Acid-base changes in blood and cerebrospinal fluid during altered ventilation

¹ Department of Medicine, Duke University Medical Center, Durham, North Carolina

Functional abnormalities of the central nervous system are observed with hypo- and hyperventilation. This study correlates changes of pH, carbon dioxide tension and carbon dioxide content in arterial and cerebral venous blood and cerebrospinal fluid during altered ventilation. With the experimental design in which ventilation was controlled and the sagittal sinus, femoral artery, and cisterna magna were cannulated, a slight metabolic acidosis was found. With 10% CO₂ inhalation acidosis occurred in both blood and spinal fluid and early in the period of inhalation, the usual cerebrospinal-arterial fluid pCO₂ gradient was reversed. With hyperventilation, pH and pCO₂ changes were more pronounced in the arterial blood but, as hyperventilation was continued, the arterial-cerebrospinal fluid difference decreased. It appeared likely that brain tissue acts as an important buffer, absorbing and releasing CO₂ during states of altered ventilation. CO₂ diffuses rapidly across cell boundaries, whereas bicarbonate crosses more slowly, thus providing an explanation for the differences noted between blood and cerebrospinal fluid. The particular clinical importance of these observations is that arterial pH, pCO₂, and CO₂ content may not accurately reflect changes within the cerebrospinal fluid or brain when ventilation is altered.