The laryngeal mask airway (LMA) may provide a better airway, with respect to ventilation and oxygenation, than a conventional mask and oropharyngeal airway (1). In addition, the LMA has been successfully used to manage difficult airways as a ventilatory device by itself and as a conduit for tracheal intubation (2–5).
Propofol appears to provide the best conditions for LMA insertion (6–9), although propofol frequently causes apnea and hypotension (5,9–11). We wanted to examine a better method for LMA insertion in uncooperative children—a method in which the onset of action is rapid but airway and spontaneous ventilation are well maintained and a mode of drug administration other than IV injection is possible. Thus, we decided to investigate ketamine for the insertion of LMA. Ketamine is well known for its airway-maintaining activity as well as for its increases in heart rate and cardiac output (12), which are favorable characteristics in pediatric anesthesia. Because it increases airway reflexes (12), however, ketamine has been regarded as inappropriate for the preparation of LMA insertion. To take advantage of airway-maintaining activity and to suppress increased airway reflexes, lidocaine spray was added to the preparation of the patients before the injection of ketamine.
Because equipotent doses of propofol and ketamine for insertion of an LMA are not known, especially in patients premedicated with midazolam, our main aim was to perform a dose-response study of the two techniques for insertion of an LMA to determine the dose(s) associated with the best balance between avoiding adverse events while maintaining airway and spontaneous ventilation. The secondary aim was to evaluate the effectiveness of lidocaine spray and IV ketamine by comparing the best doses of the two techniques.
After IRB approval, informed consent was obtained from all parents. Children (n = 90) of ASA physical status I or II and aged from 3 to 12 yr, who appeared to have normal airways by examination and history, were studied during elective surgical procedures that ordinarily involve tracheal intubation. Those who were >20% different from the ideal body weight for their height and those with neuromuscular or psychiatric disease, seizure disorder, upper respiratory tract infection, or a history of allergy or asthma were excluded. Patients were randomly allocated by the stratified randomization for age to 1 of 2 main groups and to subgroups of 10 to receive predetermined doses of either propofol (total, n = 40) or ketamine with pretreatment of lidocaine spray (total, n = 50). Four subgroups received 2.5, 3.0, 3.5, or 4.0 mg/kg of propofol (Zeneca Ltd, Macclesfield, UK), whereas 5 received 2.0, 2.5, 3.0, 3.5, or 4.0 mg/kg of ketamine (Yuhan Corp., Seoul, Korea).
Five minutes before the induction of anesthesia, midazolam 0.05 mg/kg and glycopyrrolate 0.005 mg/kg were injected IV as premedication. Standard monitors, such as an electrocardiogram, percutaneous arterial oxygen saturation by pulse oximetry, and a noninvasive blood pressure (NIBP) monitor, were used. One minute before the induction, lidocaine spray 10 mg (Xylocaine® 10% spray; Astra Pharmaceutical Production AB, Södertälje, Sweden) was applied only to the ketamine group. The spray was directed to the oropharynx while the tongue was depressed with a pressor. When body weight was between 10 and 20 kg, spray was applied twice; when body weight was between 20 and 30 kg, spray was applied 3 times; and when body weight was more than 30 kg, spray was applied 4 times. Anesthesia was induced IV with propofol or ketamine; both induction drugs were prepared in a 10-mL syringe. An exactly calculated dose was ready at the bedside before the induction, and an additional dose of 1 mg/kg was also prepared for an unsatisfactory induction. An assistant blinded to the induction drug was ordered to inject the predetermined drug under a given protocol. Just after the start of the IV injection, another physician blinded to the study protocol was allowed to enter the operation room and assess the conditions and responses.
Propofol was injected via a three-way stopcock for 15 s, and ketamine was injected for 1 min. Immediately after injection, 3 mL of saline was used to flush the drug from the IV line, the dead space volume of which was <1 mL. After 1 min, LMA was inserted, and ventilation was then assisted or controlled with oxygen/isoflurane (2%) for 2 min. Systolic and diastolic blood pressure and heart rate were measured just before the induction, just before LMA insertion, and 1 and 2 min after insertion. NIBP measurements usually take approximately 30 s, so every NIBP mea-surement started 30 s before the designated time.
Loss of the lid reflex was checked every 10 s after the completion of injection. Immediately after loss of the lid reflex, a face mask was gently put onto the face with 3 L/min of oxygen. The respiratory rate was monitored by impedance pneumography and ETco2 wave form. If apnea (cessation of breathing for >20 s) occurred, controlled ventilation was instituted.
The LMA was inserted by an experienced anesthesiologist according to the manufacturer’s recommendations (13). If spontaneous ventilation was lost, LMA position and airway patency were checked by gentle manual ventilation. If spontaneous ventilation was active, LMA position and airway patency were clinically checked by regular, rhythmic reservoir bag movement and ETco2 display.
Conditions for LMA insertion (Table 1) and patient responses were assessed by the same physician, who was unaware of the kind and dose of the induction drug and the study protocol. The overall result was considered satisfactory if all criteria were satisfactory. The outcome was considered acceptable if all were better than acceptable, but at least one acceptable criterion was included. Otherwise, the result was considered unsatisfactory; i.e., at least one of the criteria was unsatisfactory. If unsatisfactory because of coughing, gagging, swallowing, biting, or tongue, head, or limb movements, anesthesia was deepened by further increments of the designated induction drug.
Laryngospasm was graded as none, mild (spontaneous relief), moderate (relieved by applying positive pressure through LMA), or severe (relieved by succinylcholine administration). Coughing, gagging, swallowing, and tongue movement were graded as absent, minimal, moderate, or severe. “Minimal” indicated some movement, but this did not affect the positioning of the LMA; “moderate” meant that holding was required, but with the inhalation of anesthetic gases was no longer necessary; and “severe” indicated that an additional dose of the induction drug was needed. Biting on the tube was also graded as absent, minimal, moderate, or severe. “Minimal” was somewhat like biting; “moderate” indicated some biting action, but with no effect on the lumen, whereas “severe” denoted obvious biting to an extent that reduced the lumen.
Head and limb movements were graded as follows: mild if no restraint was necessary; moderate if some restraint was necessary but could be discontinued within 30 s of inhalation of anesthetic gases, which made 1-min NIBP measurement possible; and severe if an additional dose of the induction drug was necessary. Secretions were graded as follows: mild if found only within the tip of the suction catheter, moderate if seen within the proximal half of the suction catheter, and severe if suctioning was required more than once.
Mild laryngospasm; the minimal or moderate level of coughing, gagging, swallowing, biting, or tongue movements; and the moderate level of head or limb movements or secretion were graded as acceptable. Responses more severe than these were graded as unsatisfactory, and less severe responses were graded as satisfactory. If present, pain or discomfort at the site of injection during the administration of the induction drug was recorded and graded by the physician, who evaluated the whole procedure, as mild, moderate, or severe according to the patient’s facial expression, arm movements, or complaints of pain.
To blind the physicians involved to the induction drug, ketamine (50 mg/mL) was diluted to 10 mg/mL with a 10% fat emulsion (Intralipose®; Green Cross Pharmaceutical Co, Seoul, Korea) prepared from refined soybean oil, egg-yolk phospholipids, and glycerin. So that the final volume of induction drug was similar in both groups and to help the design of the blinded study, the concentration of this mixture was the same as that of commercially prepared propofol. To reduce pain when propofol was injected and to control the condition, 1 mL of 1% lidocaine was added to each 100 mg (10 mL) of propofol (6,11) and ketamine preparation.
An intention-to-treat analysis was used, with subjects analyzed according to their initial assignment and not according to the total drug administered. Statistical differences in patient characteristics were assessed by the Kruskal-Wallis test, and overall results were assessed by the χ2 test. The overall results between the best subgroups were compared by using Fisher’s exact test. Systolic and diastolic blood pressure and heart rate were compared by using repeated-measures analysis of variance, followed by Duncan’s multiple range test. P < 0.05 was considered significant.
There were no significant differences between groups in age or body weight distribution. In no case was removal and reinsertion of the LMA necessary. A satisfactory or acceptable range was achieved only with 3.0 or 3.5 mg/kg of ketamine (Table 2). With 2.0–3.5 mg/kg of ketamine, there was no apnea or airway obstruction. However, apnea or airway obstruction was experienced after every dose of propofol (Table 2). In 15 of 21 apnea cases, spontaneous respiration returned after LMA insertion (Table 2). No propofol dose was completely satisfactory or acceptable (Table 2). Overall, the ketamine groups (41 satisfactory or acceptable cases out of 50) showed better results than the propofol groups (14 satisfactory or acceptable cases out of 40) (P < 0.001;Table 2). If we compare the best ketamine doses and the propofol dose similar to that reported effective (14), the number of satisfactory or acceptable cases for 3.0 or 3.5 mg/kg of ketamine (20 of 20) was larger than that for 3.5 mg/kg of propofol (5 of 10) (P = 0.002;Table 2).
Pain on injection was seen in 72.5% (29 of 40) of the propofol group; in 1 patient, it was severe; in 10, moderate; and in 18, mild. During ketamine injection, however, the incidence of mild pain was only 10%. Unlike the propofol group, oral suction was sometimes needed in the ketamine group, even though glycopyrrolate had been injected before surgery (Table 2).
Depending on whether 3.0 or 3.5 mg/kg of ketamine or 3.5 mg/kg of propofol was used, all the systolic and diastolic blood pressures of the ketamine subgroups were higher than those of the propofol subgroup after the anesthesia induction and 1 and 2 min after LMA insertion (P < 0.05), but heart rate was not different. There were no cases of delayed recovery or postoperative problems.
For children premedicated with midazolam and glycopyrrolate, pretreatment with lidocaine spray and ketamine may be better than propofol for the insertion of LMA because spontaneous ventilation is maintained and the airway is secured. The ketamine dose of 3.0–3.5 mg/kg appears to be ideal.
Comparisons have been made between propofol and other induction drugs with reference to LMA insertion (6–8). The thiopental (4.0 mg/kg) and fentanyl (1 μg/kg) groups showed a much more frequent incidence of gagging (8). Even after premedication with diazepam and mask ventilation with oxygen, N2O, and isoflurane (2%) for two minutes, thiopental (5.0 mg/kg) resulted in more frequent laryngospasm and even failure (incidence of 11%) to insert an LMA because of inadequate relaxation (7). Pretreatment with topical lidocaine before the injection of thiopental (5.0 mg/kg) made no difference to LMA insertion with propofol (2.5 mg/kg), except that the thiopental group had a shorter mean apneic time of 96.1 seconds if compared with that of propofol (184.9 seconds) (6). In our study, apnea or airway obstruction was observed in all the propofol groups and in the ketamine 4.0 mg/kg subgroups.
As previously stated, for patients premedicated with oral midazolam 0.5 mg/kg, the 90% effective dose required for satisfactory LMA insertion was 3.6 mg/kg (14). Thus, 2.5–4.0 mg/kg of propofol was used for comparison with the ketamine and lidocaine spray regimen. IM injection of ketamine is a possible mode of anesthesia induction, which can be regarded as another favorable point compared with any other IV induction drugs.
The LMA can be used as a routine airway and as a conduit for tracheal intubation during difficult airway management. Passage of a fiberoptic bronchoscope through an LMA is nearly 100% successful in most studies (3,5,15), and LMA insertion seems to be easier than any other airway-maintaining method (16). Ketamine after pretreatment with lidocaine spray may be a good approach to LMA insertion for the management of difficult airway in children.
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