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© 2001 International Anesthesia Research Society
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Abstract |
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We describe video imaging as a technique for assessing neuromuscular blockade at the larynx. We sought to determine the stability and reproducibility of this technique and to compare the effect of succinylcholine at the adductor pollicis and the larynx. Ten patients were studied. Anesthesia was induced and maintained with propofol. The recurrent laryngeal nerve was stimulated superficially and movements of the vocal cords were recorded on videotape by using a fiberoptic bronchoscope passed via a laryngeal mask airway. Neuromuscular function was recorded at the adductor pollicis by using a mechanomyograph. Twenty images of the vocal cords were examined repeatedly by one investigator and by ten independent observers. The mean difference between the two sets of observations was 0.86 degrees with a correlation coefficient (r) of 0.997. For 3 min before the administration of relaxant the coefficient of variation in the cord movement during supramaximal stimulation ranged from 1%–4% (median 2.7%). After the administration of succinylcholine 1 mg · kg-1 the times to loss of T1 at the larynx and hand were 63 ± 15 s and 63 ± 12 s respectively. Times to 25% recovery were 215 ± 36 s at the larynx and 436 ± 74 s at the hand and times to 75% recovery were 285 ± 55 s and 525 ± 85 s respectively. These results indicate that video imaging may be a useful research technique for estimating neuromuscular blockade at the larynx and that the time to onset of succinylcholine at the larynx is similar to that at the hand, whereas the duration of blockade is significantly shorter at the larynx.
Implications: Assessment of neuromuscular blockade at the larynx is possible by using a video imaging technique. By using this technique, the time to onset of neuromuscular blockade at the larynx is similar to that at the hand after the administration of succinylcholine; this finding is different from previously published data obtained by using a cuff pressure measurement technique.
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Introduction |
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The effects of neuromuscular relaxants on the adductor muscles of the larynx have been studied by measuring the pressure change in the cuff of an endotracheal tube placed between the vocal cords on superficial stimulation of the recurrent laryngeal nerve (1–4). Recently, electromyography of the laryngeal muscles using surface electrodes and a specially designed endotracheal tube has also been used. There are significant differences in the time to onset of neuromuscular blockade at the larynx as compared with the adductor pollicis by using these two techniques (1,5,6). In a recent study, we have shown that the resting pressure in the cuff of an endotracheal tube placed between the vocal cords decreases significantly at the onset of neuromuscular blockade; this phenomenon may confound the estimation of the depth of blockade (7). We also presented preliminary data on an alternative technique, video imaging. This technique measures the movement of the vocal cords caused by superficial stimulation of the recurrent laryngeal nerve and does not involve placement of an endotracheal tube between the cords. The mean level of blockade as determined by video imaging was consistent with the cuff pressure measurement only when the resting cuff pressure was maintained at a constant level (7). However, further investigation of video imaging is required before it can be recommended to assess neuromuscular blockade at the larynx. The aims of this present study are:
1. To evaluate the stability of the video imaging set-up and the reproducibility of the measurement of vocal cord movement at varying degrees of neuromuscular blockade and
2. To assess the use of video imaging as a monitor of the onset, duration of action and recovery of neuromuscular blockade at the larynx after succinylcholine.
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Methods |
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Ten patients were recruited to the study, which had been approved by the hospital ethics committee. All patients gave written, informed consent. All were scheduled for elective ear, nose, and throat procedures that did not involve pathology of, or surgery to, the larynx. None of the patients were taking any drugs or had any other intercurrent disease that would affect neuromuscular function. Anesthesia was standardized for all the patients. None received premedication, and anesthesia was induced with propofol 2–3 mg · kg-1 supplemented with alfentanil 10 µg · kg-1 and maintained with a propofol infusion at 10 mg · kg-1 · hr-1. After the induction of anesthesia, a laryngeal mask airway was positioned. Assisted ventilation of the lungs was performed with 100% oxygen to maintain the end-tidal PCO2 in the normal range [35–42 mm Hg].
A force transducer (7) was then attached to the right thumb with a 300 g preload applied and a nerve stimulator (Digitimer DS7; Digitimer Ltd, Hertfordshire, UK) attached to Ag/AgCl electrodes at the right wrist. The arm was wrapped and the forearm temperature monitored. The supramaximal stimulus at the adductor pollicis was determined by using a single twitch stimulus at 5-s intervals with current increasing from 0 mA to 100 mA in 5 mA steps. The stimulus generated by the nerve stimulator had a square wave formation and duration of 200 ms. A flexible fiberoptic bronchoscope was introduced via the laryngeal mask airway and the glottis viewed. A nerve stimulator, identical to that used at the hand and synchronized with it, was attached to the suprathyroid notch and the forehead or sternum via Ag/AgCl electrodes to stimulate the recurrent laryngeal nerve superficially as described by Donati et al. (2). A continuous recording of the vocal cords was made on videotape for subsequent analysis.
Recurrent laryngeal nerve stimulation was commenced with a single stimulus at 20 mA at 5 s intervals. Every 15 s this was increased in 5-mA increments to a maximum of 100 mA to determine the supramaximal stimulus. The current that was judged visually to be associated with maximal cord movement was then noted, and a train-of-four stimulus was applied at 15-s intervals for 10 min at a current 10% more than the maximal stimulus–the supramaximal stimulus. This stimulating current was used for the remainder of the study. To determine whether the cord movement relative to stimulus intensity changed with time, the supramaximal stimulus was reevaluated after a 10-min period of stimulation and again after recovery of neuromuscular blockade at the hand.
After the second supramaximal current determination, a further 3-min period of stabilization was allowed before the administration of succinylcholine 1 mg · kg-1 IV. Subsequently, the coefficient of variation was calculated for the cord movement for each patient during this period of supramaximal stimulation. Peripheral nerve stimulation continued at both the hand and larynx at 15-s intervals until the adductor pollicis contraction had recovered to its maximum plateau.
Video frames showing cord position immediately before stimulation and on maximal cord adduction after the first stimulus (T1) and the last stimulus (T4) of each train-of-four were later captured from the videotape by using the commercial software package Adobe CaptureTM (Adobe, San Jose, CA). These images were then stored digitally. The anterior angle between the cords in each of the images was measured by using a commercially available scientific image measurement software package (SigmaScanTM; Jandel, San Rafael, CA). The anterior 2/3 of the cords, i.e., the membranous cord anterior to the arytenoid cartilage, in each of the images was selected for measurement. All image measurements were repeated five times and the mean of the five measurements was calculated.
The mean difference between the angle measured between the cords at rest and maximal adduction was calculated for each level of stimulation current applied in all the subjects. A comparison was made between the vocal cord movements during the supramaximal stimulus assessment at commencement of the study, before the administration of relaxant and after recovery from the blockade by using repeated measures analysis of variance.
To determine the intra- and interobserver variability of this technique, 20 frames were selected for repeated analysis. These were taken from two patients selected randomly to include 10 baseline and 10 maximal adduction images.
The 20 selected frames were examined by one of the investigators (KG) on 10 different occasions in a random order to obtain the primary measurements. On each occasion, every frame was measured five times without the investigator being aware of the previous measurement. The mean of the five measurements was used for subsequent analysis. The mean and standard error of the mean of the repeated measurements were calculated for each of the images.
Ten other observers, none of whom were involved with the project, were asked to measure the same 20 images. Each was given an instruction sheet detailing how to use the software along with five sample images to allow them to become familiar with the measurement technique. They were then asked to measure each of the 20 images five times. The mean and standard error of the mean for each of the 20 images were calculated. The data obtained by 10 independent observers were compared with that obtained by the primary observer by using simple regression and a Bland-Altman plot.
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Results |
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The ten subjects studied were six men and four women aged 29 ± 7 yr and weighing 72 ± 18 kg (mean ± SD for both).
The angles measured between the cords to determine intra- and interobserver variability ranged from 5 (cords adducted) to 50 (cords relaxed) degrees and the SEM ranged from 0.13–0.77 for the single observer and from 0.21–1.51 for the 10 observers for the 20 images. The correlation coefficient (r) between the angles measured by the independent and primary observers was 0.997. The Bland Altman plot showed that the mean difference between the two sets of observations was 0.86 degrees (95% confidence limits -0.25 and 1.75) and all the points lie within ± 2 SD. The limits of agreement for this plot are that the angles measured by the independent observers may be 3 degrees less than or 4 degrees greater than those measured by the primary observer.
In seven of the 10 patients adduction at the larynx reached >95% of its maximal value within 3 min of stimulation. In the remainder, this value was achieved by 6 min of stimulation. For the 3 min before the administration of relaxant the coefficient of variation in cord movement ranged between 1% and 4% (median 2.7%). The T4:T1 ratio over the same time period was 0.9.
Figure 1 shows the value of T1 as a percentage of the maximal contraction for the ten subjects, as measured at stimulation currents ranging from 20 to 100 mA at the commencement of the study, after stabilization, before the administration of relaxant, and after recovery of the cords and hand to >95% of baseline values. The mean supramaximal stimulus estimated from visualizing cord movement during the study was 78 ± 8 mA. The supramaximal stimuli calculated subsequently offline were 82 ± 12 mA at commencement of the study, 74 ± 13 mA after the 10-min period of stabilization and 72 ± 19 mA after recovery from neuromuscular blockade. On average, the supramaximal stimulus applied during the study was 4 mA more than that calculated subsequently from the angles measured after the 10-min period of stabilization. In no subject was the supramaximal stimulus calculated offline before the administration of relaxant more intense than the current actually applied. Repeated measures analysis of variance indicated no significant differences between the contractions at stimuli between 20 and 75 mA when the presuccinylcholine data were compared with that obtained after recovery (P = 0.8).
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Figure 2 shows the change in T1 at both the hand and larynx with time after the administration of succinylcholine 1 mg · kg-1. It can be seen that there is no difference in time to loss of T1 (63 ± 15 s at the larynx and 63 ± 12 s at the hand); however, recovery occurs significantly earlier at the larynx than at the hand. Times to 25% recovery are 215 ± 36 s at the larynx and 436 ± 74 s at the hand and times to 75% recovery are 285 ± 55 s and 525 ± 85 s respectively.
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Discussion |
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After encouraging preliminary results by using video imaging in a previous study (7), we decided to investigate video imaging as a tool for assessment of neuromuscular blockade at the larynx. We have shown that video imaging:
1. Provides a stable preparation for assessment of neuromuscular blockade at the larynx when assessed over a period of 10 minutes on surface stimulation of the recurrent laryngeal nerve.
2. Provides a reproducible assessment of neuromuscular blockade at the larynx that is not observer-dependent.
3. Can be used successfully to determine the supramaximal stimulus at the larynx, which is consistent before onset of and after recovery from neuromuscular blockade.
4. Can be used to record a complete profile of onset, duration of action, and recovery from neuromuscular blockade including loss of all cord movement during paralysis.
Strict criteria for good clinical research have been published for the assessment of neuromuscular blockade at the hand (8). These include a stable response time for 10 minutes (with variation of <2% for at least 3 minutes before the administration of relaxant), the ability to record the supramaximal stimulus and demonstrate stable supramaximal response at the end of the experiment, the ability to record complete profile of onset, duration of action, and recovery of neuromuscular blockade, and documentation of stability by adequate (80%–120%) return of T1 after recovery. Although the cuff pressure and electromyographic methods of assessing neuromuscular blockade at the larynx are analogous to those used at the adductor pollicis, we are unaware of any attempts to examine these techniques critically in light of the above criteria when applied to the vocal cords. Our previous study showed that at the onset of neuromuscular blockade the resting cuff pressure decreases significantly, thereby confounding further assessment (7). This led us to develop video imaging as an alternative technique. The principal advantage of video imaging is that this allows direct visualization of the effects of neuromuscular blockade at the larynx rather than the "blind" nature of cuff pressure measurement. Vocal cord position and movement play an essential part in determining ease of tracheal intubation and the results obtained by video imaging, in theory, provide a reliable estimation of the time taken for the cords to stop moving to a stimulus after the administration of a neuromuscular relaxant.
Although it is tempting to apply the criteria of research practice and standards described for measurement of blockade at the hand to the assessment of neuromuscular blockade at the larynx, it is important to realize that the two sites are inherently different. Surface stimulation of the recurrent laryngeal nerve is less precise than stimulating the ulnar nerve and often causes contraction of additional groups of muscles (particularly the strap muscles). This has implications for the stability of the setup. Unlike the hand, the larynx is difficult to fix in position, and this may allow changes in the baseline position of the cords through the study period. The concept of preloading, as standardized for the hand, may also not be strictly applicable to the laryngeal adductor muscles. Whereas the thumb is constrained only proximally, the vocal cords are tethered at both extremes. The cuff pressure technique uses a semi-inflated cuff placed between the cords. This will apply some preload to the cords, but there are no described methods to standardize it and keep it constant through the study period. If the cords move apart after the administration of relaxant, as evidenced by a decrease in resting cuff pressure, the subsequent twitch height measured on stimulation of the recurrent laryngeal nerve will inevitably be decreased unless the force of contraction also increases. During video imaging the adductor muscles are not preloaded as such, but stimulation of the recurrent laryngeal nerve causes simultaneous contraction of the abductor muscles of the larynx that may provide a further unquantifiable counterload. However, video imaging cannot either measure or standardize this element. In view of such site-related differences, we believe that work is required to formulate what can be expected at best from any technique assessing neuromuscular blockade at the larynx.
The results of this present study regarding the onset and recovery times after succinylcholine 1 mg · kg-1 at the larynx are very similar to those described by d’Honneur et al. (1) by using electromyography. However, this is in distinction to the studies by Wright et al. (5) and Meistelman et al. (6) and a widely held belief that the onset of neuromuscular blockade at the larynx is significantly faster compared with the onset at adductor pollicis. Both latter studies used the cuff pressure technique. The most probable reason for the difference between the findings of these studies and ours is the change in the resting cuff pressure after the administration of a relaxant, as we have discussed in a previous paper (7). However, this suggestion can only be proven if the studies using cuff pressure could be repeated with the resting cuff pressure maintained at a constant value throughout the study. We are not aware of any such technique.
The present study has a number of limitations, some of which can be attributed to the study design and others to the technique itself. In this study, we would ideally have wanted to stabilize the "muscle preparation" for a prolonged period of time. However, the practical limitations of conducting this study in patients constrained us to a 10-minute period. The fact that the value of T1 after recovery of neuromuscular blockade at currents ranging from 20 to 100 mA is very similar to that before the onset of blockade indicates that the preparation must have achieved a reasonable degree of stability, at least for the duration of studies such as this. Criticism could also be made of the dose of succinylcholine used. However, we had three purposes in mind. First, we wanted to show that vocal cord movement could be abolished completely, as estimated by this technique, and that the large currents used were not responsible for direct muscle stimulation that may confound attempts to estimate maximal effect of other relaxants. Second, from an entirely pragmatic point of view, 1 mg · kg-1 is the dose of succinylcholine commonly used in our institution and we wanted to determine the effects of this dose at the two sites studied. Third, it allowed us a direct comparison with previous work performed at the larynx with alternative techniques.
Video imaging, as described in this study, is restricted by the frame rate of the videotape, which runs at 25 frames per second. Thus, consecutive images are 40 milliseconds apart. This could result in the peak contractions being missed and thus, overestimation of the blockade if only one contraction were analyzed. A high-speed videorecorder or taking the average of repeated measurements will overcome this problem. In this study, all the baseline and steady state data presented are the means of multiple measurements to attempt to address this problem. Onset and recovery of neuromuscular blockade is by necessity a dynamic process that changes within the 15-second intervals between stimuli and hence multiple measurements are not possible. Video imaging measures movement rather than force of contraction. Force is related to distance traveled in a manner such that both mass and time taken to travel distance can affect the relationship (force = mass x distance/time x time). Assuming that with increasing of depth of blockade, the mass remains constant, the change in time to reach the peak can affect the relationship between force and distance. It is difficult to predict or evaluate what effect this might have on the estimation of block. It is, therefore, possible that the percentage change in angle may not be linearly related to the percentage change in force of contraction. Further work would be required to address these issues. However, despite these limitations, the findings of this study are consistent with electromyography, and the limits of agreement between video imaging and cuff-pressure measurement are acceptable if the resting cuff pressure is maintained (7).
In conclusion, we have shown in this study that video imaging can be used to determine the effects of neuromuscular relaxants at the larynx. We have shown that the laryngeal muscles respond in a predictable way to increasing stimulation currents. We have also shown that with regards to the time of onset of neuromuscular blockade at the larynx compared to the hand, results obtained by using video imaging are very similar to those described recently using electromyography, but are different from those derived from the cuff pressure technique. Further studies are required using video imaging to compare the effects of nondepolarizing relaxants at the larynx and hand using subparalyzing doses. This would allow additional comparisons of the effects of muscle relaxants at the larynx as determined by the various techniques now available.
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References |
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TopAbstractIntroductionMethodsResultsDiscussionReferences |
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- d’Honneur G, Kirov K, Slavov V, Duvaldestin P. Effects of an intubating dose of succinylcholine and rocuronium on the larynx and diaphragm. Anesthesiology 1999; 90: 951–5.[Web of Science][Medline]
- Donati F, Plaud B, Meistelman C. A method to measure elicited contraction of laryngeal adductor muscles during anesthesia. Anesthesiology 1991; 74: 827–32.[Web of Science][Medline]
- Donati F, Meistelman C, Plaud B. Vecuronium neuromuscular blockade at the adductor muscles of the larynx and adductor pollicis. Anesthesiology 1991; 74: 833–7.[Web of Science][Medline]
- Meistelman C, Plaud B, Donati F. Rocuronium (ORG 9426) neuromuscular blockade at the adductor muscles of the larynx and adductor pollicis in humans. Can J Anaesth 1992; 39: 665–9.[Web of Science][Medline]
- Wright PMC, Caldwell JE, Miller RD. Onset and duration of rocuronium and succinylcholine at the adductor pollicis and laryngeal adductor muscles in anesthetized humans. Anesthesiology 1994; 81: 1110–5.[Web of Science][Medline]
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Meistelman C, Plaud B, Donati F. Neuromuscular effects of succinylcholine on the vocal cords and adductor pollicis muscles. Anesth Analg 1991; 73: 278–82.
[Abstract/Free Full Text] -
Girling KJ, Bedforth NM, Spendlove JL, Mahajan RP. Assessing neuromuscular block at the larynx; The effect of change in resting cuff pressure and a comparison with video imaging in anesthetized humans. Anesth Analg 1999; 88: 426–31.
[Abstract/Free Full Text] - Viby-Mogensen J, Engbaek J, Eriksson LI, et al. Good clinical research practice (GCRP) in pharmacodynamic studies of neuromuscular blocking agents. Acta Anaesthesiol Scand 1996; 40: 59–74.[Web of Science][Medline]
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  T. M. Hemmerling, F. Donati, and K. Girling Video Imaging of the Larynx Needs Careful Evaluation Response Anesth. Analg., July 1, 2001; 93(1): 243 - 244. [Full Text] [PDF]
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