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Departments of *Anesthesia and Perioperative Care, and Nursing, University of California, San Francisco, California
Address correspondence and reprint requests to Dr. Eshima Mckay, Department of Anesthesia, C-450, University of California, San Francisco, CA 94143-0648. Address e-mail to eshimar{at}anesthesia.ucsf.edu.
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Abstract |
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Introduction |
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We tested this hypothesis in the present study, using the patient’s ability to swallow 20 mL of water without coughing or drooling to indicate recovery of protective airway reflexes. We performed our tests at predetermined time intervals after the first appropriate response to command, independent of the time required to awaken and follow commands after anesthesia. In addition, we took the opportunity to make comparative measurements of clinically useful variables such as the rapidity of awakening.
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Methods |
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Informed consent was obtained from each patient on the day of surgery. Before anesthesia, each patient was given 20 mL of water to swallow approximately 30 min before induction of anesthesia; all could swallow this volume without coughing or drooling. The attending anesthesiologist determined premedication (midazolam), propofol, and fentanyl doses for induction of anesthesia. Once the laryngeal mask airway was positioned and spontaneous ventilation resumed, desflurane or sevoflurane (assigned randomly) was administered in a target background of approximately 50% nitrous oxide at a maintenance total gas flow of 1 L/min. Desflurane and sevoflurane concentrations to be used were at the discretion of the attending anesthesiologist, usually between 0.3–1.0 minimum alveolar anesthetic concentration (MAC), as revealed by end-tidal measurements of respired gases. MAC values were assumed to be 6% desflurane and 1.85% sevoflurane (6). Additional propofol (boluses) and/or opioid (usually fentanyl) were prescribed as determined by the attending anesthesiologist. Although the usual opioid used was fentanyl, one patient received morphine (desflurane group) and one received meperidine (sevoflurane group). We assumed that 10 mg of morphine and 75 mg of meperidine were equivalent to 100 µg of fentanyl (7). Some patients (n = 25) also received regional blockade (10 lumbar plexus, 4 fascia iliaca, 4 femoral/sciatic, 5 axillary, and 2 ankle blocks). All patients received 12.5 mg of dolasetron and 4 mg of dexamethasone after induction of anesthesia.
A blinded observer recorded the patient’s vital statistics (age/weight/height), significant diseases, smoking history, and duration of anesthesia. The observer was blinded to the drug the patient received (the vaporizers were covered by a towel and were visible only by the attending anesthesiologist). The blinded observer determined the time to first appropriate response to command, asking the patient to "open his/her eyes" every 30 s after discontinuation of anesthetic administration. Exactly 2 min after the patient responded appropriately to command, he or she was asked to swallow 20 mL of water. Successful swallowing was defined as ingestion of the 20 mL without coughing or drooling. If swallowing was successful, this ended the patient’s participation in the study. If it was unsuccessful, the patient was asked to swallow at 6, 14, 22, and 30 min, with termination of participation after successful swallowing.
For ordinal data, unpaired two-tailed Student’s t-tests were applied, and significance was accepted at P < 0.05 without correcting for multiple comparisons. 
2 analysis was applied as indicated.
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Results |
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The demographics, including the types of surgery, did not differ between the groups, except that fewer patients in the desflurane group received regional block, and more patients in the sevoflurane group underwent orthopedic surgery (Table 1).
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After discontinuation of anesthetic administration, patients given desflurane responded to command in 3.4 ± 1.9 (mean ± sd) min, whereas those given sevoflurane took significantly longer (5.3 ± 3.1 min, P < 0.01). All 31 patients given desflurane drank the 20 mL of water without coughing or drooling 2 min after responding to command; 16 of 33 patients given sevoflurane were able to swallow the 20 mL of water without coughing or drooling (Fig. 1; P < 0.001). The patient given morphine and the patient given meperidine both successfully swallowed at 2 min. At 6 min, 27 of 33 patients given sevoflurane were able to swallow without coughing or drooling (P < 0.05). One patient given sevoflurane repeatedly failed the test of swallowing (out to 30 min; not significant).
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Sevoflurane anesthesia was associated with a longer, and more variable, time from discontinuation of anesthetic administration to an ability to swallow 20 mL of water without coughing or drooling (Fig. 2) than was desflurane (9.2 [7.0, 12.2] min [median, quartiles] versus 4.9 [4.0, 6.4] min; P < 0.01).
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The use of regional anesthesia did not affect the time between discontinuation of anesthetic administration and the time until patients could respond to commands. For patients receiving desflurane plus a regional anesthetic (n = 8), the time to respond to commands was 3.2 ± 1.9 versus 3.4 ± 1.9 min in those not given regional anesthesia (n = 23). Patients in the sevoflurane group with a regional anesthetic (n = 17) required 6.2 ± 3.7 min to respond to commands versus 4.7 ± 2.6 min for those without a regional block (n = 16). A regional anesthetic also did not delay the return of airway reflexes. Among the patients in the sevoflurane group, 7 of 17 (41%) receiving regional anesthesia swallowed successfully 2 min after following commands whereas 8 of 16 (50%) who did not receive regional anesthesia successfully swallowed at 2 min (P = 0.73); by 6 min, 13 of 17 (76%) receiving regional anesthesia swallowed successfully whereas 14 of 16 (88%) without regional anesthesia swallowed successfully (P = 0.66).
Finally, the weight of the patient did not correlate with time to respond to command or the time to recovery of protective airway reflexes (Figs. 3 and 4).
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Discussion |
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Why might desflurane allow a more rapid return of protective reflexes? The lower solubility of desflurane in blood and tissues (8–10), and the consequent kinetics supplies one explanation. It is also possible that a pharmacodynamic explanation would apply—that desflurane is inherently less depressant to protective airway reflexes, but this speculation is unproven.
Our results are consistent with the work of Sundman et al. (11) who demonstrated that pharyngeal dysfunction could occur at 25% of MACawake for sevoflurane or isoflurane (and, presumably, desflurane). That is, the cerebral partial pressure of inhaled anesthetics must decrease to <8% of MAC to restore protective airway reflexes. Such a decrease requires a longer period of time with more soluble anesthetics.
Although the observer was blinded to the identity of the anesthetic delivered, we cannot exclude the possibility that the observer guessed whether the patient received desflurane or sevoflurane. Guessing the anesthetic identity on the basis of awakening might seem reasonable, but the "minutes to follow command" were 3.35 ± 1.92 for desflurane and 5.49 ± 3.13 for sevoflurane. That is, some patients took longer to awaken after desflurane anesthesia than the average patient given sevoflurane, and some patients awoke after sevoflurane anesthesia more rapidly than the average patient given desflurane. Yet all patients could swallow without aspiration two minutes after desflurane, but less than half the patients given sevoflurane could swallow without aspiration. Further to this point, we evaluated the time to respond to command after sevoflurane anesthesia for those who aspirated at two minutes and those who did not. The times did not differ between groups (328 ± 169 [n = 15] versus 334 ± 215 seconds [n = 18]), again indirectly suggesting that the observer remained blinded/objective.
The results have clinical implications. Patients at risk of pulmonary aspiration need to be guarded more closely after awakening, particularly when a more soluble anesthetic is used. Perhaps patients given more soluble anesthetics should be turned to the lateral position for transport. The need to closely guard the patient is notable because the time at which such extra care is needed is the time often used to transport the patient from the operating room to the postanesthesia care unit, a time when monitoring equipment and suction may not be available and the anesthesiologist may be distracted by tasks related to transportation.
The authors appreciate the several suggestions made by Dr. Edmond I Eger II, MD, who is a paid consultant to Baxter Healthcare Corp.
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