It is important to consider the influence on two other physiological mechanisms, autoregulation and CO 2 -CBF relationship. The normal autoregulatory mechanism is gradually abolished as the concentration of the volatile agent is increased, CBF becoming blood pressure dependent. Thus as blood pressure rises, CBF increases and cerebral vasodilation occurs. Idealised curves of the effect of a progressively increased dose of a volatile anaesthetic agent on CBF autoregulation. The CO 2 -CBF relationship is also affected by the volatile agents, the curve being shifted to the left.
Hypocapnia is still able to reduce cerebral blood flow and therefore to oppose the vasodilation.
Effects of anaesthetic agents on the CBF-CO 2 response curve. The slope of the thin line indicates that CBF-CO 2 sensitivity is increased with agents which increase CBF, such as volatile agents. Following the head injury there will be some cerebral swelling because of contusion. Compensation will have taken place and thus the patient may not appear to have any significant decompensation. In addition blood pressure may fall as well, the combination dramatically reducing cerebral perfusion.