The Rigid Tube as an Alternative in Controlling the Problematic Airway

Summary

Presented here is a protocol of tracheal intubation over an intubating tube introducer using a rigid tube for laryngoscopy with an attached light source. The main characteristics of this maneuver are the retromolar approach and the use of the thyroid cartilage as a landmark while advancing the rigid tube.

Abstract

The unexpected problematic airway represents a large proportion of anesthesia-related morbidity and mortality. The retromolar or paraglossal approach is an alternative to the majority of the rigid instruments used for tracheal intubation, which follow the midline to access the glottis. This single-center, prospective case-series study offers an option to conventional laryngoscopy in case of a poor glottic view, introducing an instrument (the rigid tube for laryngoscopy) that uses the retromolar approach to accomplish tracheal intubation. If after anesthesia induction, the modified Cormack-Lehane glottis view grade >2b, the intubation is carried further with the rigid tube. The tube follows the direction of the thyroid cartilage while advancing from the labial commissure, displacing the tongue to the contralateral side. Adjusting the position of the larynx with the nondominant hand by gently pushing the thyroid cartilage and following an imaginary line towards it while advancing it improves the time needed for proper glottis visualization. Once the epiglottis is in sight, the practitioner progresses slowly, lifting the epiglottis and aiming the tip of the tube more anteriorly. When the glottis appears in the visual field, the intubating tube introducer is placed in the trachea, and a lubricated cuffed intubating tube is advanced over the introducer after the rigid tube is extracted. This tool was tested on 30 patients with an unsatisfactory glottic view when using the Macintosh laryngoscope and obtained excellent results with respect to intubation time and complications. The reduced visual field is the main limitation of this method, which requires a training period for reasonable expertise. This simple, robust, and cheap instrument could be a rescue option in case of a difficult airway.

Introduction

Airway management incidents represent a large proportion of anesthesia-related mortality and severe morbidity despite the development of new tools for airway control and extensive educational resources¹. The video laryngoscope and flexible or rigid endoscopes are big steps forward, but they require a supplementary investment in addition to training^2,3. Difficult intubation is often unanticipated, so it is the practitioner's responsibility to have a backup plan and use the appropriate tool from those available⁴. This study aims to show that a cheap and straightforward tool, the rigid tube for laryngoscopy (RTL), could be efficiently used for tracheal intubation in patients with poor glottis visualization during conventional laryngoscopy.

The rigid tube for laryngoscopy (Figure 1) is a 15-25 cm long, straight, round hollow tube with a 5-20 mm diameter and a bevel end distally. It resembles a rigid bronchoscope or esophagoscope, but shorter⁵. A connective piece that has a prismatic light deflector with connection to a light source is attached to the proximal end of the rigid tube when in use, and the image is obtained directly, looking through this assembly. It is an instrument used by ear, nose, and throat specialists to visualize the larynx and its vicinity. In a pilot study, 20 patients with supraglottic pathology and anatomically challenging airways were intubated with the rigid tube, and the results were encouraging⁶.

This research included patients with anatomical criteria for difficult intubation and a modified Cormack-Lehane⁷ glottis view grade ≥2b when using a curved blade laryngoscope. The purpose was to demonstrate that laryngoscopy using the rigid tube with a retromolar approach could offer a quick glottic view in cases with poor glottic visualization at conventional laryngoscopy and the right conditions to intubate the trachea over the intubating tube introducer.

Protocol

We designed a prospective, observational, single-center study evaluating the feasibility of tracheal intubation using the RTL with the approval of the University Ethics Committee no 432/24.11.2016 and registered with ClinicalTrials.gov NCT03341507. The study involved adult patients with ASA physical status 1–3, requiring general anesthesia for ear, parotid, and rhino-sinus surgery and having a presumed anatomically difficult airway as calculated with The Simplified Airway Risk Index (SARI) score⁸, with a score ≥ 5, and no airway pathology.

NOTE: The pre-anesthetic exam focused on airway evaluation aiming to select patients with a modified Cormack-Lehane⁷ grade of glottic visualization ≥2b. The Cormack-Lehane classification defines the grade of glottis view during laryngoscopy, ranging from unrestricted glottic visualization-grade 1 to inability to see any laryngeal structure-grade 4. The SARI score⁸ is a multivariate risk score for predicting difficult tracheal intubation. A SARI score of 4 or above, 12 being maximum, enhances the chances of difficult intubation. Seven parameters contribute to the SARI score: mouth opening, thyromental distance, movement of the neck, Mallampati score, the ability to create an underbite, body weight, and previous intubation history.

1. Patient and equipment preparation

1. Premedicate with 1 or 2 mg of midazolam intravenously for 10 min upon patient transportation in the operating theatre.
2. Have the patient lie on the operating table with their head placed at a tilt in a sniffing position.
3. Apply standard hemodynamic and respiratory monitoring: ECG, noninvasive arterial blood pressure, oxygen saturation, respiratory frequency, end-tidal CO₂. Monitor patient anesthesia depth using bispectral index monitoring.
4. Make sure that an experienced anesthetist or ENT physician is available in case of an emergency.
5. Prepare a suction device with a suction probe in place and a difficult airway kit at hand. Prepare and check the connective piece with the light source attached with a set of rigid tubes of various sizes: three tubes that are 15-20 cm long with a 1.2-1.8 cm diameter for women and three tubes that are 20-25 cm long with a 1.2-2 cm diameter for men.
6. Identify the thyroid cartilage (Adam's apple) by palpation of the anterior neck. Ask the patient to swallow and gently palpate the anterior neck with the thumb, the index, and the middle finger. The thyroid cartilage is the structure that moves while the patient swallows.
   1. Ask the patient to extend the head and measure the distance between the labial commissure and the thyroid cartilage in a straight line using the thumb and the middle finger. Hold a rigid tube in the other hand while performing the measurement. The mandible impedes the use of a ruler to measure this distance.
   2. Note the length obtained on the rigid tube prepared, starting from its tip. Do this by directly translating the two fingers used to measure the distance mentioned in the previous step on the rigid tube.
   3. Expect to find the glottis when using the RTL at a depth equal with the distance between the labial commissure and thyroid cartilage. For a better grip and a useful lever effect, use a tube with a length of about double the above-measured distance.

2. Induction of general anesthesia and conventional laryngoscopy

1. Preoxygenate the patients for 5 min and start anesthesia induction with 2-3 µg/kg fentanyl, 2-3 mg/kg propofol, and 1-1.5 mg/kg succinylcholine. Monitor the depth of anesthesia by checking the bispectral index level.
2. After 1 min of mask ventilation with a pause during generalized muscular fasciculation provoked by succinylcholine, perform standard laryngoscopy with a curved blade laryngoscope, and register the modified Cormack-Lehane glottis visualization grade.
   1. If the noticed degree is <2b, resume the tracheal intubation conventionally with the curved blade laryngoscope.
   2. If the glottis view grade is ≥2b, meaning that, at the best, part of the arytenoids and the epiglottis are visible, withdraw the Macintosh laryngoscope, and maneuver further with the RTL.
      CAUTION: If the mask ventilation is impossible or not efficient, the airway should be secured promptly following the institutional guidelines.

3. Tracheal intubation with the rigid tube

1. Cover the upper molars with a rubber teeth protector or a cotton swab on the side of the approach.
2. Introduce the RTL in the oral cavity at the level of the right or left labial commissure with the bevel facing the superior arcade, the prismatic light deflector, and a functioning light source attached, held with the dominant hand with the light deflector inside the palm and the index following the tube.
   1. Start with a 1.5-1.8 cm diameter tube to maximize the visual field.
   2. Displace the tongue gently to the contralateral site.
   3. If the pathology does not require otherwise, a righthanded practitioner should use the right retromolar space for approach.
3. Use the nondominant hand to open the patient’s mouth, hyperextend the patient's head, and advance the RTL towards the pharynx.
4. Apply gentle pressure on the proximal end of the RTL against the protected superior molars as the tip of the tube is reaching the hypopharynx. That way, the tip of the RTL orientates anteriorly on the direction of the glottis.
   NOTE: The thyroid cartilage is a landmark when moving the tip of the RTL from the retromolar space towards the midline.
   1. If the epiglottis, the glottis or part of it is not getting into sight, withdraw the tube. At the second attempt, insert the tube in the oral cavity at the level of the incisors as that may make the tongue base and epiglottis easier to access. Once the epiglottis is in sight, slide the tube towards the labial commissure, keeping its tip in place, and advance further.
   2. Aspirate the secretions as needed with a flexible suction probe inserted through the RTL.
   3. Adjust the position of the larynx with the nondominant hand by gently pushing the thyroid cartilage and follow an imaginary line towards the thyroid cartilage as it improves the time needed for proper glottis visualization.
      NOTE: When moving the cartilage while looking through the tube, the motion translates to the inner structures around the glottis and helps the practitioner orientate to reach his target.
5. Advance the RTL slowly until it reaches the epiglottis, and then lift the epiglottis with the tip of the device. Make sure that the bevel is oriented posteriorly. At that point, the vocal cords are visible at the distal end of the tube.
   1. If, after lifting the epiglottis, the glottis is not in the visual field, consider using a smaller diameter RTL.
6. Once the glottis is in sight, place the intubating tube introducer through the RTL into the trachea. Do not insert the introducer too vigorously because the bronchial injury may occur.
   1. Do not advance any more as soon as the glottis is visible after lifting the epiglottis because of the risk of vocal cord injury.
7. Extract the rigid tube.
8. Place a standard cuffed lubricated intubating tube over the introducer into the trachea. Do not use force and gently rotate the intubating tube while advancing.
   1. Make sure there is only a small gap between the inner diameter of the intubating tube and the introducer to reduce the chance of impingement.
9. Once the intubating tube is at the proper depth, 20-24 cm from the labial commissure, remove the intubating tube introducer leaving the intubating tube in place.
10. Inflate the cuff and confirm tracheal intubation through lung auscultation and capnography.
11. Inspect the upper lip and the teeth for damage. Be aware that the intubation over an intubating tube introducer carries the risk of vocal cords injury. Check the patient for sore throat and voice disturbance after surgery.
    CAUTION: Stop the maneuver if it exceeds 100 s or if the patient desaturates to 80% and resume mask ventilation or an alternative method of securing the airway.

Representative Results

Over 24 months, we included 64 patients with a SARI ≥5, predictive for difficult intubation (Supplemental File 1). Thirty of them presented a modified Cormack-Lehane glottis view grade ≥2b during the laryngoscopy performed with the curved blade laryngoscope, so they were attempted to be tracheal intubated with the RTL (Table 1). In all cases, with one exception, the procedure with the rigid tube was successful without any incidents occurring. The patient we did not intubate, with a grade 3 Cormack-Lehane glottis view, desaturated to 80% in less than a minute, so we decided to place a laryngeal mask since the patient would have ear surgery.

In most of the cases (two-thirds), the intervention was straightforward, and the glottis was in sight in less than 25 s from the start of the maneuver with the RTL with a median total time for intubation of 50 s (Figure 2). The advancement of the intubating tube over the intubating tube introducer lasted between 20 and 30 s and was uneventful also. We did not find a statistical correlation between the time needed for intubation and the Cormack-Lehane glottis view grade at conventional laryngoscopy.

The median oxygen saturation after tracheal intubation was 95%, and the lowest desaturation was 83%. We did not notice any significant complication during or after the procedure: five patients complained of sore throat, and three cases presented minor injury of the upper lip.

Figure 1: The rigid tube for laryngoscopy with the connective piece. Please click here to view a larger version of this figure.

Figure 2: Intubation time with the rigid tube.
The extreme values are indicated on the bottom respectively on the top of the chart, while the median value and the interquartile range values are filled with grey. Please click here to view a larger version of this figure.

Total,n	30
Age mean±SD	47.8 ±10.5
Female sex, n	4
BMI mean±SD	33.2 ±4.5
ASA (1/2/3),n	5/14/11
SARI score median	6
Baseline oxygen saturation (%) median (IQR)	98(97-99)
Cormack-Lehane (2b/3/4),n	16/12/2.

Table 1: The essential pre-anesthetic features of the patients included in the study. ASA-American Society of Anesthesiologists; BMI- Body Mass Index; SARI- Simplified Airway Risk Index; IQR- Interquartile Range

Supplemental File 1: Experimental Flow Diagram. Please click here to download this file.

Discussion

The most prevalent mistake when dealing with an unexpected problematic airway is to insist on the same tool and the same technique only to realize that it is not working. This makes the situation only worse by promoting edema or bleeding⁹.

We had the idea of testing this tool when we anesthetized a patient with a grade 4 glottic visualization at conventional laryngoscopy, according to the Cormack-Lehane classification. Since the rigid tube was prepared to investigate this patient, and we did not have another backup device at hand, we tried it using the retromolar approach and obtained a perfect glottis view.

Tracheal intubation with the RTL using the retromolar approach, resembling the Bonfil's Rigid Endoscope intubation technique¹⁰, offers an alternative when curved blade laryngoscopy is unsatisfactory. The improved view obtained with this technique may result from the reduced pressure applied on the tongue base and the consecutive decrease in the chance of backward displacement of the epiglottis¹¹. In addition, because of the tubular shape of the RTL, the intrusion of the soft structures into the line of sight is eliminated. The reduced soft tissue compression, as compared with the Macintosh technique, comes from a better airway alignment during the retromolar approach in case of the more anterior position of the glottis. Using either side is an advantage for lefthanded and in the case of pharyngeal or cervical masses⁶. Patients with protruding upper teeth and a reduced inter-incisors gap may also benefit from this technique⁶.

In our research, the median intubation time with the rigid tube was 50 s. In a study carried by Bein et al., 25 patients with a poor glottis view at conventional laryngoscopy were intubated with the Bonfil's rigid endoscope in a median time of 47 s¹². Another paper comparing two types of rigid stylets used in a retromolar manner reported 35-40 s to accomplish tracheal intubation¹³.

The thyroid cartilage being a landmark means that one should take care that the rigid tube orientates towards it during the RTL advancement. Accomplishing that requires a reasonable sense of spatial orientation as the investigator is alternatively looking through the device and following its progression in the direction of the thyroid cartilage. Once the tip of the RTL reaches the pharynx, the investigator adjusts the rigid tube direction, not looking through the tube but following with the tip of the device the location of the thyroid cartilage. When the tube and the thyroid cartilage are aligned, the operator is searching for the epiglottis and glottis looking through the instrument. From experience, following these steps will reduce the time needed for laryngoscopy. Inserting the tube in the oral cavity at the level of the incisors and sliding retromolar once the epiglottis is visible represents a viable alternative at the second attempt.

RTL is a compact instrument easy to decontaminate, resistant to long term use, and requires a minimal setup time; its cost of maintenance is lower than that of most of the tools used for airway control. The aspiration of saliva or blood proves facile by inserting a suction probe through the tube, and there is no risk of fogging since there is no distal optical system involved.

The main limitation of this method is the reduced visual field, proportional to the RTL diameter. Aiming the tip of the RTL towards the thyroid cartilage surpasses this deficiency in part. Dental injury and bleeding of the pharyngeal structures during the procedure are potential complications, especially if embedded with excessive force. The use of intubating tube introducers carries the risk of airway perforation if it is inserted too deep, vocal cord injury, and the impossibility of advancing the intubating tube in the trachea¹⁴. The technique needs a training period for reasonable expertise and regular use for proficiency preservation.

Although we did not find data in the literature regarding this method of airway approach, we are confident that this simple, cost-efficient technique, at hand in a hospital with an ENT department, might prove useful as a rescue option when a practitioner encounters difficult intubation.

Materials

Name	Company	Catalog Number	Comments
Airway management set	Karl Storz	11300 B3
Anesthesia machine	Draeger
Anesthetic drug: Fentanyl	Chiesi
Anesthetic drug: propofol	Fresenius
Anesthetic drug: succinylcholine	Takeda
Anesthetic drugs: midazolam	Aguettant
Intubation Tube	Touren		A 7-7.5 mm diameter is suitable for tracheal intubation over the bougie
Intubation Tube Introducer (Bougie)	Ontex
Lubricating gel	Dynarex
Mcintosh laryngoscope	Heine
Rigid tube with a removable connective piece that has a prismatic light deflector attached	Explorent GMBH		The connective piece serial number is: 650021 We did not find a current commercial offer for the rigid tube and the connective piece we used in this study.
Table top light source with light cable	Karl Storz	20134001, 61594GW