Abstract
Objective
Auditory evoked potential (AEP) index is one of the several physiological parameters for assessing the depth of anesthesia. The purpose of this study was to investigate whether the AEP monitoring could provide a better information for assessment of anesthesia level in classic laryngeal mask airway (C-LMA) insertion than the use of clinical signs in general anesthesia with single standard dose of intravenous propofol and fentanyl.
Methods
One hundred and seventy adult patients requiring general anesthesia for minor surgery were recruited and randomized to receive AEP monitoring (group A) or judgment of clinical signs (group B) for assessment of anesthesia depth and optimal condition to insert the C-LMA. The insertion conditions, including jaw relaxation, movements, presence of airway trauma and airway reflex, successful insertion rate and induction time were recorded and compared.
Results
The two groups were demographically similar. In group A, baseline heart rate was slower than group B (74 ± 14 vs. 78 ± 14 beats/min, p = 0.0267) and persisted throughout the whole study period. There was no significant difference in the change of heart rate during induction of general anesthesia between both groups. The incidence of movement was reduced in group A patients with AEP monitoring in comparison with group B patients (2.4% vs. 28.2%, p < 0.0001); of the unwanted events, swallowing was 0% versus 7.1%, p = 0.0126; laryngospasm was 0% versus 4.7%, p = 0.0430 and emergence of airway reflex was 1.2% versus 11.8%, p = 0.0050; the successful insertion rate was 100% versus 94.1%, p = 0.0232; and jaw relaxation was 83.5% versus 70.6%, p = 0.0448. There were no differences between both groups in trauma and induction time.
Conclusion
This study demonstrated that AEP index provided better information for C-LMA insertion with higher successful rate, less emergence of airway reflex and lower incidence of movement during induction of general anesthesia with single dose of intravenous propofol and fentanyl.
Keywords
evoked potentials, auditory; anesthesia, general; laryngeal masks;
1. Introduction
The classic laryngeal mask airway (C-LMA) has been widely used with increasing day-case surgery. In clinical practice, the signs of loss of eyelash reflex and jaw relaxation are used for assessing optimal level of anesthesia for insertion of the C-LMA, but its reliability is in doubt.1, 2, 3, 4 Some patients show inadequate anesthesia with movements of the body or presence of airway reflex after LMA insertion. There are several physiological parameters under investigation to evaluate the status of anesthesia, some of which have been introduced in clinical practice.5, 6, 7 Auditory evoked potential (AEP) monitoring has been used to quantify the pharmacokinetic and pharmacodynamic action of anesthetic agents that may detect the transition from consciousness to unconsciousness.5, 7, 8, 9, 10, 11, 12, 13 The A-Line™ AEP monitor, a new device for AEP measurement, converts the morphological change of AEP to a numerical index (A-Line ARX Index™ [AAI]). Some investigations have shown that AEP is able to predict the reactions of patient with well-defined stimulation such as insertion of LMA and skin incision.10, 11, 12
The aim of this study was to investigate whether AEP monitoring could be superior to the assessment by clinical signs as indicator in discriminating the anesthetic depth and determining the optimal timing for C-LMA insertion following induction of general anesthesia with single dose of intravenous propofol and fentanyl.
2. Methods
This study was approved by the ethics committee of Sin Lau Christian Hospital and written informed consents were obtained from all patients. Patients with ASA physical status I and II, aged 18–70 and scheduled for elective minor surgery under general anesthesia, were assigned randomly into two groups using a computer-generated table of random digits for assessment of adequate depth of anesthesia for insertion of C-LMA. Patients in group A underwent A-line™ AEP monitoring. The AEP signals were converted into A-line ARX-index (AAI) generated by the A-Line™ AEP monitor version 4.1 (Danmeter A/S, Odense, Denmark). The AAI scales ranged from 100 (awake) to 0, and a value between 15 and 30 indicated adequate anesthesia.10 The time for insertion of C-LMA was determined by an AAI value less than 30 and persisting for 3 s or more in group A patients or by clinical signs including loss of eyelash reflex and jaw relaxation in group B patients. Exclusion criteria included neurological disorders, impairment of hearing acuity, mental impairment, medications with hypnotics, and antidepressants, substance abuse, a history of gastrointestinal reflux, reactive airway diseases, obesity (body mass index > 30 kg/m2) and potential difficult airway (Mallampati score > 2, mouth opening < 25 mm, and cervical spine disease). Finally, 170 patients (85 in group A and 85 in group B) were recruited for study.
Pre-anesthetic medication was omitted. On arrival at the operation room, standard monitoring was applied including pulse oximetry, continuous ECG and noninvasive blood pressure (BP). Baseline BP and heart rate (HR) were recorded prior to induction of anesthesia and measurements were repeated, 1 min and 5 min after insertion of C-LMA. Venous access was established with a 20-G cannula for the intravenous fluid infusion and administration of induction agents. All patients were hydrated with normal saline. All patients were pre-oxygenated with 100% oxygen at tidal volume ventilation. Anesthesia was induced with intravenous fentanyl 1 μg/kg, followed by a bolus dose of intravenous propofol 2.5 mg/kg 2 min later, given over 30 s in conjunction with lidocaine 40 mg for reducing the painful injection of propofol.14 Manually controlled positive pressure ventilation with 100% oxygen was applied when apnea was noted after propofol injection. In group A, additional dose of propofol 1 mg/kg was given if the AAI did not reach the designated value within 2 min. In group B, after propofol injection, the eyelash reflex of the patient was sought by continuously stroking the eyelashes after the patient had closed the eyes spontaneously and upon loss of eyelash reflex, jaw relaxation was checked until the mouth could be opened enough for insertion of the C-LMA. Additional dose of propofol 1 mg/kg was given when jaw relaxation was not enough to insert the C-LMA within 2 min. Extra-dose of propofol was given to those in whom obviously inadequate anesthesia was noted after C-LMA insertion. The size of C-LMA was 3 for women and 4 for men. The insertion of C-LMA was performed by an anesthesiologist who had experience of C-LMA anesthesia for at least 3 yr. Proper position of C-LMA was confirmed by observing the respiratory movement and chest expansion, and gas leak was not evidenced as the airway pressure was below 25 cmH2O, otherwise, the C-LMA was removed and reinserted with reinforced dose of propofol 1 mg/kg if necessary. The number of attempts for successful insertion of LMA was recorded. After 1-min observation for successful insertion, anesthesia was maintained according to the clinical requirement. The manually controlled ventilation was adjusted to keep the end-tidal carbon dioxide tension between 30–35 mmHg. During the surgery, C-LMA cuff pressure was monitored intermittently and maintained less than 50 cmH2O by sphygmomanometer.
The insertion conditions of C-LMA was evaluated by the anesthesiologist in-charge with six variables by which jaw relaxation was graded as full or limited, head or limb movement as absent or present, coughing or gagging as absent or present, swallowing as absent or present, laryngospasm as absent or present, and trauma as absent or present. Bodily movement was defined as moving of the head or limbs. Laryngospasm was defined as evidence of upper airway obstruction or the presence of strider which was relieved by deepening of anesthesia or administering muscle relaxant. Coughing, gagging, swallowing and laryngospasm were defined as airway reflexes. Jaw relaxation was graded as full and limited (full = no resistance, limited = tight but could be opened just enough to insert C-LMA). The induction time was defined as the space of from the time after propofol injection to the time when LMA insertion was decided. The insertion conditions were evaluated on the first attempt. Reinsertions of C-LMA were designated as LMA insertion failures. At the end of surgery, upon removal of C-LMA, the pharyngolaryngeal trauma was considered as bloody secretion seen on the C-LMA.
All values are expressed as mean ± SD or number (percent) unless otherwise stated. The Pearson’s chi-square test was used for categorical variables, whereas 1-way analysis of variance was used for continuous variables. A p value < 0.05 was considered to be statistically significant. Data analysis was performed using the JMP software program (SAS Institute Inc, Cary, NC).
3. Results
Patients in groups A and B were demographically similar, as shown in Table 1. During the procedure, three patients in group A needed another dose of propofol to decrease the AAI below 30 and the insertion time of C-LMA were 150 s, 180 s and 240 s, respectively. No patient in the group B needed extra-dose of propofol to offer proper clinical conditions for LMA insertion, but seven patients who experienced severe coughing, swallowing and laryngospasm needed extra-dose of propofol to deepen the anesthetic level to secure a patent airway after LMA insertion. All patients did not recall their anesthetic events.
Serial changes of hemodynamic variables during study period are presented in Fig. 1. There were no differences in baseline systolic BP and diastolic BP between both groups (136 ± 19 vs. 139 ± 20 mmHg and 72 ± 13 vs. 74 ± 15 mmHg). The baseline HR in the group A was slower than that in the group B, with lingering throughout the whole induction period. HR as measured 1 min and 5 min after C-LMA insertion was significantly lower in the group A (74 ± 14 vs. 78 ± 14 beats/min, p = 0.0267; 67 ± 9 vs. 71 ± 11 beats/min, p = 0.0036; 64 ± 9 vs. 68 ± 10 beats/min, p = 0.0046; respectively). The systolic BP as measured 1 min after C-LMA insertion was significantly lower in the group A (103 ± 13 vs. 109 ± 16 mmHg, p = 0.0046) than in the group B. There were no significant differences in diastolic BP and in the changes of HR 1 min and 5 min after C-LMA insertion between both groups (7 ± 10 beats/min vs. 7 ± 10 beats/min and 9 ± 12 vs. 10 ± 12 beats/min). Patient responses to C-LMA insertion between groups A and B are shown in Table 2. Compared with group B, patients in group A showed higher incidence of full jaw relaxation (83.5% vs. 70.6%, p = 0.0448), successful insertion rate (100% vs. 94.1%, p = 0.0232) and reduced incidents of bodily movement (2.4% vs. 28.2%, p < 0.0001), laryngospasm (0% vs. 4.7%, p = 0.0430), swallowing (0 vs. 7.1%, p = 0.0126), emergence of airway reflex (1.2% vs. 11.8%, p = 0.0050), and presence of inadequate insertion condition (18% vs. 39%, p = 0.0046). There were no significant differences between both groups in coughing or gagging, induction time and trauma.
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4. Discussion
Adequate mouth opening and suppression of upper airway reflexes are necessary for successful LMA insertion. Clinical signs assessment for the depth of anesthesia with jaw relaxation and loss of eyelash reflex may be unreliable. Signs of insufficient anesthesia, including movement of the body, coughing or gagging, and laryngeal spasm, may appear with more intense stimuli. AEP monitoring has been reported to provide an index of adequate depth of anesthesia for successful LMA insertion.10, 11, 12, 13 In the present study, AEP monitoring provided a reliable indicator for successful LMA insertion to reduce the incidences of movement (2.4% vs. 28.2%, p < 0.0001), swallowing (0 vs. 7.1%, p = 0.0126), and laryngospasm (0% vs. 4.7%, p = 0.0430) as compared with clinical signs assessment. These findings supported our hypothesis that the AAI as an indicator for insertion of LMA enables prediction of adequate concentration of anesthetics at the effect site as depth of anesthesia is concerned to reduce the incidence of untoward effects of upper airway reflex, about which clinical signs assessment is inferior. One minute after C-LMA insertion, SBP was lower in the group A than in the group B (103 ± 13 vs. 109 ± 16 mmHg, p = 0.0046; respectively). This result suggested that AEP monitoring, compared with clinical signs assessment can better predict the adequate anesthetic level so as to offer better inhibition of airway reflex, to cause sympathetic and hemodynamic excitation during C-LMA insertion.
Several induction agents have been investigated whether they could provide a reliable condition for LMA insertion,15, 16, 17 of which propofol has currently been widely used, because it provides a greater inhibitory effect on airway reflex than thiopental.16, 17 Should a larger dose of propofol be needed as a sole agent for induction, however, profound cardiovascular depression may be induced.18 Various adjuncts have been dose investigated to back up the action of propofol to facilitate LMA insertion, of which clonidine, opioids, lidocaine, and midazolam stand out.19, 20, 21, 22, 23, 24, 25, 26, 27 Besides, there are many techniques of administration of propofol for induction of anesthesia, such as target-controlled infusion,28 intravenous infusion, and bolus injection. In the study by Doi et al., on general anesthesia induced with propofol and alfentanyl movement after LMA insertion in 50% or 5% of patients the AAI value as determined was 45.4 or 33.1, respectively, calculated directly from the best-fitting logistic curve,10 the anesthetist was blinded to the value of AEP index. In this study, we used AAI index less than 30 as an indicator for C-LMA insertion facilitated by bolus propofol combined with fentanyl. This study showed that the incidence of movement was 2.4% for C-LMA insertion, a result which supported the conclusion by Doi et al. that AAI value less than 33 may predict a level of anesthesia deep enough to cause movement in less than 5% of patients subjected to the stimulation of C-LMA insertion although we used fentanyl instead of alfentanyl and bolus injection instead of target-controlled infusion of propofol.
In the investigation by Hui et al., insertion conditions between coadministration of alfentanyl-propofol and fentanyl-propofol for C-LMA insertions were compared.29 Narcotic agents followed by propofol 2.5 mg/kg were injected intravenously over 10 s and 90 s prior to the C-LMA insertion. The incidence of swallowing, coughing or gagging, and laryngospasm in the fentanyl-propofol group was 33%, 24%, and 15%, respectively, in comparison with our results in the group B, in which the incidence in above sequence was 7.1%, 2.4%, and 4.7%, respectively. We used the similar anesthetic agents and dosages, but achieved a better insertion condition, the reason of the discrepancy of which might be due to the different time of fentanyl injection. According to the investigation of Ko et al.,30 the optimal time for fentanyl administration to attenuate the hemodynamic response by tracheal intubation as determined was 3-5 min. In the present study, patients were pre-treated with fentanyl 1 μg/kg 2 min before propofol administration. The insertion time of C-LMA in the present study was around 3.5 min after fentanyl injection, which might provide a sufficient time for fentanyl to reach its peak effect for blocking the noxious stimulus induced by C-LMA insertion and so improved insertion conditions were facilitated. Our results confirmed the report by Hui et al. that fentanyl injection given 90 s after of propofol injection, was not sufficient for fentanly to reach the peak effect.29
There was no significant difference in induction time between both groups. But a longer time to secure the airway was needed in the group B because we took more attempts to reinsert the LMA and necessarily deepened the anesthetic level for arresting airway reflex (1.2% vs. 11.8%, p = 0.0050). The average induction time between both groups was not different. It supported the fact that the pharmacokinetics and pharmacodynamics are usually constant for patients in both groups with similar demographics. If considered individually, the pharmacokinetics and pharmacodynamics may be affected by sex, age, BMI and general condition. AEP monitoring can detect the individual difference and monitor the anesthetic status serially to find a proper anesthetic level. Loss of eyelash reflex and jaw relaxation are only the clinical surrogate markers for LMA insertion, they do not represent an adequate anesthetic level to prevent airway reflex from the stimulation of C-LMA insertion.
In group B, all patients fulfilled the insertion criteria, but some patients needed another dose of propofol to complete the induction because of obviously inadequate anesthesia, and some patients even needed muscle relaxant to relieve the laryngospasm after C-LMA insertion. These might be due to inadequate anesthetic dose or inappropriate timing or condition to insert the C-LMA because the effective concentration of drug distribution to effect-site was not achieved. In group A, three patients needed reinforced medication to fulfill the insertion criteria before C-LMA insertion. It might imply that a dose of propofol 2.5 mg/kg may be insufficient to achieve an optimal insertion condition for every patient. AEP monitoring could tell who will need more dose of propofol before LMA insertion, whereas assessment by clinical signs was unable to predict the necessity of extra-dose of propofol.
In conclusion, insufficient anesthetic depth for insertion of C-LMA may result in untoward effects, such as emergence of airway reflex and side morbidity during induction of general anesthesia. AAI measured by A-Line™ AEP monitor as an indicator is superior to clinical signs to discriminate different anesthetic depth and provides a more optimal condition for insertion of C-LMA.