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

Abstract

Brain tissue partial oxygen pressure (PbtO₂) 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). 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. 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 (PPbtO₂ (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 PbtO₂ decreased while AP and ICP increased. In 77% of events, the direction of TOI changes was concordant with PbtO₂. NIRS 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.