Brain glucose and lactate levels during ventilator-induced hypo- and hypercapnia

Objective: Levels of glucose and lactate were measured in the brain by means of microdialysis in order to evaluate the effects of ventilator-induced hypocapnia and hypercapnia on brain metabolism in healthy non-brain-traumatized animals. Design and setting: Prospective animal study in a university laboratory. Subjects: Eight adult Landrace/Yorkshire pigs. Interventions: The microdialysis probe was inserted in the brain along with a multiparameter sensor and intracranial pressure (ICP) probe. The animals were ventilated in a pressure-controlled mode according to the open lung concept with an inspired oxygen fraction of 0.4/1.0. Starting at normoventilation (PaCO₂ ± 40 mmHg) two steps of both hypercapnia (PCO₂ ± 70 and 100 mmHg) and hypocapnia (PaCO₂ ± 20 and 30 mmHg) were performed. Under these conditions, brain glucose and lactate levels as well as brain oxygen (PbrO₂), brain carbon dioxide (PbrCO₂), brain pH (brpH), brain temperature and ICP were measured. Results: At hypercapnia (PaCO₂ = 102.7 mmHg) there were no significant changes in brain glucose and lactate but there was a significant increase in PbrCO₂, PbrO₂ and ICP. In contrast, at hypocapnia (PCO₂ = 19.8 mmHg) there was a significant increase in brain lactate and a significant decrease in both brain glucose and PbrCO₂. Conclusions: Hypocapnia decreases brain glucose and increases brain lactate concentration, indicating anaerobic metabolism, whereas hypercapnia has no influence on levels of brain glucose and brain lactate.