Positive end-expiratory pressure (PEEP) and cerebrospinal fluid pressure during normal and elevated intracranial pressure in dogs.

The effect of up to 15 cm H2O positive end-expiratory pressure (PEEP) on cerebrospinal fluid pressure (Pcsf) was investigated in five anaesthetised, mechanically ventilated dogs during normal and then elevated (40-50 cm H2O) intracranial pressure (ICP). Stepwise elevations of PEEP in 5 cm H2O increments resulted in small rises in Pcsf at normal ICP and in significantly larger rises when ICP was elevated. The regression equations for the relationships between Pcsf and end-expiratory pressure (EEP) were as follows: Pcsf = 12.95 + 0.82 EEP for normal ICP, and Pcsf = 46.41 + 2.06 EEP for elevated ICP. Mean PaCO2 rose from 39.7 +/- 2.5 to 47.6 +/- 5.0 torr during normal ICP, and from 34.2 +/- 2.9 to 50.9 +/0- 5.3 torr at elevated ICP as PEEP was elevated to 15 cm H2O. We conclude that PEEP raised Pcsf, and that this increase is more severe under conditions of elevated ICP. The rise in Pcsf due to PEEP may be explained by either the rise in intrathoracic pressure or the rise in PaCO2, or both.