To study brain‐sparing physiological responses in a rodent model of birth asphyxia which reproduces the asphyxia‐defining systemic hypoxia and hypercapnia.

Steady or intermittent asphyxia was induced for 15‐45 minutes in anaesthetized 6‐ and 11‐days old rats and neonatal guinea pigs using gases containing 5% or 9% O₂ plus 20% CO₂ (in N₂). Hypoxia and hypercapnia were induced with low O₂ and high CO₂ respectively. Oxygen partial pressure (PO₂) and pH were measured with microsensors within the brain and subcutaneous (“body”) tissue. Blood lactate was measured after asphyxia.

Brain and body PO₂ fell to apparent zero with little recovery during 5% O₂ asphyxia and 5% or 9% O₂ hypoxia, and increased more than twofold during 20% CO₂ hypercapnia. Unlike body PO₂, brain PO₂ recovered rapidly to control after a transient fall (rat), or was slightly higher than control (guinea pig) during 9% O₂ asphyxia. Asphyxia (5% O₂) induced a respiratory acidosis paralleled by a progressive metabolic (lact)acidosis that was much smaller within than outside the brain. Hypoxia (5% O₂) produced a brain‐confined alkalosis. Hypercapnia outlasting asphyxia suppressed pH recovery and prolonged the post‐asphyxia PO₂ overshoot. All pH changes were accompanied by consistent shifts in the blood‐brain barrier potential.

Regardless of brain maturation stage, hypercapnia can restore brain PO₂ and protect the brain against metabolic acidosis despite compromised oxygen availability during asphyxia. This effect extends to the recovery phase if normocapnia is restored slowly, and it is absent during hypoxia, demonstrating that exposure to hypoxia does not mimic asphyxia.