Continuous cerebral compliance monitoring in severe head injury: its relationship with intracranial pressure and cerebral perfusion pressure
Background. Cerebral compliance expresses the capability to buffer an intracranial volume increase while avoiding a rise in intracranial pressure (ICP). The autoregulatory response to Cerebral Perfusion Pressure (CPP) variation influences cerebral blood volume which is an important determinant of compliance. The direction of compliance change in relation to CPP variation is still under debate. The aim of the study was to investigate the relationship between CPP and compliance in traumatic brain injured (TBI) patients by a new method for continuous monitoring of intracranial compliance as used in neuro-intensive care (NICU). Method. Three European NICU’s standardised collection of CPP, compliance and ICP data to a joint database. Data were analyzed using an unpaired student t-test and a multi-level statistical model. Results. For each variable 108,263 minutes of data were recorded from 21 TBI patients (19 patients GCS≤8; 90% male; age 10–77 y). The average value for the following parameters were: ICP 15.1±8.9 mmHg, CPP 74.3±14 mmHg and compliance 0.68±0.3 ml/mmHg. ICP was ≥20 mmHg in 20% and CPP<60 mmHg for 10.7% of the time. Compliance was lower (0.51±0.34 ml/mmHg) at ICP≥20 than at ICP<20 mmHg (0.73±0.37 ml/mmHg) (p<0.0001). Compliance was significantly lower at CPP<60 than at CPP≥60 mmHg: 0.56±0.36 and 0.70±0.37 ml/mmHg respectively (p<0.0001). The CPP – compliance relationship was different when ICP was above 20 mmHg compared with below 20 mmHg. At ICP<20 mmHg compliance rose as CPP rose. At ICP≥20 mmHg, the relation curve was convexly shaped. At low CPP, the compliance was between 0.20 and 0.30 ml/mmHg. As the CPP reach 80 mmHg average compliance was 0.55 ml/mmHg., but compliance fell to 0.40 ml/mmHg when CPP was 100 mmHg. Conclusions. Low CPP levels are confirmed to be detrimental for intracranial compliance. Moreover, when ICP was pathological, indicating unstable intracranial equilibrium, a high CPP level was also associated with a low volume-buffering capacity.