Optimal anaesthetic depth for LMA insertion.

PURPOSE: A fixed dose of propofol administered rapidly can be insufficient or in excess resulting in airway complications and haemodynamic disturbances. This study is designed to assess whether loss of motor response to jaw thrust can be a reliable clinical indicator of anaesthetic depth for laryngeal mask airway (LMA) insertion.
METHODS: One hundred and twenty ASA I and II patients scheduled for general anaesthesia on day care basis were randomly allocated into two groups. Following pre-oxygenation, anaesthesia was induced to accomplish LMA insertion either with a 3 mg/kg propofol (Group CD, n=60) or in dose to abolish jaw thrust response (Group JT, n=60). Mean arterial pressure (MAP) and heart rate were continuously monitored while LMA insertion conditions were recorded using 6 variable, 3 point score.
RESULTS: 85% patients developed apnea in group CD when compared to 2% in group JT, P<0.0001. Despite similar insertion score, propofol consumption was significantly more in group CD when compared to group JT. More than 20% fall of MAP from baseline was noted in group CD after induction but there was no significant hypotension at any time in group JT.
CONCLUSION: Loss of motor response to jaw thrust provides satisfactory LMA insertion conditions.

RCT Entities:   Population Interventions Outcomes

INTRODUCTION

The laryngeal mask airway (LMA) insertion without the use of neuromuscular blocking agents requires an anaesthetic depth sufficient enough to obtund the airway reflexes. Propofol depresses both pharyngeal and laryngeal reflexes more than thiopentone and is therefore preferred.[1-3] However, the dose of propofol required to obtain adequate anaesthetic depth varies between patients. Administering propofol in a fixed dose can be insufficient or in excess resulting in hazardous airway complications and haemodynamic disturbances. Dose to target a clinical response rather than a fixed dose will allow better titration to achieve appropriate anaesthetic depth for LMA insertion and thereby minimise the incidence of such complications.[4-7]

Loss of verbal contact, loss of motor response to jaw thrust[8-10] and apnoea[11] are the clinical indicators often utilised for insertion of LMA. A clinical indicator of anaesthetic depth that will provide satisfactory insertion conditions and preserve spontaneous breathing is a safer option. Trapezius squeezing test has been used as a criterion of adequate anesthetic depth for LMA insertion in children without neuromuscular blocking agents.[12] However, forward jaw thrust imitates the stimulus that is caused by insertion of LMA;[8] therefore, loss of motor response to jaw thrust is a reliable clinical indicator of appropriate anaesthetic depth to allow uncomplicated insertion of LMA.[8-10]

This study is designed to evaluate whether propofol dose titrated to loss of motor response to jaw thrust is a good clinical indicator of adequacy of anaesthetic depth for LMA insertion versus a fixed dose of propofol for the same.

METHODS

This study was conducted between October 2005 and June 2007 after approval of institutional research ethics committee. 120 ASA 1 and 2 adults aged between 18 and 60 years scheduled for elective day care surgery under general anaesthesia were recruited for the study after obtaining a written informed consent. Patients with predicted difficult intubation, predicted difficult mask ventilation, and history of allergic drug reactions were excluded from the study.

The current study is designed to test the hypotheses that loss of motor response to a full jaw thrust is a safe and reliable end point of induction with propofol for LMA insertion. The sample size was calculated using an alpha level of 5% and power of 90%. A difference of 20% in the total propofol dose requirement among both the groups was observed according to our institutional pilot study (where in various fixed doses of propofol of 2, 2.5 and 3 mg/kg were tried. Though the lower doses were haemodynamically stable, it had unacceptable insertion conditions. Hence the dose of 3 mg/kg as fixed dose, as choosing a lower dose, despite knowing that insertion conditions would be sub optimal, would have been ethically unacceptable. Participant's allocation sequence was accomplished according to a computer generated list in blocks of ten. The patients were randomly assigned to group JT (loss of motor response to jaw thrust, n=60) and group CD (3 mg/kg lean body weight, of propofol, n=60) by sealed envelope technique by a person other than the anaesthesiologist involved in the study.

All patients were premedicated with intravenous metoclopromide (10 mg) and ranitidine (50 mg) given 15 min before surgery. In the operating room, baseline cardio-respiratory parameters (non-invasive blood pressure, oxygen saturation at room air and electro-cardiogram) were recorded. Fentanyl (1 μg.kg-1) was given intravenously 4 min prior to induction of anaesthesia. Following preoxygenation for 3 min, induction of anaesthesia was achieved with propofol @6.6 ml.min-1 (max. available infusion rate in the syringe pump) using a syringe driver pump. During the process of induction, jaw thrust manoeuvre was applied[8] by progressively lifting the jaw forwards when the verbal contact with the patient was lost. Jaw thrust was relaxed to a previously tolerated level if a motor response was noticed and resumed 5 s later until there was no motor response to a full forward thrust which was defined as the end point of induction to perform LMA insertion in group JT. In group CD, propofol infusion was stopped after administration of the calculated dose was completed and the motor response to jaw thrust was abolished. However, propofol infusion was continued in those patients where motor response to jaw thrust persisted. Once the desired end points of induction were reached in respective groups, the total dose of propofol and the time taken to reach the end point for LMA insertion was noted. LMA (size 3 or 4 classic LMA chosen based on the body weight of the patient, as per the manufacturer's recommendation) insertion[13] was performed by an anaesthesiologist who was not associated with the study, who was blinded to as to how the end point was achieved and having at least two years of experience. The person then performed the LMA insertion and recorded the no. of attempts and the insertion conditions using a 6 variable 3 point scoring system, [Table 1].[9]

Table 1

Laryngeal mask airway insertion score

LMA cuff was inflated with air and connected to the Bain's circuit with a total fresh gas flow of 100 ml/kg body weight. Appropriate placement and ventilation were determined by chest wall movement, auscultation of breath sounds, a square-wave capnograph trace, and no oropharyngeal leak with peak airway pressures of 16 cm H2O. Maintenance of anaesthesia was continued with 66% nitrous oxide in 33% oxygen and propofol infusion @ 200 mg.kg–1.min–1 for first 10 min. Vecuronium bromide 0.06 mg.kg–1 was used for muscle relaxation and ventilation of lungs was continued through Bain's circuit. Cardio-respiratory variables were recorded at every minute for first 10 min after insertion of LMA. Incidence of complications was recorded in both the groups.

Data were analysed using the SPSS statistical software, version 13. The incidence of apnea was analysed using Chi-square test. Parametric and non-parametric values were analysed using Student's unpaired t-test and Mann–Whitney's U test respectively. Inter-group differences between the variables recorded at different time points were analysed by a two-way repeated measure analysis of variance (ANOVA) using group as the independent samples factor and time as the repeated measurement factor. A significant group-by-time interaction was followed by tests of significance using Tukey's method to compare the two groups at various points in time. P<0.05 was considered as significant.

RESULTS

The demographic characteristics were similar in both the groups but the baseline heart rate and mean arterial pressure were found to be significantly different, P<0.05 [Table 2]. Despite a significant higher mean consumption of propofol (29 mg) and a longer mean time (26 s) taken to achieve end point of induction in group CD (P<0.05), the insertion conditions and the time required for LMA insertion in the first attempt were similar in both the groups [Table 3].

Table 2

Demographic and physical characteristics

Table 3

Induction and LMA insertion characteristics

The incidence of apnea was significantly more in group CD (51/60) when compared to group JT (3/60), P<0.001. The median SpO2 in both the groups were similar (98% vs 99% in group CD and group JT respectively), [Table 3].

Hypotension (>20% fall of MAP from baseline) was noted in group CD between 2nd and 6th min after induction while there was no significant hypotension at any time in group JT [Figure 1]. However, a significant fall in MAP from the baseline value was seen in both the groups after LMA insertion that lasted for 8 min, P<0.05. There was no significant difference in the heart rate at any point between the two groups [Figure 2]. We did not note any significant complication, like inability to place the LMA, desaturation, or bradycardia.

Figure 1

An error bar graph indicating the mean percentage change in mean arterial pressure (MAP) in both the groups at baseline, at insertion of LMA, and every minute for the first 10 min after insertion of LMA. –●–: Mean MAP in group A at various time intervals. --■--: Mean MAP in group J at various time intervals. B = Baseline, O = Insertion of LMA. A clinically significant hypotension was noted between 2nd and 6th min in group A

Figure 2

An error bar graph indicating the mean percentage change in heart rate (HR) in both the groups at baseline, at insertion of LMA, and every min for the first 10 min after insertion of LMA. –●–: Mean HR in group A at various time intervals. --■--: Mean HR in group J at various time intervals. B = Baseline, O = Insertion of LMA

DISCUSSION

We have demonstrated that loss of motor response to full forward jaw thrust is a safe and a reliable endpoint to propofol induction for LMA insertion. Jaw thrust manoeuvre is considered to imitate the stretch-induced stimulus that is caused by insertion of LMA. Thus, loss of motor response to jaw thrust may indicate a level of anaesthesia deep enough to allow uncomplicated insertion of LMA while preserving spontaneous breathing.[8] Greater depth of anaesthesia (fixed dose) would subject patients to adverse respiratory and haemodynamic effects of the higher dose without further improving conditions for LMA insertion. Drage et al.,[8] demonstrated that satisfactory LMA insertion conditions could be achieved in 87% patients who had loss of motor response to jaw thrust with a propofol dose of 2.55 (0.46) mg.kg–1. However, we could achieve satisfactory insertion conditions in all our patients belonging to group JT with a lesser dose of propofol; 2.27 (0.4) mg.kg–1. Reduction in propofol requirement in our study to achieve the same end point can be attributed to the use of slightly lower rate of infusion 6.6 ml.min–1 against a rate of 10 ml.min–1 used by them.[8] Differing rates of intravenous drug administration will result in variations in the early stages of drug distribution. Rapid injection leads to an early peak in plasma concentration providing a large gradient for both uptake into the central nervous system (CNS) and for drug redistribution to the other tissues. The early establishment of a large gradient for CNS uptake would explain the reduction in induction time with increasing infusion rate and the use of bolus doses. The slow increase in plasma concentration associated with a slow infusion provides a more sustained gradient for even drug delivery throughout the CNS.[4-7] The measurement of plasma concentration of propofol would bring in more objectivity than calculation of plasma concentration using anaesthesia simulators, but both are limited in their clinical utility due to limited availability of facilities to do so at the point of delivery, including our institute as well as other premier institutes in India. Therefore, we could not do so. The use of bispectral index (BIS) monitor would be a clinically relevant alternative to assess depth of anaesthesia for airway manipulation, this could not be used due to non availability during the time when the study was conceptualised and this would be the limitation of this study. However we did get to use it in three patients towards the end of the study when the monitor was made available to us for demonstration. Two of the patients in Group CD became apneoic which corresponded to a BIS value of 20 and 25 respectively, while in a single patient in Group JT loss of motor response to jaw thrust was noted at a BIS value of 35. This finding would need further validation. Time to onset of drug action depends on both the circulation time and the biophase kinetics. The physico-chemical properties of propofol are such that it requires a finite transport time to reach the biophase. If it is assumed that the” biophase delay” is the rate limiting process regulating the concentration of propofol at its site of action, then the slower infusion rates could easily reach the necessary concentration at the site of action with a lower total dose than the faster infusion rates. This concept implies that higher infusion rates will shorten the induction time but invariably lead to an over dose of propofol;[6] thus higher propofol consumption resulted in hypotension that occurred from 2nd to 6th min after LMA insertion in group CD.

We therefore conclude that loss of motor response to jaw thrust provides satisfactory LMA insertion conditions.