Effect of hyperventilation, mannitol, and ventriculostomy drainage on cerebral blood flow after head injury

Therapies to lower intracranial pressure (ICP) after traumatic brain injury (TBI) include hyperventilation (HV), intravenous mannitol (IM), and cerebrospinal fluid drainage from a ventriculostomy (DV), To determine the effects of these therapies on cerebral blood flow (CBF), fiberoptic oximetry was used to measure jugular venous O-2 saturation (SjvO2) as an index of the CBF to cerebral metabolic rate for O-2 (CMRO(2)) ratio after IM (25 g IV for more than 5 min), DV (3 min), or HV (increase respiratory rate by 4) therapy for elevated ICP, Assuming CMRO(2) is constant, changes in SjvO2 reflect changes in CBF, Continuous measurements of SjvO2, ICP, blood pressure, artery rial O-2 saturation, and end-tidal CO2 were obtained in 22 patients with a Glasgow Coma Scale score of 5.3 +/- 0.4 (mean +/- SD) in the first 5 days after TBI, Therapy was initiated a total of 196 times when ICP was > 15 mm Hg for > 5 minutes, and measurements made at 20 minutes after treatment were compared with those made just before, After DV, ICP fell in 90% of the observations by 8.6 +/- 0.7 mm Hg (mean +/- SEM, n = 119); after IM, ICP fell in 90% of the observations by 7.4 +/- 0.7 mm Hg (n = 43); and after HV, ICP fell in 88% of the observations by 6.3 +/- 1.2 mm Hg (n = 14), In patients where ICP fell, SjvO2 increased by 2.49 +/- 0.7% saturation (from 68.0 +/- 1.3%) with IM, but only by 0.39 +/- 0.4% saturation (from 67.2 +/- 0.9%) with DV, When HV resulted in a decrease in ICP, SjvO2 decreased by 7.7 +/- 1.0% saturation (from 63.4 +/- 2.5%), The magnitude of the effect was assessed by considering the change in SjvO2 resulting from a measured change in ICP (SjvO2/ICP). For each decrease in ICP of 1 mm Hg, SjvO2 increased 0.53 +/- 0.14% saturation with IM, 0.10 +/- 0.10 with DV, and decreased by 1.95 +/- 0.53 with HV, In this study DV, IM, and HV effectively reduced elevated ICP after cerebral injury, For a given change in ICP, however, LM increased the CBF to CMRO(2) ratio nearly five times as much as DV, whereas HV decreased this ratio in all cases, These data suggest that mannitol improves CBF while reducing ICP; HV, although effective at reducing ICP, results in marked lowering of CBF and should be used cautiously.