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Asymmetry of critical closing pressure following head injury

¹ University of Cambridge Clinical School, Addenbrooke’s Hospital, Cambridge, UK
² Academic Neurosurgery Unit, Addenbrooke’s Hospital, Cambridge UK

Correspondence to:
Correspondence to:
Dr M Czosnyka
Academic Neurosurgery Unit, Box 167, Addenbrooke’s Hospital, Hills Road, Cambridge, CB2 2QQ, UK; mc141{at}medschl.cam.ac.uk

Objective: Critical closing pressure (CCP) is the arterial pressure below which the vessels collapse. Hypothetically it is the sum of intracranial pressure (ICP) and vessel wall tension in the cerebral circulation. This study investigated transhemispherical asymmetry of CCP by studying its correlation with radiological findings on computed tomography (CT) scans in head injury patients.

Method: ICP, arterial blood pressure, and middle cerebral artery blood flow velocity were recorded daily in 119 ventilated patients. Waveforms were processed to calculate CCP. CT scans were analysed according to a system based on the Marshall classification.

Results: Left–right differences in CCP correlated with midline shift on the CT scan (r = 0.48; p<0.02). 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<0.006). 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. Cerebral perfusion pressure calculated as the difference between mean arterial pressure and CCP did not correlate with outcome, but "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 ICP.

Abbreviations: ABP, arterial blood pressure; CCP, critical closing pressure; ICP, intracranial pressure; FV, flow velocity

Keywords: cerebrovascular circulation; computed tomography; head injury; transcranial Doppler