Comparison of vital capacity induction with sevoflurane to intravenous induction with propofol in adult patients

Abstract

Background:

Various techniques exist for gaseous induction in adults. Vital capacity induction (VCI) is a special technique for gaseous induction of anesthesia.

Aims:

We compared the two methods for time of induction, vital parameters at induction, and suitability for laryngeal mask insertion and its effects, if any on the emergence and postoperative cognitive function tests.

Settings and Design:

A total of 60 adult American Society of Anesthesiologists grades I and II gynecological patients were randomly allocated into two groups of 30 each.

Materials and Methods:

Group “S” received VCI with 8% sevoflurane in 8 l/min oxygen and group “P” received intravenous induction with propofol after premedication. Laryngeal mask airway (LMA) was inserted and anesthesia maintained with oxygen, nitrous oxide, and sevoflurane in both groups. Induction time, corresponding entropy, vital parameters, and emergence time were noted. Postoperative cognitive and psychomotor functions were noted with P-deletion test, digit symbol substitution test, and finger nose test.

Statistical Analysis Used:

Unpaired t-test and Fisher exact test.

Results:

Time for induction was 61 ± 32 s and 31 ± 10.8 s for VCI (group S) and propofol (group P), respectively. The difference was statistically significant (P = 0.001). About 70% patients in VCI had excellent conditions for LMA insertion when compared with 76% in propofol group (P = 0.3855). The incidence of airway complications, emergence times, and recovery of postoperative cognitive functions was not significantly different in both groups.

Conclusions:

VCI provides an induction and recovery comparable to propofol induction.

INTRODUCTION

Single breath vital capacity induction (VCI) technique was first introduced by Bourne in 1954.[1] Various gaseous induction techniques have received a renewed interest after the introduction of sevoflurane in clinical practice. Sevoflurane has a low blood gas solubility and relative absence of pungency, making it suitable choice for rapid VCI. Rapid emergence also makes it suitable agent for ambulatory anesthesia and short surgical procedures. However, it remains a less practiced technique. We assessed its feasibility as an alternative anesthesia technique for short surgical procedures by comparing it with propofol induction.

MATERIALS AND METHODS

After approval from the local Ethical Committee, 60 American Society of Anesthesiologists (ASA) grades I and II adult gynecological patients posted for short day care procedures such as diagnostic hysteroscopy, abdominal tubal ligation, cystocoele repair, etc., were selected. Patients excluded were those having history of adverse reaction to inhalational agents or propofol, those on chronic sedatives, antipsychotic treatment and ASA grades III and IV patients due to comorbid medical illness and patients with compromised airway. A written informed and valid consent was taken from the selected patients. They were randomly allocated into two groups by sequentially numbered sealed envelopes.

After preanesthesia evaluation, the patients were kept nothing by mouth for 6 h prior to surgery. Group “S” received VCI with sevoflurane and group “P” received intravenous (IV) induction with propofol. On the day of surgery, the patients were explained cognitive and psychomotor tests in the preoperative room; namely digit symbol substitution test (DSST), P-deletion test, and finger nose test. They solved one set of the tests in preoperative room, which served as a control for their postoperative test results.

A 20 G IV access was established in the operating room, and a ringer lactate solution started. Noninvasive blood pressure, pulse oximeter, electrocardiogram, end tidal CO₂, and entropy (state entropy [SE] and response entropy [RE]) monitors were applied. A respiratory gas monitor (RGM) was also incorporated.

All patients were premedicated with injection midazolam 0.03 mg/kg, injection fentanyl 1 mcg/kg, injection glycopyrollate 0.2 mg, and injection ondensetron 4 mg IV.

Patients were preoxygenated with an alternative source of oxygen with face mask in group S by using another anesthesia machine.

Preinduction vital signs and SE and RE were recorded.

Patients in group S were explained to practice vital capacity maneuver. In this maneuver, the patient is first asked to first exhale fully, and then inhale fully to maximum possible inspiration from an unprimed mask during practice. This is similar to vital capacity measurement performed in pulmonary function tests. After this, patient is asked to hold the breath as long as possible followed by normal tidal volume breathing if not induced in first vital capacity breath.

The anesthesia circuit was primed with 8% sevoflurane with oxygen at 8 l/min by occluding the patient end of the circuit until the RGM showed >6% sevoflurane concentration. During the time when the circuit was primed, the patient was preoxygenated for 3 min with alternative oxygen source as the anesthesia machine and circuit were being primed. After preoxygenation, the O₂ mask was removed, and the primed anesthesia circuit mask was applied. The patient then performed VCI maneuver.

Patients in group P received 2 mg/kg of propofol with a 20 ml syringe as a bolus.

Time required for induction in group S was measured as time from start of VCI maneuver until the loss of eyelash reflex. For group P, induction time was measured from start of injection until the loss of eyelash reflex. The eyelash reflex was checked every 2-3 s in both groups. The corresponding value for RE and SE were noted at induction. Proseal laryngeal mask airway (LMA) of appropriate size was inserted. Additional depth was provided with incremental doses of 20 mg of propofol boluses titrated to patient's requirement only if necessary in both groups. Conditions for LMA insertion were subjectively noted as excellent, acceptable or poor based on scales used in some previous studies.[2,3]

Excellent: If there was no reaction from patient. No rescue anesthetic was required for any purposeful or involuntary movements.

Acceptable: If there as some movement or minimal reaction from the patient and rescue anesthetic was required, but there were no respiratory complications.

Poor: If insertion as associated with marked movements or respiratory complications in spite of rescue anesthetic. Complications such as coughing, breath holding, gagging, laryngospasm, etc., were noted.

An increase or decrease in pulse rate by >20% of baseline value was noted as a significant change, which may need appropriate intervention. Similarly, a fall or rise in mean arterial pressure by >20% of baseline was noted as significant requiring intervention. Hypotension was managed by IV fluids and vasopressors like mefenteramine 6 mg. Increase in heart rate or blood pressure was managed by increasing the depth of anesthesia. Bradycardia if hemodynamically significant was treated with anticholinergic drugs. Patients from both groups were maintained on spontaneous respiration with oxygen, nitrous oxide (40:60), and sevoflurane throughout the procedure.

At the end of the procedure, both the inhaled anesthetics were simultaneously discontinued, and oxygen flow was set to 8 l/min. Emergence time was then measured by blinded observer from the end of anesthesia to eye opening to verbal commands. The LMA was then removed, and the patient was shifted to postoperative room. Postoperatively, the cognitive and psychomotor tests DSST, P-deletion test, and finger nose test were performed at 15, 30, 45, 60, 75, 90 min, and thereafter half hourly until the test results returned to preoperative baseline value.

Statistical analysis

For calculation of sample size, the type 1 error α was taken as 0.05 together with a power of 0.80. From the expected difference in mean induction time from previous studies[4] of approximately 12s, the minimal sample size of 28 in each group was calculated. Continuous data are presented as mean ± SD. The analysis was performed using GraphPad software. P < 0.05 was considered as significant.

RESULTS

The demographic parameters and duration of surgery were comparable in both groups [Table 1].

Table 1.

Demographic parameters and duration of surgery

The induction time, corresponding entropy values and emergence time are shown in Table 2.

Table 2.

Induction and emergence parameters

The induction time was significantly longer in VCI group, whereas there was no significant difference in the emergence time in both groups.

During induction, pulse rate was within 20% of baseline value in 90% patients of group S and all 100% patients of group P. A fall in mean arterial pressure by more than 20% of baseline occurred in eight patients (26%) in propofol group as against only one patient (3%) in sevoflurane group in our study [Table 3].

Table 3.

Hemodynamic changes during induction

Subjective assessment of conditions for LMA insertion as mentioned in Table 4.

Table 4.

Conditions for LMA insertion

Untoward effects in the “poor” category were coughing and breath holding, the later was seen in group S [Table 5].

Table 5.

Conditions for LMA insertion

There was no significant difference in the time required for return of postoperative cognitive test, that is, DSST and P-deletion test to baseline in both groups. Finger nose test showed a clinically small difference in both groups, 29 min in group S as against 24 min in group P (P = 0.05) [Table 6].

Table 6.

Time required for return of postoperative cognitive and psychomotor tests to baseline

DISCUSSION

In this study, we compared the induction time and quality of a single breath VCI with sevoflurane to that with IV propofol. VCI has also been studied with halothane. However, the time for induction is longer, with a higher incidence of movements and higher incidence of arrhythmias as compared to sevoflurane.[1] We preferred sevoflurane due to its favorable features like fast onset and hemodynamic stability. The induction time was measured as the time until loss of eyelash reflex, and it was correlated with SE and RE. The induction time in VCI group of 61.93 ± 16 s in our study was longer than that reported by Philip et al.[5] which was 43 ± 4 s. They had used a circuit primed with 8% sevoflurane with 75% N₂O and oxygen, but had not premedicated their patients. In another study by Agnor et al.,[1] the induction time was 41 ± 16 s with sevoflurane and 66% N₂O in oxygen. As reported by El-Radaideh and Al-Ghazo,[4] who used 8% sevoflurane in 66% nitrous oxide and oxygen at 9 l/min (6:3), the time was 51 ± 4 s. Similarly, Yurino and Kimura[6] also used 7.5% sevoflurnae and 66% N₂O had induction in 41 ± 16 s. Nitrous may aid faster induction by second gas and concentration effects.[5] The difference in the findings may be attributed to using N₂O in the primed circuit, which was not used in our study. In study by Liu et al.,[7] using 8% sevoflurane in oxygen at 6 l/min, the induction time for VCI with sevoflurane was 43.8 ± 13.4 s. Again the patients who were induced in first vital capacity breath were the ones who could hold their breaths until loss of consciousness had an average induction time of 51 s. Others required further tidal volume breaths and had a higher average induction time of 72 s. This difference was also appreciated by Philip et al.,[5] where 59% patients who were able to hold breath until loss of consciousness had the most rapid induction. In study by Lin et al.,[8] two-third patients required second or third breath.

The entropy values with loss of eyelash reflex were 59.36 ± 22.3 (SE) and 70.3 ± 19.8 (RE) in sevoflurane group. These are in agreement with another study by Hasak et al.,[9] where both SE and RE were more than 60 in 72% of patients induced on sevoflurane correlating with absence of response to voice and mild shaking. The values are clearly higher than that stated for adequate depth of anesthesia.

We chose propofol as a comparator induction agent as it is widely used for ambulatory anesthesia.[10,11] The induction time with propofol was 31.1 ± 5.4s. This is in agreement to many studies,[4,12] although few studies showed higher induction times.[5] The entropy value for propofol that correlates with loss of eyelash reflex was SE 51 ± 5 and RE 63 ± 5. In study by Tesniere et al.,[13] the SE and RE that correlated with loss of eyelash reflex was 51 ± 23 and 58 ± 24, respectively for induction with propofol-remifentanil. In yet another study using propofol induction[14] the SE and RE values that correlated with loss of consciousness were 59 and 63 in 50% of patients and 37 and 44 in 95% of patients. Comparison between bispectral index (BIS) and entropy with propofol induction; the RE and SE values correlated with BIS at the time of taking incision (40 ± 13, 38 ± 12, and 39 ± 11, respectively); however, their correlation at the time of loss of eyelash reflex was not specified.[13]

The conditions of LMA insertion showed no statistical difference between the two groups. About 70% (n = 21) and 76% (n = 23) patients of groups S and P had excellent conditions for LMA insertion by requiring no rescue agent and no adverse airway reaction (P = 0.3855). In the study by Philip et al.,[5] the airway side-effects were less in VCI with sevoflurane and N₂O as compared with propofol induction. In yet another study by El-Radaideh and Al-Ghazo,[4] sevoflurane produced fewer side-effects than propofol for efficient LMA insertion. However, LMA insertion was faster with propofol requiring fewer attempts. In our study, we found acceptable LMA inserting conditions with seven patients in group S and five in group P (P = 0.3740). This was mostly taken care of by giving a rescue dose of propofol titrated to patient's requirement. Only 2 (6%) patients in group S and 2 (6%) patients in group P had poor conditions due to airway complications coughing, hiccoughs, and breath holding (P = 0.694). Among inhalational anesthetics sevoflurane was found to have minimal airway irritation and hence suitable for inhalational induction as compared with halothane.[1,15] In other studies, there was higher incidence of airway side-effects with sevoflurane induction, while using lower initial concentrations, slower induction techniques and nonprimed techniques.[6,16,17,18] Thus, priming of circuit with faster induction using VCI made the conditions for LMA insertion comparable to propofol.

In our study, during induction 26% patients in propofol group had significant hypotension as against 3% in sevoflurane group managed by IV fluids.

The emergence time in sevoflurane group was 267 ± 118 s as against 248 ± 96 s in propofol group. Since all the patients were maintained on oxygen sevoflurane and N₂O, and surgical duration being comparable in both groups, this may have outweighed the effect of initial inducing agent.

The structure of the study also provided us with the opportunity to study some aspects of postoperative cognitive and psychomotor recovery. The duration of surgery was short and comparable in both groups. Second, we did not use muscle relaxants as the type of surgery did not mandate its use. Furthermore, the types of surgery were associated with minimal postoperative pain. DSST is a widely used test for assessing postoperative cognitive recovery and was found to be more sensitive than other tests like digit span test and perceptual speed test.[19] Our results were similar to those of Philip et al.,[5] who also found no difference in DSST score in two groups. P-deletion test is a letter cancellation test described by Dixon and Thornton.[20] It is again used in many studies for assessing postoperative cognitive recovery.[21,22,23] Our results were in agreement with those of Dashfield et al.[23] where there was no significant difference in the P-deletion scores between the two groups, that is, VCI with sevoflurane and IV induction with propofol. Finger nose test is commonly used hand coordination test. It was used for assessing psychomotor recovery by Schäffer et al., in ophthalmic patients who received neurolept anesthesia.[24] Others like Dashfield et al.,[23] have used complex computer simulated tests for measuring hand coordination as a measure of psychomotor recovery. We used finger nose test owing to its simplicity and easy measurability. There was a statistically significant difference, though clinically small one (29 min in group S as against 24 min in group P) (P = 0.05).

CONCLUSION

The induction time with VCI was higher than propofol. However, the conditions for LMA insertion, associated airway complications, hemodynamics, emergence, and postoperative cognitive functions were comparable to propofol. Thus, VCI with sevoflurane can be employed for induction as an acceptable alternative to propofol, for LMA insertion.