The lower limit of cerebral blood flow autoregulation is increased with elevated intracranial pressure

BACKGROUND:: The cerebral perfusion pressure that denotes the lower limit of cerebral blood flow autoregulation (LLA) is generally considered to be equivalent for reductions in arterial blood pressure (ABP) or increases in intracranial pressure (ICP). However, the effect of decreasing ABP at different levels of ICP has not been well studied. Our objective in the present study was to determine if the LLA during arterial hypotension was invariant with ICP. METHODS:: Using continuous ventricular fluid infusion, anesthetized piglets were assigned to 1 of 3 groups: naïve ICP (n = 10), moderately elevated ICP (20 mm Hg; n = 11), or severely elevated ICP (40 mm Hg; n = 9). Gradual hypotension was induced by inflation of a balloon catheter in the inferior vena cava. The LLA was determined by monitoring cortical laser-Doppler flux. RESULTS:: The naïve ICP group had an average CPP at the LLA (LLACPP) of 29.8 mm Hg (95% CI: 26.5-33.0 mm Hg). However, the moderately elevated ICP group had a mean LLACPP of 37.6 mm Hg (95% CI: 32.0-43.2 mm Hg), and the severely elevated ICP group had a mean LLACPP of 51.4 mm Hg (95% CI: 41.2-61.7 mm Hg). The LLA significantly differed among groups, and the increase in LLA correlated with the increase in ICP. CONCLUSIONS:: In this atraumatic, elevated ICP model in piglets, the LLA had a positive correlation with ICP, which suggests that compensating for an acute increase in ICP with an equal increase in ABP may not be sufficient to prevent cerebral ischemia.