INTRODUCTION
Double-lumen tubes (DLTs) are the gold standard for lung isolation during operations in the thoracic cavity. However, they are longer and larger in diameter as compared to normal endotracheal tubes, which makes their insertion challenging. Complications such as mucosal injury, hoarseness, and sore throat are higher with DLT placement owing to decreased vision on insertion.[1 2 3 4
MATERIALS AND METHODS
The study was conducted between May and August 2017 in the operating rooms of the department of thoracic surgery in a tertiary care teaching hospital on patients requiring lung isolation. The study was designed to be an open-label prospective randomized controlled clinical trial with block randomization. Recruitment commenced after institutional review board approval (IRB Min. No. 10550 dated February 15, 2017) and registration of the trial with the Clinical Trials Registry of India (CTRI) - REF/2017/08/015114. All eligible patients were recruited after obtaining written informed consent on the day prior to the operation. There were no changes made to the trial design or protocol during the course of the study.
Seventy-three consenting adults of American Society of Anesthesiologists (ASA) physical status (PS) classes I and II, between the ages of 18 and 70 years, with no anticipated difficult airway (El Ganzouri Risk Index ≤4), who were scheduled to receive left DLT, were recruited.[5
Patients were randomly assigned to receive CMAC D-blade (Group D) or Macintosh (Group M). Permuted block randomization of size 2, 4, or 6 was used to generate the random sequence using SAS 9.1.3 software SAS (Statistical Analysis System), Cary, North Carolina, United States. This computer-generated randomization was placed in serially numbered, opaque, sealed envelopes in the operating theater. The sealed envelopes were opened, and the appropriate intervention was informed to the primary anesthesiologist before the induction of anesthesia by another anesthesiologist not involved in the study. The primary anesthesiologist could not be blinded to the laryngoscope used. Postoperative complications such as sore throat and hoarseness were evaluated by a staff nurse in the ward who was blinded to the primary intervention. The statistician analyzing the outcomes was blinded as well.
Interventions
A standardized anesthetic protocol was followed. All patients received ASA standard monitoring with invasive blood pressure monitoring. After preoxygenation (end-tidal EtO2 >90%), anesthesia was induced with intravenous (i.v.) injection of propofol (1.5 mg.kg−1 ) and fentanyl (2–3 μg.kg−1 ). Additional propofol boluses (maximum up to 3 mg.kg−1 ) were titrated to loss of verbal response. Tracheal intubation was facilitated by i.v. vecuronium (0.1 mg.kg−1 ). Intubation was carried out as per the randomization, either with the conventional Macintosh laryngoscope or with the CMAC D-blade laryngoscope using the appropriately sized DLT.
All participating anesthesiologists were trained for DLT insertion with both laryngoscopes, ten times each on TruCorp AirSim Bronchi mannequin (TruCorp© 2017, Ireland) and five times each in patients, prior to participation in the study. The selection of the DLT (Mallinckrodt Medical, Athlone, Ireland) was based on sex, height, and the left main bronchus diameter on computed tomography scan as per standard recommendations.[6
The distal bronchial curve of the DLT was adjusted to match the shape of the laryngoscope with the stylet [Figure 1 ]. After laryngoscopy was performed with the allotted laryngoscope, and the glottis was visualized, the DLT with the stylet was introduced with the distal concavity facing anteriorly. Once the blue cuff was beyond the vocal cords, the stylet was removed, and the tube was rotated 90° counterclockwise and inserted until mild resistance was perceived.
Figure 1: Curvature of DLT obtained with the specified stylets of the different laryngoscopes (Macintosh, CMAC-Macintosh, and CMAC D-blade). DLT = Double-lumen tube
It was stated in the study protocol that in the event of an unanticipated difficult airway, the patient would be excluded from the study and standard Difficult Airway Society guidelines would be followed.[2
Outcomes
The primary outcomes were the time taken for visualization of the glottis and the time for intubation. The time for visualization of the glottis was defined as the time from the introduction of the laryngoscope into the mouth until an optimal view of the glottis was obtained as decided by the intubating anesthesiologist. The time for intubation was defined as the time from the introduction of the laryngoscope into the mouth until three complete capnographic cycles were visualized on the monitor. This was measured by an independent observer, using the timer on the anesthesia monitor. The position of the DLT was confirmed with a fiber-optic bronchoscope for every case.
The subjective difficulty of intubation was measured using the IDS,[7
Complications, such as esophageal intubation, airway trauma evidenced by blood on the laryngoscope blade, or obvious lacerations after intubation, were documented by the independent observer. Patients were also checked by the primary investigator for up to 3 postoperative days to assess for soreness of the throat and hoarseness of voice.
Sample size
The sample size was calculated to show a minimum difference of 10 s in the mean intubation time. This is considered clinically important when using CMAC D-blade as compared to Macintosh blade based on previous studies.[8
Statistical methods
Summary data were presented as mean (standard deviation) for continuous variables and as numbers and percentages for categorical variables. The characteristics of patients in Group D and Group M were compared using t -test for continuous data. Categorical data were compared using Chi-square/Fisher's exact test as appropriate. Statistical significance was defined as P < 0.05. All analyses were performed using SPSS version 25.
RESULTS
A total of 82 patients were screened, of which 9 patients were excluded as they did not meet the inclusion criteria or did not consent to participate in the study. Of the 73 consenting patients, 36 were assigned to Group D and 37 to Group M. The flow of participants is shown in the CONSORT diagram [Figure 2 ]. Four consultants and three trainees who were trained prior for the use of CMAC and DLT performed all the intubations. All patients received left DLT as part of the protocol. Both the groups were comparable in terms of demographic parameters such as age, sex, height, BMI, and baseline airway characteristics as classified by the El Ganzouri Index [Table 1 ].
Figure 2: CONSORT flow diagram
Table 1: Baseline demographic parameters and El Ganzouri multivariate indicator for airway difficulty
The outcome measures are summarized in Table 2 . The mean time taken to visualize the glottis was significantly less in Group D as compared to Group M (4.56 ± 2.39 s vs. 7.27 ± 4.891 s, P = 0.01). However, the mean intubation time was comparable between Groups D and M (55.92 ± 18.749 s vs. 51.08 ± 15.269 s, P = 0.61).
Table 2: Comparison of outcome measures
The first-attempt success rate was 83.3% (30/36 cases) in Group D, and 81.1% (30/37) in Group M, with no statistically significant difference (P = 0.689).
The use of the Macintosh blade was associated with a significantly higher incidence of poor laryngoscopic grade (64.9% vs. 94.4%, P = 0.007), the requirement of ELM (43.2% vs. 19.4%, P = 0.043), and difficulty of intubation as estimated by the IDS (3.39 vs. 2.86, P = 0.046) [Table 2 ].
The time taken to intubate can vary with the experience of the person who performs the intubation. The consultant anesthesiologists took less time to intubate, with either of the laryngoscopes than postgraduate trainees, but this difference was significant only with the CMAC D-blade. A subgroup analysis showed that the consultants took significantly less time to intubate with D-blade (41.6 ± 4.83 s vs. 48.36 ±, P = 0.047) compared to the trainees (61.42 ± 19.34 s vs. 52.23 ± 16.6 s, P = 0.043) [Table 3 ].
Table 3: Time to intubate and first-attempt success across operators of varied experience
The incidence of soreness of throat and hoarseness was significantly less in Group D (11.1% vs. 27.9%, P = 0.045) as compared to Group M (5.6% vs. 21.6%, P = 0.047) [Table 4 ].
Table 4: Complications
There was one case of esophageal intubation in Group M. None of the patients in either group had lacerations or blood on the laryngoscope blade. There were two instances of cuff tears in Group D. All the patients were intubated within two attempts.
DISCUSSION
The key findings of the study were that the time taken to visualize the glottis was less in patients who were intubated using the CMAC D-blade (Group D) compared to those intubated using a Macintosh blade (Group M). However, this did not eventually translate to a faster intubation time. There were fewer patients in Group D with poor laryngoscopic grade, high intubation difficulty scores, and requiring ELMs to assist in tube placement [Table 2 ].
Shah et al . compared the Macintosh and CMAC D-blade in the setting of onco-surgical patients and had similar intubation times (37.41 + 18.8 vs. 32.27 + 11.13).[9 P = 0.047). Shah et al . had significantly less first-attempt success (53% with Macintosh and 87% with CMAC D-blade) as compared to our study (81.1% with Macintosh and 83.3% with D-blade). Moreover, a majority of their failures requiring reintubation were associated with longer intubation times (more than 180 s) which can result in morbidity in this high-risk population. Unlike our study, they chose onco-surgical patients, where 20% of the patients were classified as having a difficult airway (5 patients in the Macintosh group and 6 patients in the D-blade group having El Ganzouri Index ≥4), accounting for a lower first-attempt success rate.
A similar observation in terms of time to intubate has been highlighted by Hsu et al . with the GlideScope as compared to the Macintosh, where two experienced consultants did all the intubations.[10
Russell et al . compared the GlideScope and the Macintosh laryngoscope for DLT insertion with 30 anesthesiologists of varied experience, and observed that the time to intubate was longer with the GlideScope (70 s vs. 32 s). These observations are similar to those in our study where trainees took a significantly longer time to intubate with CMAC D-blade as compared to the Macintosh blade (61.42 ± 19.34 s vs. 52.23 ± 16.6 s, P = 0.043). It must be noted here that introduction and advancement of a DLT using an angulated blade is technically more difficult despite a better CL grade, especially for inexperienced operators.[11
A recent meta-analysis by Liu et al .[12
Both in the meta-analysis and in our study, the time taken to intubate was less with videolaryngoscopes where the operators had significant experience in the use of the videolaryngoscope for DLT insertion. This illustrates the importance of training in videolaryngoscopy to overcome the learning curve.[13 11 et al . observed progressive improvement in first-pass success of tracheal intubation with the GlideScope over a 7-year period.[13
In their meta-analysis comparing different videolaryngoscopes for DLT insertion, Kim et al . found that the Macintosh laryngoscope was significantly better in terms of time to intubate.[14 P = 0.043), as once the three axes were aligned and the glottis visualized, the intubation with DLT was fast. However, the consultants who had significant experience with the use of D-blade, were able to perform DLT intubation significantly faster with it.
Videolaryngoscopes, theoretically, enable quick tube placement, as the wide viewing angle (80° in case of the CMAC D-blade) preempts the need to align the oral, laryngeal, pharyngeal axes of the airway.[15 et al .) have studied videolaryngoscopes as channeled, nonchanneled and video stylets, but none of the studies have focused on differences between the traditional Macintosh laryngoscope and hyperangulated blades.[16 12 Figure 3 ]. The superior glottic view offered by the D-blade did not translate into faster intubation time. This is because the tip of the DLT is angulated to match the curve of the hyperangulated D-blade, which impinges in the subglottis, necessitating manipulations and resulting in increased intubation times. This is easily overcome by the withdrawal of the stylet by 0.5–1 cm behind the tip of DLT, which significantly reduces impingement in the subglottis. The removal of the hyperangulated stylet immediately after the blue cuff of the DLT passes the glottis, followed by a 90° counterclockwise rotation, facilitates smooth intubation [Figure 3 ]. Bustamante et al . described a successful maneuver of sequential rotation with the removal of the stylet after entering the glottis using a GlideScope.[17 et al . which showed significant malpositions with videolaryngoscopes. We did not do the 180° rotation as described by Bustamante et al ., as a rotation of more than 90° increases the risk of intubation of the opposite side, necessitating repositioning under the guidance of a fiber-optic bronchoscope.[17
Figure 3: X-ray showing the hyperangulated DLT abutting the anterior tracheal wall. DLT = Double-lumen tube
Angadi and Frerk proposed that videolaryngoscopes and Macintosh laryngoscope should not be compared on the CL grade alone and the IDS should be used instead.[18 P = 0.046) despite similar first-attempt success rate and intubation times. This is because the use of the D-blade resulted in better CL grades (94.4 vs. 64.9 CL1), with less lifting force, thus resulting in significantly less external laryngeal manipulations (19.4% vs. 43.2%). Our results were similar to Kim et al . who found that fewer external laryngeal manipulations were required when a videolaryngoscope was used.[14
The incidence of sore throat and hoarseness was significantly less with the CMAC D-blade (11.1% vs. 27.9%). Our results are similar to those observed in the meta-analysis by Liu et al . and may be attributed to a magnified glottic view available on the monitor screen during intubation.[12
Two cases in the CMAC D-blade had cuff tears. There are two possible explanations for this. First, the flange height of the CMAC D-blade is less (1.6 cm) as compared to the Macintosh laryngoscope (2.5 cm). Second, the anesthesiologist's attention may have been diverted away from the mouth to the screen during the initial manipulation of the tube, resulting in the cuff inadvertently impinging against the teeth. We corrected this immediately by re-emphasizing that the correct technique of video laryngoscopy which is described as (a) to look into the mouth during the introduction of the D-blade, (b) view the screen to obtain the desired view of the laryngeal inlet, (c) to look again into the mouth during initial introduction of the tube till the tracheal cuff passes the teeth, (d) to increase the lifting force to reoptimize the view and intubate. Adherence to this technique helped us to keep the incidence of airway injuries and DLT cuff damage to a minimum in our study.[19
The limitations of our study were that first, it was impossible to blind the anesthesiologist to the type of laryngoscope being used, and this could have introduced bias.
Second, the level of difficulty of insertion of the DLT was assessed using the IDS, which included the subjective feel of lifting force required or the need for external laryngeal manipulations; all of these are operator dependent.
Third, we did not use Percentage of Glottic Opening Score to grade the laryngoscopic view, which is more relevant for videolaryngoscopes.[20
CONCLUSION
The CMAC D-blade videolaryngoscope resulted in a shorter time to visualize the glottis, with a significantly better glottic view, superior IDS score, and reduction in sore throat and hoarseness. However, this did not translate to a faster intubation time or a better first-attempt success rate in our study. We conclude that there is no advantage in the routine use of CMAC D-blade for DLT insertion in patients with no anticipated airway difficulty, especially in resource-poor settings.
Financial support and sponsorship
This study was financially supported by the Research Fund of Christian Medical College, Vellore.
Conflicts of interest
There are no conflicts of interest.
Acknowledgment
We would like to thank all the supporting technical staff, the department of anesthesia, and our patients who made this study possible.
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