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From the Department of Anesthesiology, University of Utah, Salt Lake City, Utah.
Address correspondence and reprint requests to Derek J. Sakata, MD, Departmentof Anesthesiology, University of Utah, 30 N. 1900 East, 3C444 SOM, Salt Lake City, UT 84132. Address e-mail to Derek.Sakata{at}hsc.utah.edu.
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Abstract |
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METHODS: We evaluated the differences in emergence time in 20 surgical patients undergoing 1 MAC of isoflurane under mild hypocapnia (ETco2 approximately 28 mmHg) and mild hypercapnia (ETco2 approximately 55 mmHg). The minute ventilation in half the patients was doubled during emergence, and hypercapnia was maintained by insertion of additional airway deadspace to keep the ETco2 close to 55 mmHg during hyperventilation. A charcoal canister adsorbed the volatile anesthetic from the deadspace. Fresh gas flows were increased to 10 L/min during emergence in all patients.
RESULTS: The time between turning off the vaporizer and the time when the patients opened their eyes and mouths, the time of tracheal extubation, and the time for normalized bispectral index to increase to 0.95 were faster whenever hypercapnic hyperventilation was maintained using rebreathing and anesthetic adsorption (P < 0.001). The time to tracheal extubation was shortened by an average of 59%.
CONCLUSIONS: The emergence time after isoflurane anesthesia can be shortened significantly by using hyperventilation to rapidly clear the anesthetic from the lungs and CO2 rebreathing to induce hypercapnia during hyperventilation. The device should be considered when it is important to provide a rapid emergence, especially after surgical procedures where a high concentration of the volatile anesthetic was maintained right up to the end of the procedure, or where surgery ends abruptly and without warning.
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Introduction |
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To prevent the decrease in Paco2 during hyperventilation, some anesthesia machines were equipped with a tank containing 100% CO2. During hyperventilation, the flow of CO2 was adjusted to maintain normal or slightly increased Paco2. In 1989, 60% of the anesthesiologists in the United Kingdom routinely administered CO2 to their patients (1). The practice is very seldom used in the United States today because of the risk of inadvertent hypercapnia (2).
We have found that adding deadspace to the patient’s airway is a simpler and safer method of controlling Paco2 during hyperventilation. Added deadspace is routinely used for the noninvasive measurement of cardiac output (NICO, Respironics Inc., Murrysville, PA) and to stimulate respiratory drive in intensive care unit patients (3). In animals, we measured a 60% reduction in emergence time when deadspace was used to maintain hypercapnia during hyperventilation (4). In the current patient study, we measure emergence times with mild hypercapnia and hyperventilation.
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METHODS |
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After IRB approval, we obtained written informed consent from 20 ASA I or II patients scheduled to receive elective surgery. All patients underwent anterior cruciate ligament repair surgery. Table 1 shows patient characteristics and duration of surgery. A coin was tossed at the beginning of the study and subsequently after every six patients were studied to decide whether the patient would receive the device. The subsequent five patients that qualified for the study were alternately assigned to one of the two groups (with and without the device). Each patient was premedicated with 1–2 mg of midazolam IV and was given a femoral nerve block before surgery. Anesthesia was induced with 150 µg of remifentanil, 2.0 mg/kg of propofol, and 1.0 mg/kg of succinylcholine. We maintained anesthesia with 1.2% isoflurane (1 MAC) for the duration of the procedure. Nitrous oxide was not used. A continuous infusion of remifentanil was titrated to meet the patient’s needs beyond 1 MAC of the inhaled anesthetic.
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We set the respiratory rate at 8 breaths/min and the tidal volume was adjusted to keep the end-tidal CO2 (ETco2) concentration at 33 mmHg (Narkomed II, North American Dräger, Telford, PA). A gas analyzer (Datex AS/3, Datex-Ohmeda, Helsinki, Finland) measured the inspired and ETco2 and anesthetic concentrations continuously. We recorded the sedation level continuously using a bispectral index monitor (BIS, Aspect Medical Systems, Newton, MA). The anesthesiologist was blinded to the BIS reading.
When the surgeon applied the first adhesive wound closure strip, we turned off the vaporizer and increased the fresh gas flow to 10 L/ min. In patients in whom the device was used, we inserted the device in the airway with the rebreathing hose fully distended to 665 mL. We increased the respiratory rate to 16 breaths/min and increased the tidal volume as needed to double the minute ventilation. We adjusted the length of the rebreathing hose to prevent the ETco2 concentration from increasing higher than 55 mmHg. In the patients in whom the device was not used, we left the tidal volume unchanged. Tracheal extubation occurred after a positive response to command to open eyes and command to open mouth.
We recorded the time from when the vaporizer was turned off until the patients opened their eyes in response to command, until the patients opened their mouths in response to command, until the normalized BIS increased to 0.95, and until tracheal extubation. The BIS data were normalized using the equation:
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where preemergence BIS is the average of all BIS recording over the min before turning off the vaporizer and maximum BIS is the highest BIS number recorded after the vaporizer was turned off.
Analysis was performed using SigmaStat version 2.03 (SPSS Inc). The two groups were compared using a two tailed Student’s t-test and the results were expressed as means ± sd. P values <0.05 were judged to be significant.
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RESULTS |
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DISCUSSION |
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TopAbstractIntroductionMETHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Other methods have been used to induce hypercapnia during hyperventilation. Vesely et al. (5) and Sasano et al. (6) added CO2 to the patient’s inspired gas during hyperventilation. Rebreathing seems to be a simpler method because it does not require a CO2 source and a controller. A CO2 absorber bypass valve found on some anesthesia machines can be used to produce hypercapnia during hyperventilation. However, the anesthetic gas is rebreathed along with the CO2 and emergence time may not be shortened unless a charcoal absorber is added (7).
Hyperventilation has been studied by Vesely et al. (5). They maintained the patient’s ETco2 at 47 mmHg and measured emergence times after isoflurane anesthesia with and without hyperventilation. They found that the time to tracheal extubation was 3.6 min in patients who were hyperventilated and 12.1 min in patients who were not hyperventilated. Sasano et al. (8) measured emergence times in dogs after isoflurane and nitrous oxide anesthesia and found that hyperventilation decreased the time to tracheal extubation from 17.5 to 6.6 min. Clearly, hyperventilation shortens emergence time when Paco2 is controlled.
Higher Paco2 during hyperventilation results in a shorter emergence time (9). Gopalakrishnan, and Sakata (10) found that hypercapnic pigs (ET co2 = 55 mm Hg) woke up 2.6 ± 0.9 min after desflurane anesthesia, whereas hypocapnic pigs (23 mm Hg) woke up after 5.8 ± 2.4 min., when all animals were hyperventilated (10). Because of its low solubility, emergence after desflurane is minimally affected by hyperventilation, but significantly shortened by hypercapnia and the resulting increase in cerebral bloodflow. In response to hypercapnia, cerebral arterial smooth muscle dilates, and cerebral bloodflow increases 6% per mm Hg change in Paco2 (11,12). The increase in bloodflow results in a more rapid clearance of volatile anesthetic from cerebral tissue, especially when combined with hyperventilation to decrease the arterial concentrations of the volatile anesthetic and increase the cerebral capillary/tissue gradient (9). Because hypercapnia increases bloodflow and hyperventilation increases the diffusion gradient, both are important in rapidly removing volatile anesthetic from the brain (13).
A third factor is our use of charcoal to eliminate rebreathing of anesthetic gases in the study group. In the control group, the fresh gas flow rate was increased to 10 L/min to reduce the amount of rebreathing, but it did not eliminate rebreathing. In a future study, we recommend using charcoal in both groups, so hypercapnia and hyperventilation can be studied independent of rebreathing.
Our study has several limitations. The observer who recorded the time when the patients opened their eyes and mouths was not blinded as to the presence or absence of the device. Also, the decision as to when to perform tracheal extubation was based on the anesthesiologist’s clinical judgment and the anesthesiologist was not blinded to the device. Fortunately, the anesthesiologist was blinded to the BIS readings and the time for the BIS ratio to increase to 0.95 paralleled the times to open eyes and mouths and time to tracheal extubation, which are less subjective measures of emergence (Fig. 2).
We used an older generation anesthesia machine in which the ventilator was not compensated for changes in fresh gas flow. When we increased the fresh gas flow to 10 L/min, the delivered minute ventilation increased from 7 to 12 L/min and the patients in whom the device was not used became slightly hypocapnic (ETco2 = 28 ± 2 mm Hg) at the time of tracheal extubation (Table 2). It would have been better to keep the control patients normocapnic so as not to bias the results in favor of the device.
Future studies are needed to define guidelines regarding the optimal amount of hyperventilation and hypercapnia for providing the safest and most rapid emergence. The amount of each will likely be dependent on the inhaled anesthetic used, the depth and duration of the anesthetic, and the patient’s physiologic state (13). Hyperventilation with large tidal volumes can produce barotrauma and lung injury and may decrease stroke volume or arterial blood pressure in critically ill patients (14,15). Hyperventilation with rapid respiratory rates (and short expiratory times) may result in air trapping and alveolar over-distension in patients with restrictive airway disease. Increasing the respiratory rate also shortens the inspiratory time, and the tidal volume decreases if the inspiratory flow rate is not changed. Should the tidal volume become smaller than the rebreathing deadspace, hypoxia could result from inadequate oxygen supply. Hypercapnia is associated with an increased risk of cardiac arrhythmias and is contraindicated in patients who have pulmonary hypertension and in neurosurgical patients in whom the increased cerebral bloodflow may cause excessively high intracranial pressure. Paco2 levels are sometimes kept at 65 mm Hg in ICU patients where benefits include higher tissue oxygen pressures (16,17). Future studies are needed to measure emergence times and other outcomes, including postoperative cognitive function, at CO2 levels above 55 mm Hg (18). Arterial blood gases should be analyzed in future studies to measure the alveolar to arterial CO2 gradient during hyperventilation. Future studies are needed in the pediatric population, in patients who are breathing spontaneously, and in patients receiving pressure support ventilation.
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CONCLUSIONS |
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Footnotes |
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Supported by National Institutes of Health and National Institute of General Medical SciencesGrant GM072661 and Axon Medical Inc., Salt Lake City, Utah.
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REFERENCES |
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This article has been cited by other articles:
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  R. Katznelson, L. Minkovich, Z. Friedman, L. Fedorko, W. S. Beattie, and J. A. Fisher Accelerated Recovery from Sevoflurane Anesthesia with Isocapnic Hyperpnoea Anesth. Analg., February 1, 2008; 106(2): 486 - 491. [Abstract] [Full Text] [PDF]
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D. J. Sakata, N. A. Gopalakrishnan, J. A. Orr, J. L. White, and D. R. Westenskow Rapid Recovery from Sevoflurane and Desflurane with Hypercapnia and Hyperventilation Anesth. Analg., July 1, 2007; 105(1): 79 - 82. [Abstract] [Full Text] [PDF]
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