ACID-BASE-BALANCE AND BRAIN

In physiological conditions, the regulation of acid-base balance in br ain maintains a noteworthy stability of cerebral pH. During systemic m etabolic acid-base imbalances cerebral pH is well controlled as the bl ood/brain barrier is slowly and poorly permeable to electrolytes (HCO3 - and H+). Cerebral pH is regulated by a modulation of the respiratory drive, triggered by the early alterations of interstitial fluid pH, d ose to medullary chemoreceptors. As blood/brain barrier is highly perm eable to CO2, CSF pH is coffected in a few hours, even in case of seve re metabolic acidosis and alkalosis. Conversely, during ventilatory ac idosis and alkalosis the cerebral pH varies in the same direction and in the same range than blood pH. Therefore, the brain is better protec ted against metabolic than ventilatory arid-base imbalances. Ventilato ry acidosis and alkalosis are able to,impair cerebral blood flow and b rain activity through interstitial pH alterations. During respiratory acidosis, [HCO3-] increases in extracellular fluids to control cerebra l pH by two main ways : a carbonic anhydrase activation at the blood/b rain and blood/CSF barriers level and an increase in chloride shift in glial cells (HCO3- exchanged for Cl-). During respiratory alkalosis, [HCO3-] decreases in extracellular fluids by the opposite changes in H CO3- transport and by an increase in lactic acid synthesis by cerebral cells. The treatment of metabolic acidosis with bicarbonates may indu ce a cerebral acidosis and worsen a cerebral oedema during ketoacidosi s. Moderate hypocapnia carried out to treat intracranial hypertension is mainly effective when cerebral blood flow is high and vascular Co2 reactivity maintained. Hypocapnia may restore an altered cerebral bloo d flow autoregulation. Instrumental hypocapnia requires a control of c erebral perfusion pressure and cerebral arteriovenous difference for o xygen, to select. patients for whom this kind of treatment may be of b enefit, to choose the optimal level of hypocapnia and to avoid any del eterious effect. If hypocapnia is maintained. over several days, an ad aptation of CSF pH may limit the therapeutic effect on the cerebral bl ood flow and the intracranial pressure.