July 2005 Anesthesiology News

Via Rebreathing/Absorption of Inhaled Anesthetics ...
Device Appears to Quicken Patients' Emergence From General Anesthesia

Miami--A rebreathing absorber designed to facilitate emergence from anesthesia induced with volatile anesthetics has been tested at the University of Utah. The device features a charcoal filter that captures and retains the anesthetic agent so that it is not rebreathed.

Computer model studies conducted to evaluate the device, revealed that the model accurately reproduced observations made in clinical trials. Patients using the rebreathing absorber device woke up 10 minutes earlier from anesthesia with isoflurane than patients who did not use the device.

"In previous studies in animals and in patients, we found a 60% decrease in time to emergence with isoflurane with use of the rebreathing absorber device," stated Joseph Orr, PhD. "Because it would cost a huge amount to test the device in many different kinds of patients and with different anesthetics, we went to the literature and found a computer model of anesthetic uptake and programmed that in. We tested the computer model with and without our new device to see what the model would predict as far as how fast patients would wake up."

At the end of a surgical procedure, the anesthesiologist may increase the patient's ventilation rate in order to rapidly remove anesthetic vapor from the lungs, Dr. Orr said in an interview with Anesthesiology News. However, hyperventilation also decreases the partial pressure of CO₂ in the arterial blood, causing decreased respiratory drive and decrease blood flow to the brain.

The aim of the rebreathing absorber device is to allow the benefits of hyperventilation while reducing the negative effect of lowering the arterial CO₂. By adding dead space to the patient's breathing circuit, the device uses partial rebreathing to increase the CO₂ level, thereby allowing an increase in the minute ventilation during emergence from inhaled anesthetics while maintaining a normal or slightly increased end-tidal partial pressure of CO₂.

The rebreathing absorber device, developed by Anecare Laboratories, Salt Lake City, uses activated charcoal to capture and retain the anesthetic agent so that it is not rebreathed, explained Dr. Orr. The device features a section of expandable breathing hose and a small canister containing activated charcoal. It is placed between the endotracheal tube and the breathing circuit Y piece at the conclusion of anesthesia. Dr. Orr is Research Assistant Professor, Department of Anesthesiology, Bioengineering Division, University of Utah School of Medicine, Salt Lake City.

In one study, which Dr. Orr reported in a poster session at the 2005 annual meeting of the Society for Technology in Anesthesia (STA), data from 20 surgical patients were used to evaluate the accuracy of the computer model. In a clinical trial, patients undergoing knee surgery were anesthetized for an average of 2.5 hours with isoflurane at a minimum alveolar concentration (MAC) of 1. Patients were ventilated at a rate of eight breaths per minute, and the tidal volume was adjusted to maintain the end-tidal partial pressure of CO₂ at 33 mm Hg.

At the completion of surgery, the rebreathing absorber device was used to speed emergence from anesthesia in 10 of the 20 patients. The device was inserted into the breathing circuit when the vaporizer was turned off, the respiratory rate was doubled, and the tidal volume was increased by about 200 ml as a result of the increase in fresh gas flow. Every 20 seconds, the anesthesiologist verbally prompted the patient to wake up. For the 10 patients who did not use the rebreathing device, the fresh gas flow was increased to 10 L/min, but the ventilation remained unchanged.

Presenting the study findings, Dr. Orr said that "in the clinical trial, the measured time to extubation was 17.7 minutes for the non-rebreathing patients and 7.2 minutes when the rebreathing device was used. [These results] compared well to the simulated emergence times of 17.7 minutes when the device was not used and 8.2 minutes with the device in place. The average difference between the actual and simulated emergence times was less than one minute for both groups."

Measured time was defined as the time between turning off the isoflurane vaporizer and extubation, he explained. Simulated emergence time was the time between turning off the simulated vaporizer and the point at which the simulated concentration of anesthetic in the brain fell below 0.4 MAC.

In a second study, also reported at the STA meeting, the investigators used the computer model to evaluate the ability of the rebreathing absorber device to speed patients' emergence from anesthesia with volatile compounds. The researchers simulated nine combinations of patient size and duration of anesthesia with each of four anesthetic gases. All subjects of the simulation studies were 30-year-old men.

The simulated patients received 1 MAC of an anesthetic for a duration of 30 minutes, two hours or eight hours. Patients were ventilated at a rate of 10 breaths per minute. The simulated tidal volume was set to maintain the end-tidal partial pressure of CO₂ at 33 mm Hg. The rate of fresh gas flow was set at 3 L/min and was increased to 10 L/min for both the non-rebreathing and rebreathing emergence tests. In the non-rebreathing emergence simulations, the respiratory rate and tidal volume remained constant. In the rebreathing simulations, the respiratory rate was doubled and the tidal volume was increased by 200 ml during emergence.

The average percentage decrease in emergence time was 59% for halothane, 54% for isoflurane, 47% for desflurane (Suprane, Baxter), and 45% for sevoflurane (Ultane, Abbott), Dr. Orr said. Overall, the greatest improvement in emergence time was seen with the more soluble anesthetics, isoflurane and halothane. The smallest percentage reduction in time (36%) was in a simulated small patient who received desflurane anesthesia for 30 minutes.

"The model simulates the metabolic production of CO₂ as a constant value that is determined according to patient weight," Dr. Orr added. "The model does not account for interpatient variability or variation in metabolic rate or cardiac output caused by surgical stimulation, pain or catecholamine release."

The investigators anticipate that the device will become commercially available in the near future.

Co-authors of the study were Derek Sakata, MD, and Dwayne Westenskow, PhD.

James H. Philip, MD, in an interview with Anesthesiology News, commented on the new technology. "There is a need for this kind of device, especially with less expensive older drugs like isoflurane and halothane, which are limited by their ability to be removed from the body. This device causes patients to breathe these gases out more quickly because the carbon dioxide that the patient would otherwise have breathed out is being brought back. The charcoal absorber eliminates the anesthetic gas from the gases that the patient breathes in."

The device can remove anesthetic gases two or three times more effectively than ordinary breathing because one can increase the patient's minute ventilation twofold or threefold. How commercially viable the device becomes will depend on its cost compared with the cost of the newer drugs, such as sevoflurane and desflurane, Dr. Philip added. He is Associate Professor, Harvard Medical School, and Director of Technology Assessment, Brigham and Women's Hospital, Boston.

--Linda Pembrook

Based on poster presentations at the 2005 annual meeting of the Society for Technology in Anesthesia and interviews with Joseph Orr, PhD, and James H. Philip, MD.