• cerebral blood flow
• intensive care unit
• phase shift
• transcranial doppler
• transfer function analysis
• traumatic brain injury

INTRODUCTION
Phase-shift (PS) between slow waves (0.005-0.05Hz) in arterial blood pressure (ABP) and middle cerebral artery blood flow velocity (FV) is an established metric of cerebral autoregulation. However, its reliability relies on the power of spontaneous waves in ABP, which varies widely. Nevertheless, in ventilated patients with traumatic brain injury (TBI), there are regular, pronounced respiratory waves (0.167-0.333Hz) present in ABP, transmitted to FV and intracranial pressure (ICP), providing robust PS metric. We hypothesised this PS is altered in patients with severe midline-shift (MLS) and worse in the affected hemisphere.
METHODS
We retrospectively analysed 104 TBI-patients’ recordings with ABP, ICP, and bilateral FV. We computed PS via cross-spectral-analysis (region 0.167-0.333Hz). We calculated PS between combinations of bilateral FV and ABP. We dichotomised recordings in two ways: “No MLS” (MLS>0mm, n=80) vs "MLS" (MLS>0mm, n=24); “focal” (n=60) vs “diffused” (n=44) pathology distribution.
CONCLUSIONS
Respiratory-wave PS metrics had an unclear relationship with MLS. Coherence between the flows seemed to be slightly lower in the group with MLS, but not statistically significant. PI presented as the most sensitive index to distinguish patients with and without MLS. Further studies are required to establish the role of respiratory PS in TBI.