What comes first? the dynamics of cerebral oxygenation and blood flow in response to changes in arterial pressure and intracranial pressure after head injury

Background: Brain tissue partial oxygen pressure (PbtO₂2) and near-infrared spectroscopy (NIRS) are novel Methods to evaluate cerebral oxygenation. We studied the response patterns of PbtO₂, NIRS, and cerebral blood flow velocity (CBFV) to changes in arterial pressure (AP) and intracranial pressure (ICP). Methods: Digital recordings of multimodal brain monitoring from 42 head-injured patients were retrospectively analysed. Response latencies and patterns of PbtO₂, NIRS-derived parameters [tissue oxygenation index (TOI) and total haemoglobin index (THI)], and CBFV reactions to fluctuations of AP and ICP were studied. Results: One hundred and twenty-one events were identified. In reaction to alterations of AP, ICP reacted first [4.3 s; inter-quartile range (IQR) -4.9 to 22.0 s, followed by NIRS-derived parameters and CBFV (10.9 s; IQR: -5.9 to 39.6 s, 12.1 s; IQR: -3.0 to 49.1 s, 14.7 s; IQR: -8.8 to 52.3 s for THI, CBFV, and TOI, respectively), with PbtO₂ reacting last (39.6 s; IQR: 16.4 to 66.0 s). The differences in reaction time between NIRS parameters and PbtO₂ were significant (P<0.001). Similarly when reactions to ICP changes were analysed, NIRS parameters preceded PbtO₂ (7.1 s; IQR: -8.8 to 195.0 s, 18.1 s; IQR: -20.6 to 80.7 s, 22.9 s; IQR: 11.0 to 53.0 s for THI, TOI, and PbtO₂, respectively). Two main patterns of responses to AP changes were identified. With preserved cerebrovascular reactivity, TOI and PbtO₂ followed the direction of AP. With impaired cerebrovascular reactivity, TOI and PbtO ₂ decreased while AP and ICP increased. In 77 of events, the direction of TOI changes was concordant with PbtO₂. ConclusionsNIRS and transcranial Doppler signals reacted first to AP and ICP changes. The reaction of PbtO₂ is delayed. The Results imply that the analysed modalities monitor different stages of cerebral oxygenation.