Force, torque, and stress relaxation with direct laryngoscopy

Pre-formed endotracheal tube and stepwise insertion for more successful intubation with video laryngoscopy

BACKGROUND

In anesthesia practice, orotracheal intubation remains the primary concern of the anesthesiologist. The introduction of video laryngoscopy (VL) has increased the success rate of orotracheal intubation; however, conflicting results have been reported regarding the usefulness of the current technique with VL in clinical practice.

AIM

To describe a modification to improve intubation with VL, followed by evaluation of the practice in vivo.

METHODS

First, a mannequin trial was conducted with operators having different experience and background. Then, a retrospective analysis was performed for an > 1-year period with patients who underwent general anesthesia with orotracheal intubation. The endotracheal tube used had been pre-formed with two curves. Stepwise intubation had been performed with direct eye vision, followed by screen assistance and rotation of the tube as needed to direct it toward the glottis. In the mannequin trial, the outcome measures were quantification of torque (force with angular acceleration during levering), need for external maneuvers, and time to intubate. In the clinical experience, orotracheal intubation used VL (pre-formed tube) or direct laryngoscopy (DL) at the anesthetist’s discretion and throat discomfort was reported by the patient.

RESULTS

In the mannequin trials using VL, there was less torque with the pre-formed tube than with a regular tube (8% and 65%, respectively). The first-pass rate was higher with the pre-formed tube (95%) than with a regular tube (81%). However, the time to intubate was longer with the pre-formed tube than with a regular tube (22 s and 12 s, respectively). In clinical practice, 562 patients underwent surgery under general anesthesia with orotracheal intubation using either VL (n = 244) or DL (n = 318) at the discretion of the attending anesthetist. VL was specifically planned in 62 of the patients, due to anticipated difficulty. Second attempts by readjustment of the curve of the tube were significantly fewer with VL than with DL (10% vs 18%). Throat discomfort was reported by fewer patients who underwent VL than those who underwent DL (6% vs 24%).

CONCLUSION

Pre-formed endotracheal tube with stepwise insertion produces less torque, fewer external maneuvers and higher first-pass success rate during VL intubation. Further, prospective studies are warranted.

Intubation Biomechanics: Clinical Implications of Computational Modeling of Intervertebral Motion and Spinal Cord Strain during Tracheal Intubation in an Intact Cervical Spine

BACKGROUND

In a closed claims study, most patients experiencing cervical spinal cord injury had stable cervical spines. This raises two questions. First, in the presence of an intact (stable) cervical spine, are there tracheal intubation conditions in which cervical intervertebral motions exceed physiologically normal maximum values? Second, with an intact spine, are there tracheal intubation conditions in which potentially injurious cervical cord strains can occur?

METHODS

This study utilized a computational model of the cervical spine and cord to predict intervertebral motions (rotation, translation) and cord strains (stretch, compression). Routine (Macintosh) intubation force conditions were defined by a specific application location (mid-C3 vertebral body), magnitude (48.8 N), and direction (70 degrees). A total of 48 intubation conditions were modeled: all combinations of 4 force locations (cephalad and caudad of routine), 4 magnitudes (50 to 200% of routine), and 3 directions (50, 70, and 90 degrees). Modeled maximum intervertebral motions were compared to motions reported in previous clinical studies of the range of voluntary cervical motion. Modeled peak cord strains were compared to potential strain injury thresholds.

RESULTS

Modeled maximum intervertebral motions occurred with maximum force magnitude (97.6 N) and did not differ from physiologically normal maximum motion values. Peak tensile cord strains (stretch) did not exceed the potential injury threshold (0.14) in any of the 48 force conditions. Peak compressive strains exceeded the potential injury threshold (-0.20) in 3 of 48 conditions, all with maximum force magnitude applied in a nonroutine location.

CONCLUSIONS

With an intact cervical spine, even with application of twice the routine value of force magnitude, intervertebral motions during intubation did not exceed physiologically normal maximum values. However, under nonroutine high-force conditions, compressive strains exceeded potentially injurious values. In patients whose cords have less than normal tolerance to acute strain, compressive strains occurring with routine intubation forces may reach potentially injurious values.

EDITOR’S PERSPECTIVE

Investigator-initiated, multicentre, open-label, two-arm, randomised controlled trial comparing intubating conditions in 25° head-up position and supine: the InSize25 study protocol

INTRODUCTION

Difficult airway management during tracheal intubation can lead to severe hypoxic sequelae. Routine intubation practice is to use a strict supine position, whereas a 25° head-up or reverse Trendelenburg position increases efficacy of preoxygenation, seems more comfortable for the anaesthetist and may also provide better intubation conditions in direct laryngoscopy. The 25° head-up position could be used for the whole population rather than only for obese patients, but there is no prospective randomised controlled trial with a robust design and large number of patients comparing strict supine against 25° intubation in operating room. The objective of the InSize25 study is to test the effect of these two patient positions on intubation conditions during laryngoscopy in scheduled surgery on non-obese patients.

METHODS AND ANALYSIS

InSize25 is an investigator-initiated, multicentre, open-label, two-arm, randomised controlled trial. The InSize25 study will randomise 1000 adult patients scheduled for surgery under general anaesthesia requiring intubation with neuromuscular-blocking drugs, candidates for direct laryngoscopy. The primary outcome variable is the view obtained during the first laryngoscopy without any external manipulation assessed using percentage of glottic opening. Important secondary outcomes are: Cormack-Lehane classification, number of attempts at laryngoscopy and at tracheal intubation, use of ancillary equipment (eg, bougies, alternative laryngoscope blades, videolaryngoscope) and manoeuvres (eg, laryngeal manipulation), comfort score for the anaesthetist, episodes of postinduction hypotension or desaturation and mechanical complications of intubation.

ETHICS AND DISSEMINATION

The trial received appropriate approval from the 'CPP Sud-Est II' ethical review board. Informed consent is required. If the 25° head-up position proves superior for tracheal intubation without more complications, it may become the routine-standard intubation position rather than only for use with obese patients. The final results will be published in a peer-reviewed journal.

TRIAL REGISTRATION NUMBER

Clinicaltrials.gov identifier (NCT03339141).

Mechanical strain to maxillary incisors during direct laryngoscopy

Background

While most Direct laryngoscopy leads to dental injury in 25–39% of cases. Dental injury occurs when the forces and impacts applied to the teeth exceed the ability of the structures to dissipate energy and stress. The purpose of this study was to measure strain, (which is the change produced in the length of the tooth by a force applied to the tooth) strain rate, and strain-time integral to the maxillary incisors and determine if they varied by experience, type of blade, or use of an alcohol protective pad (APP).

Methods

A mannequin head designed to teach and test intubation was instrumented with eight single axis strain gauges placed on the four maxillary incisors: four on the facial or front surface of the incisors and four on the lingual or back, near the insertion of the incisor in the gums to measure bending strain as well as compression. Anesthesiology faculty, residents, and certified registered nurse anesthetists intubated with Macintosh and Miller blades with and without APP. Using strain-time curves, the maximum strain, strain rate, and strain time integral were calculated.

Results

Across the 92 subjects, strain varied 8–12 fold between the 25th and 75th percentiles for all four techniques, but little by experience, while strain rate and strain integral varied 6–13 fold and 15–26 fold, respectively, for the same percentiles. Intubators who had high strain values with one blade tended to have high strains with the other blade with and without the APP (all pairwise correlation rho = 0.42–0.63).

Conclusions

Strain varies widely by intubator and that the use of the APP reduces strain rate which may decrease the risk of or the severity of dental injury.

Previsional space during direct laryngoscopy: Implication in the difficult laryngoscopy

The laryngoscope should displace oral soft tissues forward out of the operator's vision. Therefore, the space in front of the view may be critical for determining the laryngoscopic view. The aim was to investigate the difference in the previsional space during difficult versus easy laryngoscopy (EL).Under general anesthesia, digital photographs of the lateral view of the head and neck were taken in the horizontal sniffing position, after head extension, and during laryngoscopy with a defined force (50 N). Three points (thyroid notch (T), maxillary incisor (I), and mandibular mentum (M)) were marked on the photograph. The previsional space was defined as the TIM triangle. We compared these areas and other variables of the TIM triangle between male patients with difficult laryngoscopy (DL: Cormack-Lehane III-IV, n = 12) versus those of age- and body mass index-matched male patients with EL (Cormack-Lehane I-II, n = 12).When the head was extended, the areas TIM triangle in DL were significantly smaller than in EL. During laryngoscopy, all values of the TIM triangle in DL, including the TIM area (16.4 ± 3.7 vs 22.6 ± 2.8 cm, P < .01), were significantly smaller than the values in EL.The previsional space was smaller in patients with DL than in those with EL. The TIM triangle could suggest new way to explain the mechanism underlying DL.

I-gel Laryngeal Mask Airway Combined with Tracheal Intubation Attenuate Systemic Stress Response in Patients Undergoing Posterior Fossa Surgery

BACKGROUND

The adverse events induced by intubation and extubation may cause intracranial hemorrhage and increase of intracranial pressure, especially in posterior fossa surgery patients. In this study, we proposed that I-gel combined with tracheal intubation could reduce the stress response of posterior fossa surgery patients.

METHODS

Sixty-six posterior fossa surgery patients were randomly allocated to receive either tracheal tube intubation (Group TT) or I-gel facilitated endotracheal tube intubation (Group TI). Hemodynamic and respiratory variables, stress and inflammatory response, oxidative stress, anesthesia recovery parameters, and adverse events during emergence were compared.

RESULTS

Mean arterial pressure and heart rate were lower in Group TI during intubation and extubation (P < 0.05 versus Group TT). Respiratory variables including peak airway pressure and end-tidal carbon dioxide tension were similar intraoperative, while plasma β-endorphin, cortisol, interleukin-6, tumor necrosis factor-alpha, malondialdehyde concentrations, and blood glucose were significantly lower in Group TI during emergence relative to Group TT. Postoperative bucking and serious hypertensions were seen in Group TT but not in Group TI.

CONCLUSION

Utilization of I-gel combined with endotracheal tube in posterior fossa surgery patients is safe which can yield more stable hemodynamic profile during intubation and emergence and lower inflammatory and oxidative response, leading to uneventful recovery.

Comparing GlideScope Video Laryngoscope and Macintosh Laryngoscope Regarding Hemodynamic Responses During Orotracheal Intubation: A Randomized Controlled Trial

BACKGROUND

To determine if the GlideScope® videolaryngoscope (GVL) could attenuate the hemodynamic responses to orotracheal intubation compared with conventional Macintosh laryngoscope.

OBJECTIVES

The aim of this relatively large randomized trial was to compare the hemodynamic stress responses during laryngoscopy and tracheal intubation using GVL versus MCL amongst healthy adult individuals receiving general anesthesia for elective surgeries.

PATIENTS AND METHODS

Ninety five healthy adult patients with American Society of Anesthesiologists physical status class I or II that were scheduled for elective surgery under general anesthesia were randomly allocated to either Macintosh or GlideScope arms. All patients received a standardized protocol of general anesthesia. Hemodynamic changes associated with intubation were recorded before and at 1, 3 and 5 minutes after the intubation. The time taken to perform endotracheal intubation was also noted in both groups.

RESULTS

Immediately before laryngoscopy (pre-laryngoscopy), the values of all hemodynamic variables did not differ significantly between the two groups (All P values > 0.05). Blood pressures and HR values changed significantly over time within the groups. Time to intubation was significantly longer in the GlideScope (15.9 ± 6.7 seconds) than in the Macintosh group (7.8 ± 3.7 sec) (P< 0.001). However, there were no significant differences between the two groups in hemodynamic responses at all time points.

CONCLUSIONS

The longer intubation time using GVL suggests that the benefit of GVL could become apparent if the time taken for orotracheal intubation could be decreased in GlideScope group.

A comparison of hemodynamic changes after endotracheal intubation by the Optiscope™ and the conventional laryngoscope

Background

Optiscope™ is a newly developed video stylet device. This study evaluated and compared the hemodynamic changes observed after endotracheal intubation with video stylet and after conventional laryngoscopic endotracheal intubation.

Methods

Fifty-eight adult patients with American Society of Anesthesiologists (ASA) physical status class 1 or 2, undergoing general anesthesia, were randomized into two groups: one group of patients were intubated using video stylet (n = 29) and the other group were intubated using direct laryngoscope (n = 29). Systolic blood pressure (SBP), mean arterial pressure (MAP), diastolic blood pressure (DBP), heart rate (HR), POGO (percentage of glottic opening) score, time for intubation and degree of sore throat were recorded.

Results

There were no significant differences in the SBP, MAP, DBP, HR, and the sore throat incidence between the two groups. Optiscope™ produced better POGO scores, but time for intubation was longer than with conventional laryngoscope.

Conclusions

Optiscope™, when compared with conventional laryngoscope for intubation, does not modify the hemodynamic response, but it provides a better view of the vocal cords.

A Comparison of the Forces Applied to a Manikin during Laryngoscopy with the Glidescope® and Macintosh Laryngoscopes

The force applied during laryngoscopy can cause local tissue trauma and can induce cardiovascular responses and cervical spine movement in susceptible patients. Previous studies have identified numerous operator and patient factors that influence the amount of force applied during intubation. There are few studies evaluating the effect of different laryngoscope blades and no study involving video laryngoscopes. In this study we measured the forces using two laryngoscopic techniques. Three FlexiForce Sensors® (A201-25, Tekscan, Boston, MA, USA) were attached to the concave blade surface of a Macintosh and a GlideScope® laryngoscope. Experienced anaesthetists performed Macintosh and GlideScope intubations on the Laerdal® Airway Management Trainer manikin. Compared to Macintosh intubations, the GlideScope intubations had equal or superior views of the glottis with 55%, 58% and 66% lower median peak, average and impulse forces applied to the tongue base. The distal sensor registered the most force in both devices and the force distribution pattern was similar between the devices. The findings suggest that the GlideScope requires less force for similar or better laryngoscopic views, at least in a manikin model.

The Influence of Gender and Experience on Intubation Ability and Technique: A Manikin Study

Female anaesthetists in early training commonly question whether their strength is adequate for tracheal intubation. This study investigated the influence of gender and experience on intubation ability and laryngoscopic technique.

A manikin model and purpose-designed force-transducing laryngoscope was used to test three cohorts at different levels of experience (novice, intermediate and experienced males and females, n = 65) for the axial force and torque exerted, best laiyngoscopic view obtained, success with and time for intubation and laryngoscopic technique.

There were no significant differences between novice or experienced female and male intubators in markers of their ability to intubate or in the forces generated. For novice females compared with novice males, mean success rate was 90% (80.2 to 99.9) versus 97% (91.1 to 100, P=0.29); and mean time to intubate 24 seconds (19 to 29 seconds) versus 18 seconds (14 to 21 seconds, P=0.057). With experience, the forces generated during intubation reduced and ability improved. Proximal laiyngoscope grips (close to the blade) generated lower forces than distal grips.

Female and male intubators did not differ in ability to intubate or in the forces they exerted during direct laiyngoscopy.