Effects of hypotensive treatment with α2-agonist and β1-antagonist on cerebral haemodynamics in severely head injured patients

Therapy of post-traumatic brain oedema often includes preservation of high arterial blood pressure to avoid secondary ischaemic injuries to the brain. This practice can be questioned since high arterial blood pressure may aggravate brain oedema through raised hydrostatic capillary pressure, causing fluid filtration across the damaged blood-brain barrier. This latter view is in agreement with our clinical experience and therefore hypotensive therapy with an α₂-adrenergic agonist (clonidine) and a β₁-adrenergic antagonist (metoprolol) has become part of our treatment protocol for severely head injured patients to decrease the post-traumatic brain oedema. The present study is an attempt to analyse whether there are any direct local cerebrovascular effects of the hypotensive agents used, which also might influence intracranial pressure. Severely head injured patients were investigated. Heart rate, mean arterial blood pressure, intracranial pressure, cerebral blood flow and arterio-venous difference in oxygen content were measured before and after a bolus dose of clonidine (six patients) and metoprolol (nine patients).
Clonidine decreased mean arterial blood pressure and cerebrovascular resistance without affecting other parameters measured. Metoprolol decreased heart rate and mean arterial pressure, but had no effect on the cerebrovascular parameters.
The results show that clonidine and metoprolol have no, or only minor, direct influence on local cerebral haemodynamics in severely brain injured patients. This implies that if there is an intracranial pressure reducing effect of these drugs, as suggested, this must be due to other mechanisms, namely a reduction in capillary hydrostatic pressure secondary to decreased arterial blood pressure and heart rate.