Journal: Anesth Analg 106(2):585-594, 2008
Reprint: Dept of Anesthesia and Perioperative Care, University of California San Francisco, 521 Parnassus Avenue, C 450, San Francisco, CA 94143-0648
Faculty Disclosure: Abstracted by R. Klotz, who has nothing to disclose.
Although recent guidelines discourage the use of long-term hyperventilation in severe head injury, hyper¬ventilation of some degree is still commonly provided to facilitate intracranial surgery. This is due to the perceived advantage of brain relaxation and a lack of apparent serious deleterious effects associated with mild-to-moderate hyperventilation. It is generally believed that cerebral blood volume and intracranial pressure (ICP) are decreased when arterial carbon dioxide tension (PaCO2) is decreased during deliberate hyperventilation. These decreases should decrease brain bulk and perhaps lessen the need for a potentially harmful retraction of the brain.
Studies have confirmed the effectiveness of hyper¬ventilation in reducing increased ICP. However, most of the data were derived from head-injured patients in the ICU setting, and therefore may not be applicable to the elective intraoperative setting. These authors con¬ducted a randomized, crossover trial to evaluate the efficacy of moderate hyperventilation in patients undergoing craniotomy for excision of supratentorial brain tumors during isoflurane or propofol anesthesia. Their null hypothesis was that neither ICP nor surgeon-assessed brain bulk was altered by deliberate hyperventilation and that the effect was independent of the anesthetic used.
Patients were randomly assigned to one of two treat¬ment sequences: hyperventilation (PaCO2 = 25 ± 2 mmHg) followed by normoventilation (PaCO2 = 37±2 mmHg) or vice versa. Patients were also randomly assigned to receive either isoflurane or propofol infusion. Anesthesia was induced with fentanyl and propofol. Vecuronium or rocuronium was adminis¬tered to facilitate tracheal intubation. Anesthesia was maintained with either isoflurane or propofol infusion per group assignment. The dose was adjusted accord¬ing to clinical judgment but with the intent that at bone flap removal, the propofol infusion would be at a rate of 100-120 mcg/kg/min or the isoflurane at an end-tidal concentration of 0.9-1.1%. The lungs were mechanically ventilated with an air/oxygen mixture at an inspiratory oxygen concentration of 0.4%.
After removal of the bone flap and exposure of dura, a 20-ga plastic cannula was inserted into the subdural space along the surface of the brain and connected to a pressure transducer. Subdural ICP was continuously recorded. An arterial blood gas sample was obtained for blood gas analysis. PaCO2 was measured and was used to determine the difference between arterial and end-tidal carbon dioxide tension (Pa-ETCO2). Ventila¬tion and PETCO2 were kept constant for at least 20 minutes. At the end of the equilibration period the first assessment was performed. Another arterial blood gas was obtained to confirm that the targeted PaCO2 was achieved. The mean subdural ICP was recorded. The neurosurgeon was asked to score the brain bulk on a 4-point scale. After this assessment, ventilation changed according to group assignment for the second assess¬ment. After another 20-min equilibration period the measurements were repeated. This was the end-point of the study. A total of 265 patients completed the study. Of these, 134 received hyperventilation first followed by normoventilation, and 131 received normoventilation followed by hyperventilation. Brain bulk assessment was completed in all patients.
Based on crossover analysis, hyperventilation signifi¬cantly decreased the number of patients with brain swelling, including those with grade 3 and brain bulk assessments, compared with brain bulk assessment during normoventilation. The absolute reduction in the incidence of brain swelling was 13.9%. There was no carryover effect and the order of treatment did not affect the results. The type of anesthetic had no mea¬surable effect on brain bulk assessment. Hyperventi¬lation decreased subdural ICP an average of 3.7 mmHg, with crossover analysis confirming an absence of period effect. The adjusted difference of ICP between hyperventilation and normoventilation was 5 mmHg. There was no difference in ICP between patients receiving propofol and isoflurane. This study demonstrates that hyperventilation decreased ICP by 24% (5 mmHg) and decreased the risk of brain swell¬ing by 14% compared with normoventilation. The choice of anesthetic had no measurable effect on surgeon-assessed operating conditions or ICP. There¬fore, this study supports the use of hyperventilation as part of the anesthetic technique to improve operating conditions in patients with brain tumors. |
