Tracheal intubation is the most important and safest technique used to provide a definite airway whenever controlled ventilation is required. Most routine orotracheal intubation is performed with the help of a laryngoscope [
Many anesthetic personnel traditionally learn how to perform laryngoscopy and intubation using the Macintosh laryngoscope. Additionally, many anesthetic personnel perform laryngoscopy with the Miller laryngoscope in the same way they would using a curved blade. Laryngoscopic competency in tracheal intubation is essential for many healthcare trainees. Failure to perform successful intubation can sometimes result in patient death. Traditional teaching on this matter has focused on certain key aspects of successful intubation, including the technical details of proper head positioning with laryngoscope blade insertion and lift, as well as a timely and atraumatic performance.
This study was designed to compare the clinical feasibility of Macintosh and Miller laryngoscopes in the hands of inexperienced users in anesthetized patients deemed to be at low risk of difficult intubation. We hypothesized that both devices perform similarly, concerning success of an endotracheal intubation. Additionally, we studied the number of insertion attempts, time needed for insertion, hemodynamic changes, and complications.
The instructional materials included a Power Point presentation and a manikin head (Laerdal Airway Management Teacher, Laerder, Denmark) on which to practice, Macintosh and Miller laryngoscopes as well as endotracheal tube (ETT, 7.5: women, 8: men, cuffed, Protex, STMS Portex Limited, UK) for the novice.
The presentation containing instructions and illustrations regarding tracheal intubation step by step was developed by the authors skilled in the use to both laryngoscopes, with reference to the manufacturer’s instructional manual and the standard text books [
The study was conducted from December 2005 to March 2006 at a large tertiary care referral center, Siriraj Hospital, Bangkok, Thailand. Patients with age at least 18 years of age who scheduled for elective surgical procedures requiring tracheal intubation were eligible for the study. The exclusion criteria were risk factors for gastric aspiration and/or difficult intubation (Mallampati class III or IV; thyromental distance less than 6 cm; interincisor distance less than 4 cm) and history of relevant drug allergy. The study was approved by the Institutional Review Board of the Faculty of Medicine Siriraj Hospital. All patients provided written informed consent for the study and the procedure.
The study is a clinical trial randomized control study. Patients were randomized into either Macintosh laryngoscope (MC) group or Miller laryngoscope (ML) group by using computerized generated randomization numbers placed in sealed envelopes. General anesthesia with endotracheal tube requiring muscle relaxation was performed in the operating room. Successful tracheal intubation was the primary outcome measured. Failure to success was defined as an inability to place an endotracheal tube after three attempts or a significant alteration of hemodynamic (severe hypertension and serious cardiac arrhythmia) and respiratory parameters (O2 saturation
Thirty nurse students in anesthesiology without intubation experience followed the instruction on the use of Macintosh and Miller laryngoscopes. Standard monitoring, including electrocardiography, noninvasive blood pressure, oxygen saturation, and end tidal carbon dioxide were continuously performed. After preoxygenation with 100% oxygen for 3 minutes, anesthesia was induced with fentanyl 1 mcg/kg followed by propofol 2-3 mg/kg. The patients’ lungs were manually ventilated with isoflurane (1.5–2.0%) in oxygen; pancuronium 0.08–0.1 mg/kg was administered. The patient’s trachea was intubated at 3 minutes after the induction of anesthesia. Anesthesia was maintained with fentanyl, pancuronium, and isoflurane (0.75–1.0%) in a mixture of nitrous oxide and oxygen 2 : 1.
The success rate of tracheal intubation in both groups was evaluated. The number of insertion attempts, intubation time needed, total time to intubation, hemodynamic change, and complications was also recorded. The intubation time defined as the time needed from insertion of the laryngoscope into the oropharynx to the time of its removal. The total time to intubation was the sum of the durations of all (as many as three) intubation attempts.
Results were expressed as mean±SD or percentage (%), when appropriate. Comparisons between Macintosh laryngoscope and Miller laryngoscope groups were compared by using with Chi-square tests (for categorical variables), Chi-square tests for trend (for ordinal variables), and two-sample independent
Of the total 119 patients randomized, 59 patients were randomized to group MC while 60 patients to group ML. Table
Patients’ characteristics (mean, SD, and percentage).
Macintosh | Miller | ||
( | ( | ||
Age (yr) (mean, SD) | 41.8 (7.9) | 42.5 (11.2) | .566 |
Gender (%): | |||
Male | 12 (20.3) | 11 (18.3) | .782 |
Female | 47 (79.7) | 49 (81.7) | |
Weight (kg) (mean, SD) | 55.2 (10.3) | 53.3 (7.4) | .105 |
Height (cm) (mean, SD) | 154.3 (6.0) | 155.8 (6.4) | .448 |
Body mass index (kg/ | 23.2 (4.0) | 22.0 (3.1) | .443 |
ASA physical status (%): | |||
I | 40 (67.8) | 38 (63.3) | .608 |
II | 19 (32.2) | 22 (36.7) | |
Mallampati score (%): | |||
1 | 42 (71.2) | 45 (75.0) | .639 |
2 | 17 (28.8) | 15 (25.0) |
Overall success rate, intubation attempt, intubation time, and the lowest oxygen saturation during intubation attempt are shown in Table
Overall success rate, intubation attempt, intubation time, and the lowest SpO2 during intubation attempt.
Macintosh | Miller | ||
( | ( | ||
Overall success rate (%) | 59 (100.0) | 47 (78.4) | |
Intubation attempt (%) | .037* | ||
1 | 59 (100.0) | 42 (70.0) | |
2 | 0 | 4 (6.7) | |
3 | 0 | 1 (1.7) | |
Intubation time (sec) (mean, SD) | 25.4 (10.7) | 46.9 (17.7) | |
Lowest SpO2 during intubation attempts (mean, SD) | 98.3 (0.6) | 98.0 (0.6) | .929 |
*Considered statistically significant.
Table
Hemodynamic parameters: systolic and diastolic blood pressure (mmHg), heart rate (beat/minute), and oxygen saturation (SpO2, %) (mean, SD).
Macintosh | Miller | ||
(59) | (47) | ||
Baseline | |||
SBP, DBP | 121.4 (11.9), 71.2 (9.6) | 122.4 (16.5), 70.5 (13.4) | .113, .082 |
HR, SpO2 | 81.0 (10.9), 99.8 (0.4) | 76.0 (11.5), 99.7 (0.6) | .148, .129 |
At insertion | |||
SBP, DBP | 131.0 (19.1), 73.2 (12.0) | 123.0 (20.0), 71.2 (14.0) | .145, .104 |
HR, SpO2 | 80.5 (13.2), 99.8 (0.4) | 78.3 (11.7), 99.9 (1.9) | .036*, .088 |
1 min | |||
SBP, DBP | 129.7 (18.1), 73.7 (10.5) | 122.4 (16.0), 72.1 (12.2) | .270, .166 |
HR, SpO2 | 80.2 (10.4), 99.8 (0.5) | 79.6 (13.2), 99.6 (0.8) | .130, .361 |
2 min | |||
SBP, DBP | 130.0 (16.4), 77.3 (9.4) | 123.8 (17.2), 73.8 (11.1) | .033*, .032* |
HR, SpO2 | 82.9 (10.0), 99.7 (0.7) | 78.0 (11.6), 99.6 (0.7) | .088, .510 |
3 min | |||
SBP, DBP | 121.5 (12.6), 71.2 (9.3) | 121.4 (16.6), 73.2 (9.7) | .315, .086 |
HR, SpO2 | 80.5 (9.9), 99.7 (0.5) | 77.6 (8.7), 99.7 (0.6) | .069, .610 |
Immediately after TT | |||
SBP, DBP | 131.4 (15.8), 76.6 (10.2) | 132.8 (14.9), 85.6 (9.0) | .709, .024* |
HR, SpO2 | 85.1 (6.3), 99.6 (0.6) | 93.7 (7.5), 99.6 (0.6) | .013*, .929 |
2 min after TT | |||
SBP, DBP | 120.8 (14.6), 74.4 (11.2) | 132.6 (17.7), 77.2 (11.2) | .019*, .244 |
HR, SpO2 | 84.3 (10.0), 99.6 (0.7) | 82.6 (8.6), 99.8 (0.4) | .007*, .323 |
4 min after TT | |||
SBP, DBP | 121.3 (17.3), 73.0 (11.0) | 128.7 (17.7), 77.1 (7.6) | .144, .172 |
HR, SpO2 | 83.5 (10.2), 99.8 (0.4) | 80.7 (7.0), 99.7 (0.4) | .142, .807 |
6 min after TT | |||
SBP, DBP | 118.3 (16.4), 70.5 (11.4) | 123.7 (12.9), 76.3 (8.2) | .352, .196 |
HR, SpO2 | 83.4 (9.5), 99.8 (0.5) | 79.7 (7.4), 99.8 (0.4) | .087, .517 |
8 min after TT | |||
SBP, DBP | 117.6 (14.5), 69.6 (9.7) | 122.5 (10.0), 76.3 (6.5) | .432, .231 |
HR, SpO2 | 83.2 (8.3), 99.7 (0.7) | 81.9 (6.2), 99.8 (0.4) | .162, .166 |
10 min after TT | |||
SBP, DBP | 120.6 (17.1), 71.9 (12.3) | 113.2 (31.4), 73.3 (9.1) | .175, .181 |
HR, SpO2 | 84.1 (7.5), 99.8 (0.5) | 84.1 (7.5), 99.8 (0.5) | .177, .763 |
SBP: Systolic blood pressure; DBP: Diastolic blood pressure; HR: Heart rate; SpO2: Oxygen.
Saturation; TT: Tracheal intubation.
*Considered to be of statistical significance.
Procedure related complications during and immediately after tracheal intubation were demonstrated in Table
Procedure related complications during and immediately after tracheal intubation (
Macintosh | Miller | ||
( | ( | ||
Overall | 2 (3.4) | 8 (13.3) | .435 |
Oral mucosa laceration | 2 (3.4) | 4 (6.7) | .414 |
Bleeding | 0 | 3 (5.0) | .082 |
Dental injury | 0 | 1 (1.7) | .319 |
*Considered to be of statistical significance.
This prospective randomized study compared clinical performances of two laryngoscopes in the hands of inexperienced users. Our study demonstrated that the Macintosh provides superior intubating conditions in the normal airway with regards to successful intubation, as judged by achieving adequate ventilation within three insertion attempts. All tracheal intubations in the Macintosh group were successful in the first attempt; whereas 13 patients (21.6%) in the Miller group were failed. In additional, the failed intubations in the Miller group could be successfully intubated by using the Macintosh laryngoscope.
Orotracheal intubation by using the Macintosh, the tip of this blade should be placed into the angle made by the epiglottis with the base of the tongue. Elevation of the laryngoscope pushes the base of the tongue upward; whereas the epiglottis is drawn upward, providing a clear view of the larynx. On the other hand, the Miller blade is straight; however, the tip should extend just behind (posterior to) or beneath the laryngeal surface of the epiglottis [
Many types of laryngoscopes have been used for tracheal intubation [
The Macintosh resulted in less stimulation of systolic and diastolic blood pressure and heart rate following tracheal intubation in comparison with the Miller laryngoscope, but not significantly different. The hemodynamic findings for direct laryngoscopy in our study were similar to those described previously [
The reported incidences of complications in both groups were very low. As might be expected in this study of patients at low risk for difficult laryngoscopy, there were no incidences of serious complications with either laryngoscope.
Our study has some limitations in many respects. First, as data were collected unblinded, some bias is possible. Second, there is no well-defined, acceptable to total time to intubation in the literature. As a consequence, for the purpose of this study, successful tracheal intubation was defined as either an ability to place the endotracheal tube within three attempts or, there was a nonsignificant alteration of the hemodynamic and respiratory parameters. Third, the traditionally laryngoscopic tracheal intubation teaching for nonanesthesia personnel using manikin alone is a controversy [
This study shows that novice laryngoscope users could successfully intubate a patient’s trachea after viewing a Power Point presentation and a manikin practicing. Orotracheal intubation by using the Macintosh laryngoscope is an effective and safe technique in nonexperienced hands with significantly increased success rate and decreased mean total time to intubation as compare to the Miller laryngoscope. However, the results of our study only apply to the selected patients deemed to have normal airways.