebral blood flow more than 1 month after the accident. Interventions addressing this challenge are warranted.