Objective: To evaluate in healthy, non-brain-traumatized animals the effects of hypo- and hyperventilation on intracranial pressure (ICP) and brain carbon dioxide, oxygen, and pH during the use of a ventilatory mode at constant mean airway pressure (MAwP).
Design and setting: Prospective animal study in a university laboratory.
Subjects: Eight crossbred Landrace/Yorkshire pigs.
Interventions: The animals were ventilated in a pressure-controlled mode according to the open lung concept with an inspired oxygen fraction of 1.0. Starting at normoventilation, a stepwise hypo- and hyperventilation was performed to PaCO2 values of 90.4+/-10.4 and 26.9+/-4.1 mmHg, respectively. The ICP and brain parenchyma values [carbon dioxide (PbrCO2), oxygen (PbrO2), and pH (brpH)] measured by multiparameter sensors were recorded continuously during these maneuvres.
Results: During hypoventilation there was a significant increase in PbrCO2 tension, PbrO2 tension, and ICP. During hyperventilation there was a significant decrease in PbrCO2 tension and ICP while the change in PbrO2 was not significant. MAwP was kept stable during the stepwise hypo- and hyperventilation, and this resulted in a constant mean arterial pressure.
Conclusions: Controlled hypo- and hyperventilation at constant MAwP in non-brain-traumatized pigs appears to induce changes in ICP and cerebral perfusion pressure which, however, do not necessarily lead to cerebral ischemia. To achieve adequate cerebral perfusion at an increased ICP level due to hypoventilation one must maintain sufficient arterial blood pressure. Hypercapnia resulted in a significant increase in brain oxygenation; however, this does not necessarily mean that permissive hypercapnia is neuroprotective.