Continuous monitoring of cerebral blood flow (CBF) and cerebral temperature in neurocritical care unit

INTRODUCTION. The balance between heat generation and heat removal by cerebral blood flow defines brain temperature in humans and other large animals. In physiological conditions brain metabolism is among the main determinants of CBF and brain temperature (BrT) affects the metabolism itself. OBJECTIVES. To correlate BrT and core temperature (cT) in neurocritical ill patients and to evaluate any variation of CBF at different levels of temperature. METHODS. 15 patients (9M, mean age 54,2 SD 10,5; 1 brain injured and 14 with subarachnoid hemorrhage -SAH) were included in the study. Core temperature was continuously monitored with a bladder catheter. Patients were sedated, mechanically ventilated. BrT and CBF were continuously monitored minute-by-minute with an intracerebral probe (Hemedex®). Patients were monitored from 3 to 6 days. Intracranial pressure was maintained < 20 mmHg. Cerebral perfusion pressure was tailored according to the state of cerebral pressure vasoreactivity. RESULTS. Mean core temperature was 37.1° (SD=0.3°). The intraclass correlation coefficient was 0.54, therefore 54% of core temperature variability was due to the “patient effect”. BrT and CBF values for any patient are summarized in figure 1. BrT ranged from 34.1 to 39.7 °C while cT ranged from 34.5 to 38.8 °C. In any patient BrT was superior to cT. Given the longitudinal nature of the data, a linear mixed regression model was fitted to assess the relationship between CBF and BrT in order to take into account the correlation among repeated measures on the same subject. The CBF effect on cerebral temperature was 0.006397°, thus meaning that any CBF increase of 1ml/100g/min the brain temperature increase of 0.006397°C. This effect did not reach statistical relevance (P-value= 0.3646). Moreover we documented 3 different trends (Figure 2. Respectively A, B, C) of correlation between CBF and brain temperature, with some patients having CBF increase with BrT increase, others having CBF increase with BrT decrease and others showed no relation between BrT and CBF.