Is cerebral perfusion pressure a major determinant of cerebral blood flow during head elevation in comatose patients with severe intracranial lesions?

Object. Head elevation as a treatment for lower intracranial pressure (ICP) in patients with intracranial hypertension has been challenged in recent years. Therefore, the authors studied the effect of head position on cerebral hemodynamics in patients with severe head injury. Methods. The effect of 0 degrees, 15 degrees, 30 degrees, and 45 degrees head elevation on ICP, cerebral blood flow (CBF), systemic arterial (Psa(Monro)) and jugular bulb (Pj) pressures calibrated to the level of the foramen of Monro, cerebral perfusion pressure (CPP), and the arteriovenous pressure gradient (Psa(Monro) - Pj) was studied in 37 patients who were comatose due to severe intracranial lesions. The CBF decreased gradually with head elevation from 0 to 45 degrees, from 46.3 +/- 4.8 to 28.7 +/- 2.3 ml . min(-1) . 100 g(-1) (mean +/- standard error, p < 0.01), and the Psa(Monro) - Pj from 80 +/- 3 to 73 +/- 3 mm Hg (p < 0.01). The CPP remained stable between 0 degrees and 30 degrees of head elevation, at 62 +/- 3 mm Hg, and decreased from 62 +/- 3 to 57 +/- 4 mm Hg between 30 degrees and 45 degrees (p < 0.05), A simulation showed that the 38% decrease in CBF between 0 degrees and 45 degrees resulted from Psa(Monro) - Pj changes for 19% of the decrease, from a diversion of the venous drainage from the internal jugular veins to vertebral venous plexus for 15%, and from CPP changes for 4%. Conclusions. During head elevation the arteriovenous pressure gradient is the major determinant of CBF. The influence of CPP on CBF decreases from 0 to 45 degrees of head elevation.