Intubation biomechanics: validation of a finite element model of cervical spine motion during endotracheal intubation in intact and injured conditions.

OBJECTIVE Because of limitations inherent to cadaver models of endotracheal intubation, the authors' group developed a finite element (FE) model of the human cervical spine and spinal cord. Their aims were to 1) compare FE model predictions of intervertebral motion during intubation with intervertebral motion measured in patients with intact cervical spines and in cadavers with spine injuries at C-2 and C3-4 and 2) estimate spinal cord strains during intubation under these conditions. METHODS The FE model was designed to replicate the properties of an intact (stable) spine in patients, C-2 injury (Type II odontoid fracture), and a severe C3-4 distractive-flexion injury from prior cadaver studies. The authors recorded the laryngoscope force values from 2 different laryngoscopes (Macintosh, high intubation force; Airtraq, low intubation force) used during the patient and cadaver intubation studies. FE-modeled motion was compared with experimentally measured motion, and corresponding cord strain values were calculated. RESULTS FE model predictions of intact intervertebral motions were comparable to motions measured in patients and in cadavers at occiput-C2. In intact subaxial segments, the FE model more closely predicted patient intervertebral motions than did cadavers. With C-2 injury, FE-predicted motions did not differ from cadaver measurements. With C3-4 injury, however, the FE model predicted greater motions than were measured in cadavers. FE model cord strains during intubation were greater for the Macintosh laryngoscope than the Airtraq laryngoscope but were comparable among the 3 conditions (intact, C-2 injury, and C3-4 injury). CONCLUSIONS The FE model is comparable to patients and cadaver models in estimating occiput-C2 motion during intubation in both intact and injured conditions. The FE model may be superior to cadavers in predicting motions of subaxial segments in intact and injured conditions.

Intubation biomechanics: laryngoscope force and cervical spine motion during intubation in cadavers-effect of severe distractive-flexion injury on C3-4 motion.

OBJECTIVE With application of the forces of intubation, injured (unstable) cervical segments may move more than they normally do, which can result in spinal cord injury. The authors tested whether, during endotracheal intubation, intervertebral motion of an injured C3-4 cervical segment 1) is greater than that in the intact (stable) state and 2) differs when a high- or low-force laryngoscope is used. METHODS Fourteen cadavers underwent 3 intubations using force-sensing laryngoscopes while simultaneous cervical spine motion was recorded with lateral fluoroscopy. The first intubation was performed with an intact cervical spine and a conventional high-force line-of-sight Macintosh laryngoscope. After creation of a severe C3-4 distractive-flexion injury, 2 additional intubations were performed, one with the Macintosh laryngoscope and the other with a low-force indirect video laryngoscope (Airtraq), used in random order. RESULTS During Macintosh intubations, between the intact and the injured conditions, C3-4 extension (0.3° ± 3.0° vs 0.4° ± 2.7°, respectively; p = 0.9515) and anterior-posterior subluxation (-0.1 ± 0.4 mm vs -0.3 ± 0.6 mm, respectively; p = 0.2754) did not differ. During Macintosh and Airtraq intubations with an injured C3-4 segment, despite a large difference in applied force between the 2 laryngoscopes, segmental extension (0.4° ± 2.7° vs 0.3° ± 3.3°, respectively; p = 0.8077) and anterior-posterior subluxation (0.3 ± 0.6 mm vs 0.0 ± 0.7 mm, respectively; p = 0.3203) did not differ. CONCLUSIONS The authors' hypotheses regarding the relationship between laryngoscope force and the motion of an injured cervical segment were not confirmed. Motion-force relationships (biomechanics) of injured cervical intervertebral segments during endotracheal intubation in cadavers are not predicted by the in vitro biomechanical behavior of isolated cervical segments. With the limitations inherent to cadaveric studies, the results of this study suggest that not all forms of cervical spine injury are at risk for pathological motion and cervical cord injury during conventional high-force line-of-sight intubation.

Intubation Biomechanics: Laryngoscope Force and Cervical Spine Motion during Intubation in Cadavers-Cadavers versus Patients, the Effect of Repeated Intubations, and the Effect of Type II Odontoid Fracture on C1-C2 Motion.

BACKGROUND: The aims of this study are to characterize (1) the cadaver intubation biomechanics, including the effect of repeated intubations, and (2) the relation between intubation force and the motion of an injured cervical segment. METHODS: Fourteen cadavers were serially intubated using force-sensing Macintosh and Airtraq laryngoscopes in random order, with simultaneous cervical spine motion recorded with lateral fluoroscopy. Motion of the C1-C2 segment was measured in the intact and injured state (type II odontoid fracture). Injured C1-C2 motion was proportionately corrected for changes in intubation forces that occurred with repeated intubations. RESULTS: Cadaver intubation biomechanics were comparable with those of patients in all parameters other than C2-C5 extension. In cadavers, intubation force (set 2/set 1 force ratio = 0.61; 95% CI, 0.46 to 0.81; P = 0.002) and Oc-C5 extension (set 2 - set 1 difference = -6.1 degrees; 95% CI, -11.4 to -0.9; P = 0.025) decreased with repeated intubations. In cadavers, C1-C2 extension did not differ (1) between intact and injured states; or (2) in the injured state, between laryngoscopes (with and without force correction). With force correction, in the injured state, C1-C2 subluxation was greater with the Airtraq (mean difference 2.8 mm; 95% CI, 0.7 to 4.9 mm; P = 0.004). CONCLUSIONS: With limitations, cadavers may be clinically relevant models of intubation biomechanics and cervical spine motion. In the setting of a type II odontoid fracture, C1-C2 motion during intubation with either the Macintosh or the Airtraq does not appear to greatly exceed physiologic values or to have a high likelihood of hyperextension or direct cord compression.

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.

Anesthesia machine checkout and room setup: a randomized, single-blind, comparison of two teaching modalities.

BACKGROUND: Novel methods for teaching are needed to enhance the efficiency of academic anesthesia departments as well as provide approaches to learning that are aligned with current trends and advances in technology. METHODS: A video was produced that taught the key elements of anesthesia machine checkout and room set up. Novice learners were randomly assigned to receive either the new video format or traditional lecture-based format for this topic during their regularly scheduled lecture series. Primary outcome was the difference in written examination score before and after teaching between the two groups. Secondary outcome was the satisfaction score of the trainees in the two groups. RESULTS: Forty-two students assigned to the video group and 36 students assigned to the lecture group completed the study. Students in each group similar interest in anesthesia, pre-test scores, post-test scores, and final exam scores. The median posttest to pretest difference was greater in the video groups (3.5 (3.0-5.0) vs 2.5 (2.0-3.0), for video and lecture groups respectively, p 0.002). Despite improved test scores, students reported higher satisfaction the traditional, lecture-based format (22.0 (18.0-24.0) vs 24.0 (20.0-28.0), for video and lecture groups respectively, p <0.004). CONCLUSIONS: Higher pre-test to post-test improvements were observed among students in the video-based teaching group, however students rated traditional, live lectures higher than newer video-based teaching.

Endoscopic study of mechanisms of failure of endotracheal tube advancement into the trachea during awake fiberoptic orotracheal intubation.

BACKGROUND: Advancing the endotracheal tube (ETT) over a flexible bronchoscope (FB) during awake fiberoptic orotracheal intubation is often impeded. The goal of this study was to identify the sites and mechanisms that inhibit the passing of the ETT into the trachea. METHODS: Forty-five consenting patients underwent a clinically indicated awake fiberoptic orotracheal intubation. After topical anesthesia, nerve block, or both, an awake fiberoptic orotracheal intubation was performed. The placement of the FB and advancement of the ETT over the FB were videotaped using a second nasally inserted FB. An otolaryngologist later reviewed the videotaped data. RESULTS: The right arytenoid or the interarytenoid soft tissues inhibited advancement of the ETT in 42 and 11% of all patients, respectively. In all cases in which the FB was located on the right side of the larynx, failure of ETT advancement almost always occurred at the right arytenoid. Withdrawing the ETT and rotating it 90 degrees counterclockwise resulted in successful intubation on the second, third, and fourth attempts in 26.6, 20, and 0.7% of patients, respectively. CONCLUSION: The right arytenoid frequently inhibits advancement of the ETT over the FB into the trachea during awake fiberoptic orotracheal intubation. The FB position in the larynx before tube advancement and the orientation of the ETT are relevant factors in failure of advancement of the ETT into the trachea. The authors recommend positioning the FB in the center of the larynx and orienting the bevel of the ETT to face posteriorly during the first attempt at intubation.

The comparative effects of sevoflurane and methohexital for electroconvulsive therapy.

The standard anesthetic agent for electroconvulsive therapy (ECT) has been methohexital. We compared sevoflurane, a short-acting halogenated anesthetic, to methohexital for induction in ECT. Twelve subjects received sevoflurane or methohexital on alternating treatment days. Seizure duration, time to administering ECT, emergence and recovery, as well as several hemodynamic measures were recorded. A total of 69 treatments were analyzed. When sevoflurane was used, seizure durations recorded by observation and by EEG, were shorter by 10 and 23 seconds, respectively. With sevoflurane, seizure duration remained, however, within a clinically acceptable range. Methohexital allowed faster administration of ECT and discharge from the recovery room (3.8 vs. 6.2 minutes and 40.8 vs. 47.0 minutes, respectively). No difference in the post-ECT hemodynamic changes was found between the two treatments. We conclude that, when indicated, sevoflurane could provide a suitable alternative treatment option to methohexital, but some limitations, including shortened seizure duration and potential side effects, should be kept in mind.

Laryngeal trauma during awake fiberoptic intubation.

IMPLICATIONS: We describe three patients with difficult airways in which fiberoptic endotracheal intubation was used to insert breathing tubes into the patients' windpipes. Airway injury occurred during the use of this technique. Although largely a safe technique, care should be exercised when anesthesiologists choose equipment and when they perform this technique.