American Society of Anesthesiologists, 2006 Annual Meeting
A852
October 16, 2006
9:00 AM - 11:00 AM
Room Hall E, Area C

Hypercapnic Hyperventilation Shortens Emergence from Isoflurane, Sevoflurane & Desflurane Anesthesia
Nishant A. Gopalakrishnan, Ph.D., Derek J. Sakata, M.D., Joseph A. Orr, Ph.D., Dwayne R. Westenskow, Ph.D.

Introduction: Emergence time from inhaled anesthetic agents could be reduced by maintaining a high PaCO₂ to accelerate the clearance of anesthetic from the brain and hyperventilation to lower the arterial concentration of the volatile agent. The objective of this study is to evaluate BIS as an indicator of anesthetic depth during emergence from isoflurane, sevoflurane and desflurane anesthesia when hypercapnia (etCO₂ = 55mmHg) is maintained during hyperventilation and mild hypocapnia (etCO₂ = 29 mmHg) is maintained during emergence using mild hyperventilation.

Methods: After IRB approval, we measured BIS rise times during emergence with hypercapnic hyperventilation or mild hypocapnic hyperventilation in 52 ASA Class I or II patients after elective surgery. Patients were randomly assigned to the hypercapnic or mild hypocapnic groups. Anesthesia was maintained using 1 MAC of agent and 0.05-0.15 mcg/kg/min of remifentanil in the patients that received isoflurane (n=20). In the group that received sevoflurane (n=16) or desflurane (n=16), anesthesia was maintained with 1 MAC of agent supplemented with the anesthetists choice of opioids and remifentanil infusion. Opioid administration was discontinued 1 hour before the anticipated emergence and remifentanil was discontinued 10 minutes prior to turning off the vaporizer.

Emergence was initiated when adhesive wound closure strips were applied. Events were recorded from the time that the vaporizer was turned off. Minute ventilation was increased in half the subjects by doubling respiratory rate from 8 to 16 breaths per minute and hypercapnia was maintained by adding dead space to the airway and using activated charcoal to remove anesthetic from the rebreathed gas. Fresh gas flow was turned up to 10 L/min during emergence. BIS was normalized using the pre-emergence BIS (average BIS 2 minutes prior to emergence) and the maximum BIS observed during emergence.

Normalized BIS =(BIS -pre-emergence BIS)/(maximum BIS -pre-emergence BIS)

The time for the normalized BIS to rise to 0.95 was compared to the time to extubation.

Results: The time for normalized BIS to rise to 0.95 was 6.5±2.4, 6.5±1.3 and 3.1±0.9 minutes during emergence from isoflurane, sevoflurane and desflurane anesthesia respectively when rebreathing was used. When rebreathing was not used the time for the normalized BIS to rise to 0.95 was 15.4±5.6, 12.6±2.2 and 6.9±2.7 minutes with isoflurane, sevoflurane and desflurane. For isoflurane, sevoflurane and desflurane, the times to extubation were 7.2±2.1, 6.1±1.4, 3.4 ±1.1 minutes respectively with rebreathing and 17.7±4.7, 12.6±1.8 and 8.5±1.5 minutes without rebreathing. BIS rise times were not statistically different from the time to extubation (p<0.05).

Discussion: BIS rose faster and more predictably when hypercapnic hyperventilation was maintained during emergence from isoflurane, sevoflurane and desflurane anesthesia. The results could possibly be explained by the faster cerebral clearance of anesthetic due to increased cerebral perfusion caused by hypercapnia.