Society for Technology in Anesthesia, 2005 Annual Meeting Abstract:
Use of a Computer Model of Volatile Anesthetic to Estimate Emergence Time, With and Without CO₂Rebreathing

Joseph Orr, Ph.D., Nishant Gopalakrishnan B.S., Derek Sakata M.D., Dwayne Westenskow Ph.D.
University of Utah, Department of Anesthesiology

Introduction: Computer modeling allows us to test a clinical intervention over a wide range of patient types and conditions. We used a computer model of anesthetic uptake to compare emergence times from isoflurane anesthesia with and without a CO₂ rebreathing/anesthetic absorber.

Methods: We implemented a model of anesthetic uptake and distribution in Borland C++ Builder version 5.0 for windows (Borland, Scotts Valley, CA). The model is based on the work of JG Lerou and describes transport of anesthetic in the arterial and venous blood as well as the flow of volatile anesthetic agents in the breathing circuit and in the pulmonary alveoli. We modified the model to simulate use of a rebreathing/absorber device. The modification allows the simulated patient to inhale CO₂ from the previous breath that was stored in the rebreathing hose. The model also assumes that the charcoal in the device absorbs all of the volatile anesthetic so that none is inhaled when the device is activated. We also modified the model to alter cerebral blood flow according to the partial pressure of alveolar (end-tidal) CO₂ , using the relationship published by Ide et al.

Data from 20 surgical patients was used to evaluate the accuracy of the computer model. In a clinical trial, patients having knee surgery (ACL repair) were anesthetized using 1 MAC of isoflurane for an average of 2.5 hours. Patients were ventilated at 8 breaths per minute and tidal volume was adjusted to maintain endtidal CO₂ at 33 mm Hg. At the end of surgery the rebreathing absorber device was used to speed emergence in 10 of the 20 patients. For ten patients the device was insert the into the breathing circuit when the vaporizer was to turned off, the respiratory rate was double, and the tidal volume was increased by about 200 ml. The anesthesiologist verbally prompted the patient to wake up every 30 seconds. For the ten patients that did not use the rebreathing device, the, fresh gas flow was increased to 10 l/min but ventilation remained unchanged.

We used the recorded details of this testing to evaluate the accuracy of the computer simulation. Patient details including age, sex, height and weight were entered into the model. Duration of the anesthetic was also entered and the simulated tidal volume was adjusted to achieve a maintenance end-tidal CO₂ of 33 as in the clinical trial. The emergence protocol was simulated for each of the patients.

Results: The graph shows the measured and modeled emergence times for the rebreathing and non-rebreathing patient groups. Measured times are the times between turning off the isoflurane vaporizer and extubation.

Simulated emergence times are the time between turning off the simulated vaporizer and when the simulated concentration of anesthetic in the brain fell to below 0.4 MAC. The measured time to extubation was 17.7 ± 4.7 minutes for the non-rebreathing patients versus the simulated time of 17.7 ± 3.4 minutes. The measured emergence time for the rebreathing patients was 7.2 ± 2.1 minutes compared to 8.2 ± 1.6 minutes for the simulation. The average difference in actual and simulated emergence time was less than 1 minute for both groups

References

JG Lerou, RD Dirksen, HH Beneken Kolmer, LHDJ Booij: A system model for closed-circuit inhalation anesthesia, computer study; Anesthesiology 75:345-355, 1991

K Ide, M Eliasziw, MJ Poulin: Relationship between middle cerebral artery blood velocity and end-tidal PCO₂ in the hypocapnichypercapnic range in humans; J Appl Physiol 95:129-137, 2003