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From the Department of Anesthesia, Toronto Western Hospital, University Health Network, University of Toronto, Toronto, Ontario, Canada.
Address correspondence to: Dr. Suntheralingam Yogendran, Department of Anesthesia, EC 2-046, Toronto Western Hospital, 399 Bathurst Street, Toronto, Ontario M5T 2S8, Canada. Phone: 416-603-5118; Fax: 416-603-6494; E-mail: Suntheralingam.yogendran{at}uhn.on.ca
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Abstract |
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Methods: Following approval of the Research Ethics Board, 124 outpatients undergoing knee arthroscopy were randomly assigned to receive either PCI or VCI sevoflurane followed by laryngeal mask airway (LMA) insertion and sevoflurane maintenance. In the PCI group, the circle circuit was not primed. The patients were asked to hold the facemask themselves and breathe normally with sevoflurane 8% in oxygen at a flow rate of 4 L·min–1. In the VCI group, the circle circuit was primed and patients were asked to take vital capacity breaths with sevoflurane 8% at an oxygen flow rate of 8 L·min–1. The LMA was inserted as soon as the patient’s jaw was relaxed. Time from induction to LMA insertion was recorded and insertion conditions rated. The amount of sevoflurane used for LMA insertion was calculated. Vital signs were monitored at one-minute intervals until ten minutes after LMA insertion.
Results: Demographic data were comparable. There were no differences with respect to LMA insertion time (PCI - 3.4 min vs VCI - 3.3 min), laryngospasm (PCI - 7% vs VCI - 5%), mean arterial pressure, heart rate, SaO2 as well as patient’s overall satisfaction.
Conclusion: PCI was comparable to VCI in sevoflurane induction with respect to the speed of induction, side effects during induction and patient satisfaction. However, PCI requires no special training and is widely applicable to all patient populations.
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Introduction |
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Inhalational induction techniques using sevoflurane include vital capacity induction (VCI) in which the patient is asked to take a deep breath and hold it for as long as possible then breathe out to residual volume.4 The other possible technique is patient controlled induction (PCI), in which the patient breathes normal tidal volume breaths and holds the anesthesia mask himself/herself. VCI was the first technique to be introduced with the emergence of sevoflurane but the high volume of sevoflurane required with this technique has restricted its use. At present, only two studies have compared these techniques with respect to the time of loss of response to command, loss of eyelash reflex and ease of laryngeal mask airway (LMA) insertion. Both studies found no difference between the two techniques.5,6 However, one author5 used a modified Bain circuit with 33% oxygen (O2) at a total flow of 6 L·min–1 [2 L of O2 and 4 L of nitrous oxide (N2O)/minute]. The other author used sevoflurane in a mixture of N2O ans O2 in a ration of 2:1 with the total fresh gas flow of 8 L·min–1 using a circle system.6 We chose induction with 100% O2 to increase the margin of safety in case of desaturation caused by laryngospasm or underlying pulmonary conditions.
We designed this prospective study of sevoflurane inhalational induction, using O2 as carrier gas and circle system, to compare VCI with PCI with respect to time needed for LMA insertion, ease of LMA insertion, patient satisfaction, and postoperative pain, nausea and vomiting.
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Methods |
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One hour before surgery, patients were premedicated with oral naprosyn 500 mg to reduce intraoperative and postoperative analgesic requirements. Patients were randomized into two groups: PCI or VCI according to a computer generated block randomization table. Instructions on both techniques were given to all patients before surgery but patients only knew what group they were randomized to in the operating room.
Upon arrival in the operating room, standard monitors were applied: electrocardiogram (lead II), noninvasive blood pressure monitoring and pulse oximetry (SaO2). Bispectral index (BIS) electrodes were applied to the forehead before the induction of anesthesia in order to obtain the preanesthetic value. A circle breathing system was used in both groups. Intravenous normal saline was started through a 20-gauge cannula. Patients received no sedative or narcotics before induction.
In the PCI group, patients were asked to hold the mask and breathe normal tidal volume breaths. Fresh gas flow (FGF) was 4 L of O2 with a dial setting of 8% sevoflurane. The circuit was not primed. In the VCI group, the anesthetic circuit was first primed with sevoflurane by completely collapsing the 2-L bag twice initially and occluding the Y piece with gauze. The circuit was flushed with 8 L of O2 with an 8% sevoflurane dial setting while keeping the adjustable pressure-limiting valve closed. The priming endpoint was an endtidal sevoflurane concentration > 6%. The time taken to prime the circuit (approximately 60 sec) was noted and was taken into account for later calculation of total sevoflurane used in both techniques.
Once in the operating room, VCI patients were instructed to practice one vital capacity breathing room air. The anesthesia mask was applied and patients were asked to take a deep breath from the residual lung volume and instructed to hold the breath as long as possible. They were encouraged to take vital capacity breaths until unconsciousness occurred. The following clinical signs of loss of consciousness were evaluated every ten seconds: loss of response to command (patients were asked to open their eyes), loss of eyelash reflex, and loss of handgrip. When the patient became unconscious, the FGF was reduced to 4 L·min–1 to prevent excessive use of sevoflurane.
Adverse events (coughing, breath holding, laryngeal spasm, movements, and excitement) were assessed by a research assistant during induction and recorded in both groups. Treatment of these side effects included manual ventilation to increase anesthesia depth. If this was insufficient, 50 mg incremental boluses of propofol were administered.
In both groups, the following variables were collected by a research assistant: heart rate, blood pressure, SaO2, BIS readings, and end-tidal sevoflurane concentration every minute for the first ten minutes postinduction, then every three minutes for the next ten minutes.
LMA insertion was attempted every 30 sec after loss of consciousness and time of successful placement was recorded. Anesthesiologists were asked to evaluate the mask insertion as smooth (full insertion), partial (gagging/coughing) or poor (need for repositioning). The number of attempts at LMA insertion was recorded for every patient. After successful LMA insertion, anesthesia was maintained in both groups with a combination of O2 and N2O (1L:1L). Sevoflurane concentration was adjusted to keep the end-tidal sevoflurane concentration between 1.5 to 2% and BIS around 50.
Boluses of 12.5 µg fentanyl were used to maintain respiratory rate between 10 to 20 breaths·min–1. At the end of surgery, the surgeons infiltrated 20 mL of 0.5% plain bupivacaine into the knee joint. The anesthesiologist then discontinued N2O and sevoflurane and increased O2 flow to 8 L·min–1. The LMA was removed when patients opened their eyes or when they coughed.
The volume of anesthetic vapour was calculated according to the following formula:
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After the end of surgery, patients were transferred to the postanesthesia care unit (PACU) and were asked to answer a standardized oral questionnaire including visual analogue scale (VAS) for pain and nausea. Pain in the PACU was treated with fentanyl and/or with two tablets of acetaminophen 500 mg, which were given at the discretion of the nurse as per routine PACU order protocol. Nausea and vomiting were treated with 1 mg of granisetron intravenously before discharge from the PACU. A research assistant asked the patients about the smell of sevoflurane, their satisfaction about induction and emergence. All patients were contacted by telephone to answer a standardized questionnaire 24 hr postoperatively.
Statistics
The number of patients required was based on data from a previous study.5 Sample size was calculated assuming an alpha value of 0.05 and a power of 0.8. If the mean difference for time taken to insert the LMA between the PCI and VCI groups is about 15%, the anticipated sample size will be 60 patients in each group. A clinically significant difference in the time needed for LMA insertion would be 30 sec.
Chi-square, Mann-Whitney and unpaired two-tailed Students t tests were used as appropriate. A P < 0.05 was considered as statistically significant.
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Results |
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).
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). There was no difference in the average percentage of sevoflurane in the expired gas for the first 15 min (Figure 1). The average volume of sevoflurane used for LMA insertion was significantly higher in the VCI group (8.0 ± 2.8 mL) than in the PCI group (4.4 ± 1.5 mL). Average time to LMA insertion was 3.4 min in the PCI group and 3.3 min in the VCI group. Average BIS readings for both groups were comparable at three-minute and four-minute intervals (Table II; Figure 2). The incidence of adverse effects during induction was comparable in both groups (Table III). Duration of anesthesia and surgery, intraoperative pain medication and fluid therapy were comparable in both groups (Table I
). The VAS score for nausea and the incidence of vomiting were not significantly different between the groups in the early phase of recovery (Table IV). There was no difference in the pain scores or the amount of rescue medication administered in the PACU in both groups. Patient acceptance in regards to smell, induction characteristics and overall satisfaction of both techniques were comparable (Table V).
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Discussion |
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The acceptance of inhalational induction in routine clinical practice in adults depends on smoothness and rapidity of onset of anesthesia, and lack of serious side effects.9 Sevoflurane has two attributes that facilitate rapid, smooth inhalational induction: a) a low blood gas solubility; and b) relative absence of pungency. The mean induction time in both groups (PCI and VCI) was comparable, despite priming of the circuit with 8% sevoflurane and use of vital capacity breaths in the VCI group. Theoretically these two factors should hasten the induction of anesthesia in VCI, however, the difference in mean induction time was not significant.
Hall et al.8 found that the time to loss of eyelash reflex in VCI was 61 ± 24 sec after 4 vital capacity breaths from a circuit primed with 8% sevoflurane and 66 % N2O. In our study, the time for the loss of eyelash reflex was longer (1.5 ± 0.6 min in PCI vs 1.5 ± 0.6 min in VCI). This difference is most probably due to lack of the second gas and concentration effects of N2O, which we did not use during induction.
Other important factors to be considered in inhalational induction are the volume of the anesthetic gas used and possible pollution of the operating room atmosphere. In this regard, PCI is superior to VCI because priming of the circuit may be responsible for increase of operating room pollution. The average amount of sevoflurane used for LMA insertion was 8 mL in the VCI group, almost double the amount used in the PCI group (4.43 mL). Hall et al.8 and Baker et al.5 have both described 6 L of FGF to prime the circuit. The difference in volume of sevoflurane in both PCI and VCI techniques would have been small if we had primed the circuit with lower FGF.
Ambulatory surgery requires a good selection of patients and appropriate care postoperatively. One of the major criteria to determine the readiness for discharge is the absence of postoperative nausea and both techniques had low incidence of this adverse event. In conclusion, the use of PCI is equivalent to VCI with regards to speed of induction, side effects during induction and patient satisfaction. However PCI requires no specific patient training.
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Acknowledgments |
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Footnotes |
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References |
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2 Smith I, Nathanson M, White PF. Sevoflurane–a long-awaited volatile anaesthetic. Br J Anaesth 1996; 76: 435–45.
3 Strum DP, Eger EI II. Partition coefficients for sevoflurane in human blood, saline, and olive oil. Anesth Analg 1987; 66: 654–6.
4 Philip BK, Lombard LL, Roaf ER, Drager LR, Calalang I, Philip JH. Comparison of vital capacity induction with sevoflurane to intravenous induction with propofol for adult ambulatory anesthesia. Anesth Analg 1999; 89: 623–7.
5 Baker CE, Smith I. Sevoflurane: a comparison between vital capacity and tidal breathing techniques for the induction of anaesthesia and laryngeal mask airway placement. Anaesthesia 1999; 54: 841–4.[Medline]
6 Yurino M, Kimura H. Induction of anesthesia with sevoflurane, nitrous oxide and oxygen: a comparison of spontaneous ventilation and vital capacity rapid inhalation induction (VCRII) techniques. Anesth Analg 1993; 76: 598–601.
7 Dion P. The cost of anaesthetic vapours (Letter). Can J Anaesth 1992; 39: 633–4.[Medline]
8 Hall JE, Stewart JI, Harmer M. Single–breath inhalation induction of sevoflurane anaesthesia with and without nitrous oxide: a feasibility study in adults and comparison with an intravenous bolus of propofol. Anaesthesia 1997; 52: 410–5.[Medline]
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