Cerebral blood flow
- Cerebral blood flow increases with an increase in PCO2 or an increase in plasma [H+] concentration. Cerebral blood flow can increase two-fold with a 70% rise in pCO2. Hydrogen ions appear to vasodilate cerebral vessels.
- This makes sense to clear unwanted CO2 and H+ from the cerebral circulation which can significantly affect neuronal function. Studies looking at local flow have shown that changes of 100 to 150% of flow can occur within seconds of increased neuronal activity for instance in the contralateral primary motor cortex on asking a subject to clench a fist.
- It has also been seen that sudden rises in systemic BP are compensated for by increased sympathetic tone constricting the large and medium-sized arteries to protect the distal circulation.
- Hypocapnia constricts normal vessels and reduces cerebral blood flow and intracranial pressure. It is used therapeutically in the ITU. Patients are intubation and hyperventilation to maintain a paCO2 of 35-40 mmHg.