1Department of Anaesthesiology, Queen Mary Hospital, Pokfulam Road, Hong Kong. 2Department of Orthopaedic Surgery, Duchess of Kent Childrens Hospital, 12 Sandy Bay Road, Hong Kong. 3Department of Anaesthesiology, Duchess of Kent Childrens Hospital, 12 Sandy Bay Road, Hong Kong*Corresponding author: Department of Anaesthesiology, The University of Hong Kong, Room 424, Block K, Queen Mary Hospital, Pokfulam Road, Hong Kong
Accepted for publication: December 4, 2001
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Abstract |
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Methods. Twenty adolescent patients were randomized into two groups of 10. One group received sevofluranenitrous oxide anaesthesia and the other received propofol i.v. anaesthesia. An alfentanil infusion was used for analgesia in both groups.
Results. Changes in anaesthetic concentration produced little effect on the latency of SSEP, but the effect on the variability of SSEP amplitude was significant (P<0.05). Sevoflurane produced a faster decrease in SSEP and a faster recovery than propofol (P<0.05). On emergence, patients who received sevoflurane tended to have shorter recovery times to eye opening (mean 5.1 vs 20.6 min, P=0.09) and toe movement (mean 7.9 vs 15.7 min, P=0.22). Those who had received sevoflurane were significantly more lucid and cooperative in recovery.
Conclusions. Sevoflurane produces a faster decrease and recovery of SSEP amplitude as well as a better conscious state on emergence than propofol.
Br J Anaesth 2002; 88: 5027
Keywords: anaesthetics volatile, sevoflurane; anaesthetics i.v., propofol; monitoring, somatosensory evoked potential; surgery, spinal
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Introduction |
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Although electrophysiological monitoring is an important breakthrough, the Stagnara wake-up test has largely remained the gold standard for assessment of neurological status during spinal surgery.5 The wake-up test may be required in circumstances in which SSEP signals are undetectable or abnormal. However, timely completion of an intraoperative wake-up test can be difficult and the critical time window for reversal of a deficit can be lost while waiting for a patient to recover consciousness. Should a wake-up test become necessary, an anaesthetic technique that facilitates more rapid, lucid recovery can be helpful. The low bloodgas partition coefficient of sevoflurane or the short, context-sensitive half-time of propofol may confer advantages in such scenarios. Previous studies have shown that recovery after sevoflurane is faster than after isoflurane,6 halothane7 and propofol.811
The aims of this study were (i) to compare inhalational sevoflurane/nitrous oxide anaesthesia with total i.v. anaesthesia (TIVA) with propofol for their abilities to preserve SSEP monitoring signals during corrective scoliosis surgery, and (ii) to compare the wake up profiles of these techniques.
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Patients and methods |
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Patients were allocated randomly into two groups of 10. Randomization was done by drawing lots from an envelope containing 20 small pieces of paper, 10 marked S and 10 marked P. Group 1 (lots marked S) received sevoflurane/nitrous oxide inhalational anaesthesia and Group 2 (lots marked P) received propofol TIVA. EMLA cream (AstraZeneca LP, Wilmington, USA) was applied to facilitate painless insertion of an i.v. cannula and patients were not premedicated. Anaesthesia was induced with alfentanil 40 µg kg1 and propofol 23 mg kg1. Atracurium 0.5 mg kg1 was given for muscle relaxation and was followed by tracheal intubation and intermittent positive pressure ventilation. Maintenance of anaesthesia in the sevoflurane group was provided with a mixture of 65% nitrous oxide and 35% oxygen with 03% sevoflurane [<1.5 minimum alveolar concentration (MAC)], titrated to clinical requirement. Anaesthesia was maintained in Group 2 by target-controlled infusion (TCI) of propofol (Diprifusor; AstraZeneca) at a plasma concentration of 25 µg ml1, titrated to clinical requirement while the patient was ventilated with 35% oxygen in air. Muscle relaxation was maintained with atracurium 0.30.5 mg kg1 h1. Intra operative analgesia was provided by an alfentanil infusion of 0.41.0 µg kg1 min1, with intermittent boluses of 150500 µg as required. No other opioid was given until after completion of the study. The infusion of atracurium was discontinued towards the end of surgery and reversed using neostigmine 40 µg kg1 and atropine 20 µg kg1. Patients were kept normothermic with forced air warmers.
Intraoperative monitoring included intraradial invasive arterial blood pressure, rectal temperature, capnography, end-tidal sevoflurane concentration, pulse oximetry, central venous pressure, urine output and peripheral nerve stimulation. Arterial blood gases and haematocrit were checked intermittently. SSEP signals were collected over Cz' (2 cm posterior to Cz; 1020 international system of EEG electrode placement) and Cv (over the cervical spinal process of C2) vs the Fz of the 1020 system (Fig. 1).12 To elicit SSEPs, a pair of stimulating electrodes was applied over the posterior tibial nerve behind the medial malleolus. The stimulation current used ranged from 10 to 30 mA and was kept constant once selected for a particular patient. The normal current density was adjusted to produce a small movement of the toes. Single-pulse stimulation with a frequency between 5.1 and 5.7 Hz and duration of 200 µs was applied. An intraoperative spinal cord monitoring system (Viking IV; Nicolet Biomedical, Madison, WI, USA) was employed to record the responses with a 203000 Hz bandpass filter. Continuous 100 times averaging was used. The initial negative and positive waves were identified in the SSEP tracings so that the latency and the peak-to-peak amplitude could be measured. The intensity changes related to anaesthesia were also noted during surgery. The mean values and standard deviations of these variables were calculated for each patient. The within-patient variability was calculated from the ratio of the standard deviation to the mean [(SD/mean)x100%].
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Results |
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Discussion |
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A sevoflurane concentration up to 1.0 MAC was still compatible with SSEP recording.3 Other studies have found that propofol/alfentanil TIVA provides a better signal-to-noise ratio than enflurane or isoflurane anaesthesia, thereby allowing more frequent and reliable intraoperative SSEP recordings.19 The TCI plasma propofol concentration that can preserve SSEP signals adequately is still to be determined. Our results also show that there is a significant change in SSEP Cz' amplitude in response to changes in anaesthetic concentration when compared with SSEP Cv amplitude. This may reflect the cerebral effect of anaesthetic drugs rather than their effect on the spinal cord.
Sometimes a higher concentration of anaesthetic may be required during surgical manipulation and a drug that allows faster recovery of SSEP after readjustment therefore permits more accurate interpretation of the data. In this regard, we found that sevoflurane had significantly shorter dose-increase and dose-decrease periods than propofol. Also, the smaller standard deviation suggests that the recovery time required is more predictable than that with propofol. In interpreting SSEP, however, it is important to realize that false negative results may occur because monitoring is pathway-specific and an injury not involving the pathway may not be detected. Failure to monitor both latency and amplitude or use of an insufficient number of recording electrodes can be responsible for false negative responses.20 Also, patients with preoperative deficits may not have detectable SSEPs. More and colleagues reported six patients in a series of 158 who had no detectable potentials before anaesthesia.21 False positive SSEP results may also occur as a result of changes in anaesthetic dose or technical problems, although the consequences are less serious. A large multicentre study of SSEPs for spinal cord monitoring found a sensitivity of 92%, specificity 98.9%, positive predictive value 42% and negative predictive value 99.93%.22 Changes in SSEP may also be produced by physiological factors such as severe hypotension, hypoxia, hyperthermia and a large decrease in haematocrit (to less than 15%).23 These did not occur in any of the patients in our study.
As well as a faster recovery time, our data from the behavioural scores showed that sevoflurane produces a better conscious state on emergence. As both groups were homogeneous with respect to age, sex, body weight, ethnicity and surgery, we can assume that this is related to the anaesthetic technique. Although there was some variation in the duration of anaesthesia and individual consumption of alfentanil, there was no correlation between the duration of anaesthesia or total dose of alfentanil per unit body weight and the recovery indices, i.e. times from cessation of anaesthesia to toe movement and eye opening. This is interesting, considering recent reports of restlessness in paediatric patients recovering from sevoflurane anaesthesia.24 25 A calm and cooperative patient is obviously preferable after spine stabilization surgery and in the event of a wake-up test being required.
Sevoflurane has been associated with more nausea and vomiting than propofol.11 In our study, two patients in the sevoflurane group developed postoperative nausea and vomiting, but none in the propofol group did so before commencement of patient-controlled anaesthesia with morphine. Although propofol appears to have intrinsic antiemetic properties, the number of patients in this study was too small to demonstrate this. One patient receiving sevoflurane had involuntary movement on wakening, but this was transient with no clinical consequence.
In conclusion, this study showed that both sevoflurane and propofol produced a significant change in SSEP amplitude but sevoflurane produced this change more quickly and with faster recovery than propofol. Patients receiving sevoflurane also had a faster, more predictable recovery from anaesthesia and a calmer, more cooperative state on emergence.
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