A new device for measuring and recording the forces applied during laryngoscopy

Design, development, and face validation of an intubation simulation device using real‐time force data feedback

Objectives

Methods

Results

Conclusion

Level of Evidence

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.

Sex-Specific Intubation Biomechanics: Intubation Forces Are Greater in Male Than in Female Patients, Independent of Body Weight

Background

Studies of head, neck, and cervical spine morphology and tissue material properties indicate that cervical spine biomechanics differ between adult males and females. These differences result in sex-specific cervical spine kinematics and injury patterns in response to standardized loading conditions. Because direct laryngoscopy and endotracheal intubation require the application of a load to the cervical spine, intubation biomechanics should be sex-specific. The aim of this study was to determine if intubation forces during direct laryngoscopy differ between male and female patients and, if so, is the difference independent of body weight.

Methods

We pooled original data from three previously published adult clinical intubation studies that used methodologically reliable intubation force measurements (measured total laryngoscope force applied to the tongue, and force values were insensitive to or accounted for other laryngoscope blade forces). All patients had undergone direct laryngoscopy and orotracheal intubation with a Macintosh 3 blade under general anesthesia. Patient data included sex, age, height, weight, and maximum intubation force. Least squares multivariable linear regression was performed between the dependent variable (maximum intubation force) and two independent variables (patient sex and patient weight). A third term was added for the interaction between patient sex and weight.

Results

Among all patients (males n=42, females n=59), the median intubation force was 42.2 N (25^th to 75^th percentiles: 31.5 to 57.4 N). While controlling for patient body weight, intubation force differed between the sexes; P=0.011, with greater intubation force in male patients. While controlling for patient sex, there was a positive association between patient body weight and intubation force; P=0.009. In addition, there was a significant interaction between patient sex and weight; P=0.002, with intubation force in male patients having greater dependence on body weight. The difference in intubation force between male and female patients who had the same body weight exceeded 5 N when body weight exceeded 75 kg, and intubation force differences between male and female patients increased as patient body weight increased. Additional analyses using robust regression and using body mass index instead of weight provided comparable results.

Conclusion

In adult patients, the biomechanics of direct laryngoscopy and intubation are sex-specific. Our findings support the need to control for patient sex and weight in future clinical and laboratory studies of the human cervical spine and head and neck biomechanics.

Intubation biomechanics: laryngoscope force and cervical spine motion during intubation with Macintosh and Airtraq laryngoscopes

INTRODUCTION

Laryngoscopy and endotracheal intubation in the presence of cervical spine instability may put patients at risk of cervical cord injury. Nevertheless, the biomechanics of intubation (cervical spine motion as a function of applied force) have not been characterized. This study characterized and compared the relationship between laryngoscope force and cervical spine motion using two laryngoscopes hypothesized to differ in force.

METHODS

Fourteen adults undergoing elective surgery were intubated twice (Macintosh, Airtraq). During each intubation, laryngoscope force, cervical spine motion, and glottic view were recorded. Force and motion were referenced to a preintubation baseline (stage 1) and were characterized at three stages: stage 2 (laryngoscope introduction); stage 3 (best glottic view); and stage 4 (endotracheal tube in trachea).

RESULTS

Maximal force and motion occurred at stage 3 and differed between the Macintosh and Airtraq: (1) force: 48.8 ± 15.8 versus 10.4 ± 2.8 N, respectively, P = 0.0001; (2) occiput-C5 extension: 29.5 ± 8.5 versus 19.1 ± 8.7 degrees, respectively, P = 0.0023. Between stages 2 and 3, the motion/force ratio differed between Macintosh and Airtraq: 0.5 ± 0.2 versus 2.0 ± 1.4 degrees/N, respectively; P = 0.0006.

DISCUSSION

The relationship between laryngoscope force and cervical spine motion is: (1) nonlinear and (2) differs between laryngoscopes. Differences between laryngoscopes in motion/force relationships are likely due to: (1) laryngoscope-specific cervical extension needed for intubation, (2) laryngoscope-specific airway displacement/deformation needed for intubation, and (3) cervical spine and airway tissue viscoelastic properties. Cervical spine motion during endotracheal intubation is not directly proportional to force. Low-force laryngoscopes cannot be assumed to result in proportionally low cervical spine motion.

Measurement of forces applied during Macintosh direct laryngoscopy compared with GlideScope® videolaryngoscopy

Laryngoscopy can induce stress responses that may be harmful in susceptible patients. We directly measured the force applied to the base of the tongue as a surrogate for the stress response. Force measurements were obtained using three FlexiForce Sensors(®) (Tekscan Inc, Boston, MA, USA) attached along the concave surface of each laryngoscope blade. Twenty-four 24 adult patients of ASA physical status 1-2 were studied. After induction of anaesthesia and neuromuscular blockade, laryngoscopy and tracheal intubation was performed using either a Macintosh or a GlideScope(®) (Verathon, Bothell, WA, USA) laryngoscope. Complete data were available for 23 patients. Compared with the Macintosh, we observed lower median (IQR [range]) peak force (9 (5-13 [3-25]) N vs 20 (14-28 [4-41]) N; p = 0.0001), average force (5 (3-7 [2-19]) N vs 11 (6-16 [1-24]) N; p = 0.0003) and impulse force (98 (42-151 [26-444]) Ns vs 150 (93-207 [17-509]) Ns; p = 0.017) with the GlideScope. Our study shows that the peak lifting force on the base of the tongue during laryngoscopy is less with the GlideScope videolaryngoscope compared with the Macintosh laryngoscope.

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.

Force and pressure distribution using Macintosh and GlideScope laryngoscopes in normal and difficult airways: a manikin study

BACKGROUND

The forces applied to the soft tissues of the upper airway may have a deleterious effect. This study was designed to evaluate the performance of the GlideScope compared with the Macintosh laryngoscope.

METHODS

Twenty anaesthetists and 20 trainees attempted tracheal intubation of a Laerdal SimMan manikin. Forces and pressure distribution applied by both laryngoscope blades onto the soft upper airway tissues were measured using film pressure transducers. The minimal force needed to achieve a successful intubation, in the same simulated scenario, was measured; additionally, we considered the visualization score achieved by using the Cormack-Lehane grades.

RESULTS

All participants applied, on average, lower force with the GlideScope than with the Macintosh in each simulated scenario. Forces [mean (sd)] applied in the normal airway scenario [anaesthetists: Macintosh 39 (22) N and GlideScope 27 (15) N; trainees: Macintosh 45 (24) N and GlideScope 21 (15) N] were lower than forces applied in the difficult airway scenario [anaesthetists: Macintosh 95 (22) N and GlideScope 66 (20) N; trainees: Macintosh 100 (38) N and GlideScope 48 (16) N]. All the intubations using the GlideScope were successful, regardless of the scenario and previous intubation experience. The average pressure on the blades was 0.13 MPa for the Macintosh and 0.07 MPa for the GlideScope, showing a higher uniformity for the latter.

CONCLUSIONS

The GlideScope allowed the participants to obtain a successful intubation applying a lower force. A flatter and more uniform pressure distribution, a higher successful rate, and a better glottic view were observed with the GlideScope.

Forces applied to the maxillary incisors during tracheal intubation and dental injury risks of intubation by beginners: a manikin study

OBJECTIVES

We wished to determine whether dental injuries during intubation would occur more frequently when performed by inexperienced beginners. We measured the laryngoscopic force exerted on maxillary teeth of a modified manikin by experienced anesthesiologists and unexperienced medical students and estimated the injury risk.

METHODS

Thirty-two anesthesiologists and 32 medical students participated in this study. Each testee performed tracheal intubation in two scenarios in a random order. In Scenario 1, the testee performed tracheal intubation for a manikin as a patient with normal dentition, in an emergency type situation. In Scenario 2, the testee performed tracheal intubation for a manikin as a patient with unstable dentition, in a routine anesthetic situation.

RESULTS

The mean peak forces in Scenarios 1 and 2 were 6.1 and 1.1 N in the experienced testee group and 7.7 and 3.8 N in the unexperienced testee group, respectively (Scenario 2, p < 0.05). The unexperienced group applied higher forces than the experienced group in the nonemergency situation. However, the maximum force applied by the inexperienced group was 40.2 N, which is substantially lower than the maximum bite force of normal incisors (150-200 N).

CONCLUSION

Our results suggest that the experience levels of the laryngoscopists are not a major determinant of dental injuries in patients with healthy dentition.

Static forces variation and pressure distribution in laryngoscopy performed by straight and curved blades

A theoretical analysis of the forces acting on the laryngoscope during the lifting of the epiglottis is carried out by applying the basic principles of statics. The static model of a laryngoscope equipped with a straight and a curved blade and the forces variation, as a function of the introduction angle and of tissue reaction application point, are described. The pharyngeal tissues and epiglottis pressure distribution on the blade is obtained, with a 1mm(2) resolution, by measurements performed in-vitro on a simulation mannequin, using straight and curved blades. The straight blade requires more effort than the curved one to obtain the same visualization of vocal cords, however forces exerted by using a laryngoscope with a curved blade do not vary linearly with the application point of tissue reaction. Average intensity of the tissue reaction has been found in the order of 32+/-11 N. Pressure distribution is maximally concentrated on the tip of curved blades (0.5 MPa on 5mm axial length), whereas it is more dispersed on straight blades (0.2 MPa on 10mm axial length). The inclination of the handle also influences the effort of the operator: for both blades, from 0 rad to 1.57 rad, the lifting force shows a total variation of about 13% of the top value, the transversal forces vary less than 6% of the top value.

[Intubation-linked dental injuries. Relevance of individually adaptable tooth protection models]

BACKGROUND

Tooth damage during anaesthesia could be reduced by using tooth protectors during endotracheal intubation. The effectiveness of different models was investigated using an upper jaw model.

METHOD

A total of 6 individual adaptable dental protectors (Endoragard and Camo, with wax or silicone filling, respectively, Beauty pink dental wax with and without tissue inserts) were examined in three different categories. The upper jaw was covered with each dental shield and then loaded with a force of 150 N via a blade of a laryngoscope. Subsequently, force reduction was measured in axial as well as horizontal directions. Furthermore, the reduction in oral view was determined by measuring the thickness of each dental shield with a micrometer.

RESULTS

The combination of Camo and silicone achieved the maximum horizontal force reduction value (39.2 N). Endoragard and silicone achieved the best axial value (21.6 N). Beauty pink wax had the thinnest dental shield (2.8 mm), whereas the combination of Camo and silicone gave the most limited view inside the oral cavity (3.8 mm).

CONCLUSION

Preformed dental shields are useful for reducing the force applied to the teeth and potentially reducing the probability of tooth damage during laryngoscopy. However, the shield with the highest force reduction capability is relatively large and expensive which makes general use almost impossible. The model Beauty pink was slightly less force reducing and could be considered as an inexpensive and yet effective tool for clinical assignment.

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.

Flexibility and light emission of disposable paediatric Miller 1 laryngoscope blades*

With the emergence of Creutzfeldt-Jakob disease and the discovery of prions in tonsillar material, there has been an increase in the number of available disposable laryngoscope blades. This has led to non-conformity over many aspects of blade design. Miller 1 disposable blades have been produced in both metal and plastic and appear to have different properties of rigidity. We examined the rigidity of 11 disposable Miller 1 blades in three different axes of force. There was a significant difference in flexibility between metal and plastic blades in both primary and torsional axis (p = 0.006). We also studied the blades' light intensity and angle of light emission, finding up to an eightfold difference in the level of illumination provided at a distance of 10 mm from the tips of the blades. The area of maximal illumination varied, with some blades providing narrow beams of light, and others provided a more dispersed field of illumination. In addition, the angle of maximal illumination varied between the blade types from a central position to one directed to the right-hand side.

Rescuer position for tracheal intubation on the ground

BACKGROUND

Emergency oral tracheal intubations in the pre-hospital setting can be more difficult because the rescuer's position with respect to a patient lying on the ground may not provide optimal conditions for intubation. Since optimal visualisation of the larynx often depends on the force generated during laryngoscopy, we measured the pressure required for intubation (P(i)) as well as the maximum pressure (P(max)) that can be generated with the laryngoscopy blade in seven intubator positions.

METHODS

Nineteen hospital personnel with intubation experience participated in this study. A modified #3 Macintosh laryngoscope blade was used to measure the pressure exerted on the tongue of a manikin placed on the ground during intubation. The following positions were studied: standard, sitting, prone, kneeling, left and right lateral decubitus and straddling.

RESULTS

Intubating in the straddling position required the lowest P(i), as a percent of P(max) (68+/-14%). This was significantly less than the prone, right lateral decubitus and sitting positions. (Tukey's W procedure, P<0.05)

CONCLUSION

The straddling position affords the intubator significantly more reserve force than the prone, right lateral decubitus or sitting position. We suggest that the straddling position may be an advantageous position for pre-hospital intubations especially when visualisation of the glottis is difficult.

Arytenoid dislocation while using a McCoy laryngoscope

Arytenoid dislocation (AD) involves either a complete disruption of the cricoarytenoid joint or a malpositioning of the arytenoid cartilages (AC) with reference to other laryngeal cartilages. In this report, we present a case of AD while using a McCoy laryngoscope. Although McCoy laryngoscope is recognized as a useful option for the cases of difficult endotracheal intubation, we are concerned that AD is likely with this device.

A biomechanical model of the upper airways for simulating laryngoscopy

This paper describes a three-dimensional finite element model of the human upper airways during rigid laryngoscopy. In this procedure, an anaesthetist uses a rigid blade to displace and compress the tongue of the patient, and then inserts a tube into the larynx to allow controlled ventilation of the lungs during an operation. A realistic model of the main biomechanical aspects involved would help anaesthetists in training and in predicting difficult cases in advance. For this purpose, the finite element method was used to model structures such as the tongue, ligaments, larynx, vocal cords, bony landmarks, laryngoscope blade, and their inter-relationships, based on data extracted from X-ray, MRI, and photographic records. The model has been used to investigate how the tongue tissue behaves in response to the insertion of the laryngoscope blade, when it is subjected to a variety of loading conditions. In particular, the mechanical behaviour of the soft tissue of the tongue was simulated, from simple linear elastic material to complex non-linear viscoelastic material. The results show that, within a specific set of tongue material parameters, the simulated outcome can be successfully related to the view of the vocal cords achieved during real laryngoscopies on normal subjects, and on artificially induced difficult laryngoscopy, created by extending the upper incisors teeth experimentally.

A comparison of the forces exerted during laryngoscopy. The Macintosh versus the McCoy blade

The forces exerted at laryngoscopy with the McCoy and Macintosh blades have been compared in 40 patients. The variables measured were the duration of laryngoscopy, the three maximally-applied forces and the mean force. The mean (SD) forces recorded were 18.9 (7.82) and 10.1 (5.33) N, respectively, with the Macintosh and the McCoy blades (p < 0.001) with a similar duration of laryngoscopy. There was a significant positive correlation between the mean force and patients' weight, height and body mass index for the Macintosh blade but only for weight and height for the McCoy blade. It is concluded that the use of the McCoy blade results in significantly less force being applied during laryngoscopy. This may be the reason for the reduction in the stress response reported previously with the use of the McCoy blade.

Force and torque vary between laryngoscopists and laryngoscope blades

Several studies have examined the effects of patient characteristics on force of laryngoscopy, but little attention has been paid to the importance of technique and equipment. This study investigated whether force, torque, head extension, and view varied significantly between laryngoscopists and compared force and torque using Macintosh 3 and Miller 2 blades. The study population consisted of ASA grade I and II patients requiring general anesthesia and endotracheal intubation for elective surgery. Force, torque, head extension, and laryngeal view were highly reproducible when laryngoscopy was repeated by the same individual, Force and torque showed great variation between laryngoscopies performed by different anesthetists, For example, peak force varied over a range of 56 newtons among patients, but could also vary as much as 30 newtons between different anesthetists repeating laryngoscopy in the same patient. Force and head extension were 30% less with Miller laryngoscope compared to the Macintosh. Thus, laryngoscopic force and torque depend on technique and equipment. Further studies of force and torque may lead to improved techniques. The force-measuring laryngoscope could be a useful tool in teaching laryngoscopy.

Force, torque, and stress relaxation with direct laryngoscopy

The anesthetist exerts axial force on the laryngoscope handle to expose the glottis. The anesthetist must also apply a perpendicular force to balance the torque on the laryngoscope. Several studies have measured axial force during direct laryngoscopy, but none has measured torque. This study used a newly designed laryngoscope handle to measure force and torque simultaneously during direct laryngoscopy of ASA grade I and II patients requiring general anesthesia and endotracheal intubation for elective surgery. In 58 patients, peak force averaged 38 +/- 2 newtons. Peak torque averaged 4 +/- 0.2 newton-meters, and the perpendicular force was estimated as 40 +/- 2 newtons. The peak torque that can be balanced by the wrist is approximately 6 newton-meters, suggesting that torque may be a limiting factor for laryngoscopy in some situations. Peak force and torque demonstrated stress relaxation, a viscous property of biologic tissues. Force and torque decreased monoexponentially to approximately 70% of peak values with a half-time of 4 +/- 0.3 s. The phenomenon occurred in spite of administration of muscle relaxants, and was probably due to stress relaxation of pharyngeal tissues that are passively stretched during laryngoscopy.