Objective
We previously demonstrated that in injured brain the level of asymmetry of CBF autoregulation between injured and uninjured hemispheres correlates with intracranial pressure, size of contusion, midline shift and larger asymmetry associates with worse outcome 1 . We extended our investigation on other indices describing cerebral haemodynamics, aiming Transcranial Doppler-derived Critical Closing Pressure (CCP). CCP is the arterial pressure below which the vessels theoretically collapse. Hypothetically it is the sum of intracranial pressure and vessels' wall tension in the cerebral circulation. Therefore, CCP has been postulated to be used as an ‘estimator’ of intracranial pressure. Other scientists also hypothesized that the measure of brain cerebral perfusion pressure would be more appropriate if it is expressed as a difference between arterial blood pressure and CCP rather than the difference between arterial and intracranial pressure 2 . We attempted to investigate these questions in clinical practice, studying trans-hemispherical asymmetry of CCP and its correlation with radiological findings on CT scans in head injury patients and comparing absolute values of CCP with various, directly monitored brain variables.
Method
Intracranial pressure (ICP), arterial blood pressure and middle cerebral artery blood flow velocity were recorded daily in 119 ventilated patients after head trauma. Waveforms were processed to calculate CCP. CT scans were analysed according to a system based on the Marshall classification. Outcome was assessed according to Glasgow Outcome Score at 6 time after injury.
Results
Left-right differences in CCP correlated with midline shift on the CT scan (r=0. 48; p<0.02) showing lower CCP at the side of brain expansion. Asymmetry of CCP also corresponded with the side of the head lesion (p<0.007) and the side of the craniotomy where it was performed (p<0.012). In both cases CCP was lower at the affected side of the brain than on contralateral side. Absolute CCP weakly correlated with brain swelling (r=−0.23; p<0.03) and arterial pressure (r=0.21; p<0.02) but did not correlate with ICP. Correlation between CCP and cerebrovascular resistance (calculated as cerebral perfusion pressure and blood flow velocity was excellent (R=0.5; p<0.001). Cerebral perfusion pressure calculated as the difference between mean arterial pressure and CCP did not correlate with outcome, while ‘traditional’ cerebral perfusion pressure (mean arterial pressure minus intracranial pressure) did.
Conclusions
Critical closing pressure is disturbed by localised brain lesions. Its asymmetry corresponds to asymmetrical findings on CT scans. CCP seems to describe vascular resistance better than to approximate intracranial pressure. Our data suggest that on side of brain contusion, brain expansion leading to the midline shift, or craniotomy vessels are more dilated than on contra lateral side, rendering cerebrovascular resistance to decrease.