The effect of hyperventilation versus normoventilation on cerebral oxygenation using near infrared spectroscopy in children undergoing posterior fossa tumor resection: A randomized controlled cross-over trial
This study examined the effects of hyperventilation versus normoventilation on cerebral oxygenation in children undergoing posterior fossa surgery.
Fifty children were enrolled and randomised into two groups; an early hyperventilation group (ETCO2 target: 26-30 mmHg) and an early normoventilation group (ETCO2 target: 31-35 mmHg). The study was a cross-over trial, so after 30 minutes in the prescribed ventilation strategy (phase 1) the patient was then switched to the alternative ventilation strategy for a further 30 minutes (phase 2). Near infrared spectroscopy (NIRS) was used to assess cerebral oxygen saturation with measurements taken at baseline and then every 5 minutes until then end of phase 2. Other measurements collected were: three intracranial pressure (ICP) measurements and a neurosurgical score of brain relaxation at the end of phase 1.
During the study period blood pressure (BP) was kept within 20% of baseline. If BP dropped below this a 10-20 ml/kg fluid bolus was administered. Any patients requiring vasopressors during the study were excluded.
The results demonstrated that cerebral oxygen saturation was significantly lower in the hyperventilation phase compared to the corresponding normoventilation phase. Linear regression showed that for every 1 mmHg decrease in ETCO2 there was a 1.4% reduction in NIRS values. There was no statistically significant difference in brain relaxation score or ICP measurements between the groups.
This study helps to clarify the utility of different ventilation strategies during neuro anaesthesia. Hyperventilation has historically been used as a strategy to emergently reduce ICP through its effect on cerebral vasoconstriction and decreased cerebral blood flow. This study demonstrates the reduction on cerebral oxygenation associated with hyperventilation but without significant beneficial effects on brain relaxation or ICP. The results are in keeping with other studies showing a variable reduction in NIRS with hyperventilation in adult and adolescent populations undergoing non neurosurgical procedures.
A limitation of this study is that patients who had significant mass effect or oedema on their preoperative MRI were excluded, suggesting the patients who arguably would most likely benefit from hyperventilation may not be represented within this cohort. Another limitation is that there was no study into the post operative outcomes of patients undergoing hyperventilation and a reduction in cerebral oxygenation. Other studies in cardiac and non-cardiac surgery have, however, demonstrated long term cognitive delay and post operative behavioural changes associated with drops in cerebral oxygenation suggesting it is not a benign finding.
Reviewed by Dr Rosalyn Boyd