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GENERAL ARTICLES

Assessing Neuromuscular Block at the Larynx: The Effect of Change in Resting Cuff Pressure and a Comparison with Video Imaging in Anesthetized Humans

University Department of Anaesthesia, University Hospital, Queen's Medical Centre, Nottingham, United Kingdom

Address correspondence and reprint requests to Keith Girling, University Department of Anaesthesia, University Hospital, Queen's Medical Centre, Nottingham NG7 2UH, UK. Address e-mail to Keith.Girling{at}nottingham.ac.uk

	Abstract

Top Abstract Introduction Methods Results Discussion Appendix 1 References

 
Neuromuscular block (NMB) at the larynx has been assessed by measuring the cuff pressure (CP) in an endotracheal tube (ETT) placed between the vocal cords. In this study, we evaluated the decrease in resting cuff pressure (RCP) after the administration of rocuronium and the effect of this decrease on the assessment of NMB, and we compared CP measurement with an alternative technique, video imaging (VI). In 20 patients, NMB was determined at the hand by mechanomyography and at the larynx initially by CP and subsequently by VI, recording images using a fiberoptic bronchoscope via a laryngeal mask. Train-of-four stimuli were applied at both sites. After baseline measurements, the ETT was replaced, and rocuronium was infused to achieve a steady-state 50% (n = 10) or 75% (n = 10) block at the hand. CP measurements were recorded before and after restoration of RCP to prerocuronium pressure, followed by further VI measurements. The mean RCP decreased from 21 ± 4 to 12 ± 5 mm Hg after rocuronium. At 50% block at the hand, the CP estimate of block at the larynx with reduced RCP was 62% ± 18%, and that after restoring RCP was 29% ± 13%; VI estimated 27% ± 14% block. At 75% block at the hand, CP and VI estimated 52% ± 11% and 46% ± 9% block, respectively (RCP maintained). We conclude that RCP decreases after the administration of rocuronium, that restoring RCP significantly alters CP estimates of NMB, and that VI is in agreement with CP measurement if RCP is maintained at prerelaxant values.

Implications: In this study, we show that a muscle relaxant-induced decrease in resting tension at the larynx may confound the assessment of neuromuscular block by cuff pressure measurement. The preliminary data suggest that video imaging may provide a suitable alternative to cuff pressure measurement to assess neuromuscular block at the larynx.

	Introduction

Top Abstract Introduction Methods Results Discussion Appendix 1 References

 
Previous workers have studied the effects of neuromuscular relaxants on the adductor muscles of the larynx by measuring the pressure change in the cuff of an endotracheal tube (ETT) placed between the vocal cords on superficial stimulation of the recurrent laryngeal nerve (1–6). In a pilot study using this technique, we found that the resting pressure in the cuff decreased after the administration of a muscle relaxant. This indicated that the preload applied to the vocal cords had changed after the administration of the relaxant. Close scrutiny of previous studies in which this technique was used indicated that this change in resting cuff pressure had been documented (1,4,6). Whether this may confound the assessment of neuromuscular block at the larynx has not been examined previously.

The aim of this study was threefold: to quantify the change in resting cuff pressure after the administration of rocuronium, to evaluate the effect of this change on the subsequent assessment of neuromuscular block at the larynx, and to compare the level of neuromuscular block as assessed by the cuff pressure technique by using an alternative method, video imaging.

	Methods

Top Abstract Introduction Methods Results Discussion Appendix 1 References

 
The hospital ethics committee approved the study protocol. Written, informed consent was obtained from all patients before their participation in the study.

Twenty patients were recruited into the study. Anesthesia was induced with propofol 2–3 mg/kg supplemented with alfentanil 10 µg/kg and maintained with a propofol infusion at 10–15 mg · kg^-1 · h^-1. After the induction of anesthesia, a Mallinckrodt 7.5-mm inner diameter ETT was positioned with the cuff placed between the vocal cords, the lungs were ventilated with 100% oxygen to normocapnia. A force transducer (see Appendix 1) was then attached to the right thumb with a 300 g preload applied, and a nerve stimulator (Digitimer DS7; Digitimer Ltd, Hertfordshire, UK) was attached to Ag/AgCl electrodes at the right wrist. The arm was wrapped and the forearm temperature was monitored. The supramaximal stimulus at the adductor pollicis was determined using a single twitch stimulus at 5-s intervals with the current increasing from 0 mA in 5-mA steps (7). The stimulus generated by the nerve stimulator had a square-wave formation and duration of 200 ms.

A second nerve stimulator (identical to that at the hand and linked to allow synchronous output) was attached to Ag/AgCl electrodes positioned over the notch of the thyroid cartilage and the sternum for stimulation of the recurrent laryngeal nerve, as described previously (1). The pilot cuff of the ETT was connected to a pressure transducer (Marquette Electronics, Milwaukee, WI), and air was introduced to the cuff to provide a resting baseline pressure of 15–25 mm Hg. The supramaximal current at the larynx was determined as described previously (1); this current was applied for the remainder of the study period.

The ETT was then removed and replaced with a laryngeal mask airway (LMA), size 3 in female patients and size 4 in male patients. A fiberoptic bronchoscope was introduced via the LMA, and the vocal cords were viewed. Images of the vocal cords were recorded (25 frames/s).

Both the ulnar and recurrent laryngeal nerves were then stimulated simultaneously for 10 min with a supramaximal train-of-four stimulus for the baseline video imaging measurements. After this period, the LMA and imaging equipment were removed, and the ETT was replaced at the larynx and teeth, and the cuff was reinflated to the same baseline resting pressure. The height of the first evoked response of the train-of-four (T1) at the larynx was confirmed with the initial supramaximal stimulus, and further stimulation continued for 5 min for baseline cuff pressure measurements. The resting cuff pressure was monitored throughout this period to ensure that there was no leak within the monitoring system. All recordings of neuromuscular function at the larynx using either technique were made during expiration to avoid any changes in vocal position/tension that may be attributed to the respiratory cycle.

The force transducer at the wrist was linked to a computer via an analog-to-digital card. The computer also controlled an infusion device driving a syringe containing rocuronium at a concentration of 1 mg/mL. Once stable recordings were achieved at the hand and larynx, rocuronium was infused via an initial bolus and closed-loop feedback system, as described in Appendix 1. In the first 10 patients, the target was 50% of baseline T1 at the hand (50% block); and in the other 10 patients, the target was 25% of baseline T1 (75% block).

In patients targeted to 50% block at the hand, the new resting cuff pressure was recorded, and cuff pressure measurements of neuromuscular block at the larynx were made when steady state (i.e., target ±2.5%) had been achieved. The resting cuff pressure was then restored to the baseline value and further measurements were recorded. The ETT was then removed, the LMA was replaced, and video images of the cord movement were recorded while the level of block remained unchanged for a further 5 min.

To exclude any effect of the study sequence on the level of neuromuscular block estimated by the two techniques, patients who received rocuronium to achieve 75% block at the hand were randomized to two groups of five patients each. In one group, the above protocol was used. In the other group, the supramaximal stimulus at the larynx was determined using the cuff pressure measurement, as described above. The LMA was then placed and video images were recorded. The LMA was maintained in position during the administration of a muscle relaxant, and laryngeal images were a continuously recorded until neuromuscular block at the hand had reached steady state for 5 min. The ETT was replaced with the cuff at the same position between the cords and with the resting cuff pressure at the baseline (prerocuronium) resting value, and further cuff pressure recordings were made.

T1 at the hand was recorded continuously by the computer, the program allowing for manual entry of events during the study. The height of T1 generated by the cuff pressure technique was recorded directly by a paper writer calibrated so that a 1-mm change was equivalent to a 1-mm Hg change in pressure. The videotape was later analyzed in the following manner. Images of the cords at maximal adduction of T1 and at rest three frames before this point were digitized by capturing the frame from the video. The images were stored on the computer hard drive. The angle between the vocal cords was measured using commercially available image measurement software (Sigmascan; Jandel Scientific, UK). The change in angle from the resting point to maximal adduction before the administration of the relaxant was taken as baseline (maximal) movement. This was measured in eight pairs of frames before the administration of relaxant, and the mean was used for analysis. The angle between the cords was also measured in eight pairs of frames (at rest and maximal adduction) at steady-state block; this mean was taken to be movement at steady-state block. To estimate the degree of block, the movement at steady-state block was compared with the prerelaxant level.

In the patients studied at 50% block at the hand, comparisons were made between the resting cuff pressure measurements before and after the administration of rocuronium by using Student's paired t-test. The level of block as determined by cuff pressure measurement at reduced and restored cuff pressure and by video imaging were compared using Student's paired t-test with Bonferroni's adjustment. For the three comparisons of neuromuscular block, P < 0.01 was considered statistically significant. Bland Altmann plots (8) were constructed to show the mean value of the difference between the two techniques at the larynx and the limits of agreement.

	Results

Top Abstract Introduction Methods Results Discussion Appendix 1 References

 
Of the 20 patients studied, 9 were male and 11 were female. Results are expressed as mean ± SD. The patients were aged 39 ± 12 yr and weighed 73 ± 16 kg. The supramaximal stimulus for the patients studied was 63 ± 5 mA at the hand and 76 ± 9 mA at the larynx. The forearm temperature was 34.8 ± 1.1°C.

To study 50% block at the hand, 10 patients (ASA physical status I) were studied. The mean cuff pressure at rest decreased from 21 ± 4 to 12 ± 5 mm Hg after administration of the relaxant (P < 0.0001). This decrease in cuff resting pressure had a significant influence on the assessment of neuromuscular block using the cuff pressure technique. The block at the larynx was assessed at 62% ± 18% with reduced cuff pressure (i.e., greater than that at the hand) and at 29% ± 13% with the cuff pressure restored (i.e., less than that at the hand). This was statistically significant (P < 0.0001). The pressure trace from one of the patients studied is shown in Figure 1.

View larger version (92K): [in this window] [in a new window]   Figure 1. Pressure tracings taken from one patient. a, The resting cuff pressure is 21 mm Hg and the height of T1 is 12 mm Hg. b, T1 at the hand is at steady state at 50% of baseline value. The resting cuff pressure is 12 mm Hg and the height of T1 at the larynx is 5 mm Hg. c, T1 at the hand is at steady state (50% block), the resting cuff pressure has been restored to the baseline value, and the height of T1 at the larynx is 9 mm Hg.

The mean differences and the limits of agreement when video imaging was compared with values obtained using cuff pressure measurement at reduced baseline and restored baseline are 35% ± 38% and 2% ± 22%, respectively (Fig. 2).

View larger version (14K): [in this window] [in a new window]   Figure 2. Bland Altman plot comparing the difference in percent block as measured by cuff pressure and video imaging and the mean percent block measured by the two techniques at 50% block at the hand. • = the reduced cuff pressure versus the video imaging, —- = the mean difference and ±2 SD from the mean, = the restored cuff pressure versus the video imaging results, - - - = the mean difference and ±2 SD from the mean for this data set.

View larger version (10K): [in this window] [in a new window]   Figure 3. Bland Altman plot showing the difference in percent block measured by cuff pressure and video imaging and the mean percent block measured by the two techniques at 75% block (25% of baseline T1) at the hand. —- = the mean difference and ±2 SD from the mean.

	Discussion

Top Abstract Introduction Methods Results Discussion Appendix 1 References

Regarding preload and assessment of neuromuscular block, for mechanomyographic monitoring systems at the hand, a variation in preload may influence the assessment of neuromuscular block (9). Perhaps this variation should be maintained at <25% during the study (7). In this study, we observed a change in resting cuff pressure that was >40%. The optimal baseline resting cuff pressure is unclear. Although some work has been performed to determine the optimal preload at the thumb (9), there has been no evaluation of the effect of different resting cuff pressures on the assessment of neuromuscular block at the larynx. However, our findings suggest that the decrease in resting cuff pressure is a consistent phenomenon that has a significant effect on the assessment of block. Continuous adjustments to the resting cuff pressure are required to maintain a constant preload at the vocal cords throughout the onset of block, which would only be possible with a complex servo system. This prompted us to seek an alternative technique for assessing block at the larynx. In this study, we attempted to use video imaging as an alternative technique and to compare this with cuff pressure measurement.

Barker et al. (10) previously described the use of video recording to determine the change in position of the vocal cords after the administration of either propofol or thiopentone. They measured the angle between the cords using a goniometer and showed that it did change after the induction of anesthesia with propofol, compared with the awake state. However, the angle measured remained within a narrow range after the initial 20 s after induction. We used a modification of this technique in an attempt to monitor neuromuscular block at the larynx.

The technique of video imaging for estimating neuromuscular block clearly has a number of limitations. No preload is applied to the cords, the technique measures movement rather than force or even acceleration, and the subsequent storage of images and measurement of the angles between the cords is time-consuming and must be performed off-line. This method would require extensive further investigation before it could be recommended as an alternative to the cuff pressure technique. Among other things, the reproducibility of the cord movement at different levels of stimulation, the adequacy of 25 frame/s video frame rate, and the stability of the cord movement over time would need to be confirmed. However, this study provides preliminary data on the use of video imaging and the levels of block as estimated by video imaging that were very close to that of cuff pressure measurement when the resting cuff pressure during neuromuscular block was maintained at prerelaxant levels.

We studied 50% and 75% block because we anticipated that any difference between the hand and larynx would be greatest at these levels (3). The cuff pressure measurement findings at the reduced resting baseline pressure at 50% block at the hand are consistent with that expected from previous published work (3); i.e., that the block is greater at the larynx than at the hand. Ideally, we would have studied more points on the curve to compare the techniques; however, this was a very invasive study requiring repeated intubations and a study time >1 h. The results from these 20 patients at two levels of block demonstrate a significant change in the resting cuff pressure after the administration of rocuronium and that video imaging is a technique worthy of further investigation. Further studies are required to compare the onset, duration, and recovery of relaxants at the hand and larynx using either a modified cuff pressure technique or video imaging.

In conclusion, after the administration of rocuronium, the resting pressure in the cuff of an ETT placed between the cords decreases. Changing the resting cuff pressure significantly alters the level of block estimated by using this technique. An alternative technique, video imaging, provides results similar to the restored cuff pressure and is worthy of further investigation.

	Appendix 1

Top Abstract Introduction Methods Results Discussion Appendix 1 References

 
The force transducer used comprised four strain gauges mounted on a length of 1-mm spring steel. The strain gauges were connected in a Wheatstone Bridge configuration, and an amplifier circuit was used to amplify the voltage when the spring steel was deflected. The lower end of the spring steel was anchored to the transducer housing, and the upper end was connected to the patient's thumb by using a length of nylon cord of sufficient gauge not to stretch when placed under tension. The transducer was extensively tested before use, and the linearity and reproducibility of the response were confirmed up to a maximum of 5 kg. The system was calibrated and accurately detected changes of force 25 g. The computer captured data at a rate of 300 Hz to ensure that the peak contraction was measured. The signal to noise ratio was <2%. Preliminary work using this device indicated that the resting tension of the adductor pollicis changed by <10% at complete neuromuscular blockade.

The closed-loop administration system used in this study comprised a computer with an analog to digital card and a digital to analog card installed. These allowed the computer to control two synchronized nerve stimulators and an infusion device via the RS232 port. Data from the force transducer were transferred to the computer at 300 Hz. Similar control systems have been well documented previously (11,12). The administration of rocuronium started with a bolus dose derived from at a rate of 1200 mL/min. The nerve stimulators produced train-of-four stimuli at 15-s intervals until the twitch height was within 20% of target or had reached a plateau for >30 s. At this point, a closed-loop infusion was commenced based on the proportional, integral, and derivative algorithm shown in . Similar algorithms have been used previously for the administration of atracurium (13,14). The rate of infusion was altered after each stimulus, and steady state was assumed when the target ± 2.5% block had been achieved for 10 min. The time to steady state was 22.5 ± 5.6 min for 50% reduction in T1 and 16.5 ± 7.0 min for 75% reduction in T1.

where K_p = proportionality constant 0.02 mg/kg, e_x = percent difference between observed and target level of block, K_d = derivative constant 0.07 mg/kg, Ki = integral constant 0.0007 mg/kg, and P = constant 1000.

	Footnotes

 
We acknowledge financial support from the NHS Executive Trent and the Special Trustees for Nottingham University Hospitals.

This work was presented in part at the Anaesthetic Research Society, Belfast, March 27,1998 and has appeared as an abstract in British Journal of Anaesthesia 1998; 81:285P.

	References

Top Abstract Introduction Methods Results Discussion Appendix 1 References

 

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9. Donlon JV Jr, Savarese JJ, Ali HH. Cumulative dose-response curves for gallamine: effect of altered resting thumb tension and mode of stimulation. Anesth Analg 1979;58:377–81.[Abstract/Free Full Text]
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11. Macleod AD, Asbury AJ, Gray WM, Linkens DA. Automatic control of neuromuscular block with atracurium. Anaesth 1989;63:31–5.
12. Assef SJ, Lennon RL, Jones KA, et al. A versatile, computer-controlled, closed-loop system for continuous infusion of muscle relaxants. Mayo Clin Proc 1996;68:1074–80.
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Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins with the assistance of Stanford University Libraries' HighWire Press®. Copyright 2006 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999