Abstract
Plateau waves are often observed in patients with “normal pressure hydrocephalus” during continuous monitoring of intracranial pressure (ICP). The authors attempted to determine whether or not hypoventilation is responsible for the plateau waves in such patients. Sixteen adult mongrel dogs were immobilized and mechanically ventilated. Hypoventilation was induced for 10 minutes by means of a 50% decrease in both tidal volume and respiratory rate, after which normal ventilation was resumed. Arterial blood gas, heart rate, mean systemic arterial pressure, mean ICP, and cerebral perfusion pressure (CPP) were monitored throughout the study. The dogs were divided into three groups: Group C (controls; n=6); Group K (dogs with kaolin-induced hydrocephalus in whom the ICP had returned to normal in the chronic stage; n=6); and Group B (animals in whom, for the purpose of comparison with Group K, a balloon was extradurally inserted and inflated and whose ICP was within the normal range; n=4) . During hypoventilation, ICP increased more and CPP in turn decreased significantly more in Groups K and B than in Group C. The increase in ICP persisted during hypoventilation, as did plateau waves. None of the parameters monitored was significantly different between Groups K and B. Thus, it appears that in these two groups ICP responded similarly to specific respiratory changes.
These results suggest that the spatial compensatory capacity in kaolin-induced hydrocephalus is lowered even if ICP is within the normal range. In such circumstances, alteration of cerebral blood volume as a result of hypoventilation can lead to elevation of ICP, which is expressed as plateau waves. It is hoped that further study will disclose whether or not hypoventilation is also a factor in the development of plateau waves in humans with normal pressure hydrocephalus.
