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An External Monitoring Site at the Neck Cannot Be Used to Measure Neuromuscular Blockade of the Larynx

Hemmerling, Thomas M. MD, DEAA; Michaud, Guillaume; Deschamps, Stéphane MSc; Trager, Guillaume MSc

doi: 10.1213/01.ANE.0000152189.85483.0E
Technology, Computing, and Simulation: Technical Communication

Using phonomyography, a new monitoring technique of neuromuscular blockade (NMB), we compared NMB after mivacurium 0.1 mg/kg at the lateral cricoarytenoid muscle (LCA) with a possible external monitoring site of the larynx. In 12 patients, data were obtained at both sites using phonomyography. Anesthesia was induced with remifentanil 0.25–0.5 μg · kg−1 · min−1 followed by propofol 2–3 mg/kg. A small piezo-electric microphone was positioned beside the vocal cords into the muscular process at the base of the arytenoid cartilage to record acoustic signals from the contraction of the LCA. A second microphone was positioned at an external site, lateral to the trachea, just below the thyroid notch. The recurrent laryngeal nerve was stimulated supramaximally using train-of-four (TOF) stimulation every 12 s. Onset, maximum effect, and offset of NMB were measured and compared. Peak effect, time to reach (T) 25%, 75%, and 90% of control twitch response, and TOF recovery to TOF ratios 0.5–0.8 were significantly longer at the external site. The onset time was not significantly different between the two sites. We used phonomyography with a microphone placed at the neck to evaluate the possibility to externally monitor NMB at the larynx. When compared with LCA, we found a more pronounced peak effect and longer offset of NMB. The acoustic signals recorded at this external site are unlikely to stem from laryngeal muscle contraction but are rather a result of contraction of the strap muscles of the neck.

IMPLICATIONS: We used phonomyography with a microphone placed at the neck to evaluate the possibility to externally monitor neuromuscular blockade at the larynx. Whereas this external site equals the onset of neuromuscular blockade at the larynx, it cannot be used as a substitute for direct laryngeal monitoring. Phonomyography records acoustic signals from strap muscles at that site.

Neuromuscular Research Group (NRG), Department of Anesthesiology, Centre Hospitalier de l’Université de Montréal (CHUM) Hôtel-Dieu, Université de Montréal, Canada

Dr. Hemmerling is recipient of the 2003 Career Scientist Award of the Canadian Anesthesiologists’ Society, which provides salary support.

Accepted for publication November 16, 2004.

Address correspondence and reprint requests to T.M. Hemmerling, MD, DEAA, Department of Anesthesiology, Université de Montréal, Hôtel-Dieu, 3580, rue St-Urbain, Montréal (Québec) H2W 1T8, Canada. Address e-mail to thomashemmerling_2000@yahoo.com.

Neuromuscular blocking drugs are used to facilitate endotracheal intubation. Neuromuscular blockade (NMB) at the larynx ensures optimal intubating conditions; complete recovery of neuromuscular transmission at the laryngeal muscles is important for airway protection at and after tracheal extubation (1). Because the larynx reacts differently from other muscles to the administration of neuromuscular blocking drugs (2), monitoring of more peripheral muscles, such as the adductor pollicis muscle, do not reflect NMB at the laryngeal muscles or other more profound muscles. However, NMB at the larynx is difficult to measure. Whereas superficial stimulation of the recurrent laryngeal nerve is simple and can be performed using routine AgAgCl-electrodes glued at the neck, direct monitoring of the larynx is challenging because all current methods require endotracheal intubation, without the aid of NMB, before laryngeal monitoring can be commenced (3–7). The cuff-pressure method (3) requires the placement of an endotracheal tube before the start of monitoring. Surface electromyography also requires the insertion of either a special endotracheal monitoring tube (4) or a routine tube equipped with a surface electrode (5–7). Even the most recent method, phonomyography (8–11), based on the fact that muscles create low-frequency sounds when contracting by the lateral movement of the muscle fibrils (12–14), requires the insertion of a microphone into the throat before the beginning of the actual monitoring.

Therefore, it would be convenient to have an external site from which to monitor the laryngeal muscles. The focus of this study was to evaluate the possibility of monitoring laryngeal NMB from the outside by placing a microphone at the neck next to the larynx. This external site was compared with internal monitoring of a laryngeal adducting muscle, the lateral cricoarytenoid muscle (LCA), to determine onset, peak effect, and offset of NMB at both monitoring sites.

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Methods

After approval of the local ethics committee and obtaining informed consent, 12 patients undergoing general surgery were included in the study. Patients with coexisting neuromuscular disease or patients on medication known to interact with neuromuscular transmission were excluded.

After arrival in the operating room, routine monitoring was started. Anesthesia was induced with remifentanil 0.25–0.5 μg · kg−1 · min−1; 2 min later, propofol 2–3 mg/kg was injected. After loss of consciousness and ventilation via face mask for 2 min with 100% oxygen, an endotracheal tube was inserted without the aid of neuromuscular blocking drugs, and ventilation was set to maintain an end-tidal Petco2 of 3.5–4.5 kPa. Anesthesia was maintained with 1–1.5 minimum alveolar anesthetic concentration of sevoflurane in a breathing gas mixture of 30% oxygen in air to maintain a target bispectral index of 50 (A2000 monitoring system; Aspect Medical Company, Newton, MA). Analgesia was provided by remifentanil 0.05–0.25 μg · kg−1 · min−1 throughout surgery.

Using a Magill forceps, a small piezo-electric microphone was inserted beside the vocal cords into the muscular process at the base of the arytenoid cartilage to record the acoustic signals from the contraction of the LCA (Fig. 1) (9).

Figure 1

Figure 1

To monitor laryngeal block externally, a second microphone was attached to the skin of the neck, lateral to the trachea, just lateral to the thyroid cartilage using a gluing tape (Fig. 2). Correct positioning of the microphones before and immediately after the observation period was controlled in all patients. The microphone signals were amplified and band-pass filtered between 0.5 Hz and 1000 Hz using an AC/DC amplifier (Model 7P122; Grass Instruments, Astra-Med, Inc., West Warwick, RI). The signals were continuously sampled at 100 Hz using the Polyview® software package (Microsoft, Redmond, WA), digitized, and stored on a portable microcomputer. The phonomyographic signals were measured peak-to-peak.

Figure 2

Figure 2

Stimulation of the recurrent laryngeal nerve was performed using two superficial electrodes placed medially at a vertical line between the sternal notch and the cricoid cartilage. Supramaximal stimulation was determined using single-twitch stimulation at 0.1 Hz. Stimulation was performed using a constant current stimulator (Innervator®; Fisher and Paykel Healthcare, Auckland, New Zealand), which generated single-twitch square pulses of 0.2 ms with a current intensity between 0 and 70 mA (train-of-four [TOF] stimulation every 12 s). After at least 5 min of supramaximal stimulation and stable baselines for all signals, mivacurium 0.1 mg/kg was injected within 5 s into a fast-flowing solution of Ringer’s lactate. Onset, peak effect, and offset of NMB were determined.

The first twitch response was used to analyze onset time (time to reach maximum decrease in twitch amplitude) and time to reach 25%, 75%, and 90% of control twitch response (T 25%, T 75%, and T 90%). The peak effect was determined as the maximum decrease of the twitch response and recorded. Time to reach a TOF ratio of 0.5, 0.7, and 0.8 were also calculated for all signals.

Sample size was calculated using an expected difference of means of 2 min at T 25% for a Power of 0.8 and α = 0.05. Data of NMB at both sites were compared using a two-sided paired t-test. P < 0.05 was considered as showing a significant difference. Bias and limits of agreement of all measured times were calculated using the Bland-Altman method. Data are presented as mean ± sd.

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Results

We were able to obtain pharmacodynamic data in 12 patients, 8 men and 4 women, with a mean age of 45 ± 12 yr and mean weight of 77 ± 11 kg. The waveform of the phonomyographic signals from LCA and the external site was similar, showing the same typical pattern found at the adductor pollicis (11): the primary depression is followed by a positive wave.

A significantly less pronounced peak effect and shorter recovery of NMB was found at the internal monitoring site (Table 1; Fig. 3). For all recovery times, the bias (LCA–external site) ranged from −3.1 to −3.8 min with unacceptably wide limits of agreement (Table 1). External monitoring at the neck does not reflect NMB of the larynx.

Table 1

Table 1

Figure 3

Figure 3

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Discussion

Using phonomyography, a new external monitoring site, situated lateral to the thyroid cartilage, was evaluated to monitor NMB at the larynx from the outside. In comparison to direct, internal monitoring of the laryngeal adducting muscles, represented by the LCA, it showed a more pronounced peak effect and longer offset of NMB. Significant bias and wide limits of agreement between the two sites interdict the interchangeable use of both sites for monitoring laryngeal NMB. The external site probably reflects NMB at the strap muscles in a nonspecific way.

The idea of searching for an external site to monitor phonomyographic signals from the larynx stemmed from research in laryngeal physiology. In a study (16) of the ratio of the spectrum of the estimated volume velocity exciting the vocal tract to the acceleration delivered to the neck wall (neck frequency response function) in nine patients after laryngectomy and four healthy volunteers, the neck acted as a low-pass filter, facilitating the pass of acoustic signals of lower frequency. However, the range of frequencies studied was more than 50 Hz (60–4000 Hz), which excludes most of the power spectrum from phonomyographic signals after evoked muscle contraction (peak frequency between 2.5 and 5 Hz, and 90% of the total signal power less than 40 Hz (10)). In another study (17), a new method was studied in two volunteers to record rapid subglottal pressure changes related to the glottal cycles using microphones placed at the skin of the jugular fossa. In those two volunteers, jugular microphone recordings using a condenser microphone were compared with direct intratracheal measurements. After extrapolation of a systematic error caused by the transmission properties of the microphone and the soft tissue, good correlation and agreement was found between the two monitoring sites. However, it must be stressed that only subglottal pressure changes, and not actual vocal cord contraction, were measured in that study.

Previously, we evaluated several possible monitoring sites at the neck by placing the microphone on the neck and taping it with a standard gluing tape. Because of the size of the microphone and its shape (Fig. 2), attachment in midline position in patients with prominent thyroid cartilage was difficult. We therefore opted for this slightly lateral position, which would bring the microphone closest to the position of the LCA muscles and the path of contraction. Neumann et al.’s study (17) indicates that sound produced next to the vocal cords could be recorded with microphones placed outside, although the number of patients in that study was small.

In a work from Tjan et al. (18), visual estimation of NMB at the strap muscles after a TOF stimulation of the recurrent laryngeal nerve was used as an indicator of good intubating conditions. The intubating conditions at onset of complete muscle relaxation of the strap muscles were found to be good or excellent. Our findings confirm that onset time of NMB at the larynx, as measured objectively using phonomyography of the LCA via a microphone placed internally, is not different from onset time of NMB measured at the strap muscles. Therefore, it seems valid to use the measurement of onset time at the strap muscles as an indication of onset of NMB at the laryngeal muscles, thus indicating the earliest moment of good to excellent intubating conditions. The small, but significant, difference of peak effect between the two sites does not seem to influence the intubating conditions. However, the significant difference of offset of NMB between the two sites and the significant bias and wide limits of agreement of all pharmacodynamic data prevent the conclusion that the external monitoring site actually measures laryngeal relaxation. Our theory is that it measures, at best, a combined response of muscle contraction of several strap muscles such as the superior belly of omohyoid, sternothyroid, and sternohyoid muscles (Fig. 2) (15) after nonspecific IM stimulation. Those muscles are innervated by the ansa cervicalis (which originates from ventral branches of cervical nerves C1, C2, and C3) and not by the recurrent laryngeal nerve. Stimulation of IM nerve branches could have led to contraction of those muscles. In addition, not all intrinsic muscles of the larynx respond in the same way to NMB drugs. The posterior cricoarytenoid muscle has a longer offset of NMB than the LCA after the administration of mivacurium 0.1 mg/kg (19). Therefore, acoustic signals recorded at the external site could also reflect signals from intrinsic muscles of larynx other than the LCA.

Phonomyography can be used to monitor NMB at an external site of the larynx, lateral to the throat, just lateral the thyroid cartilage. Acoustic signals from strap muscles of the neck are thought to be the principal component of this signal. When compared with LCA, NMB at this site is more pronounced and shows longer recovery. However, onset of NMB at these strap muscles is not significantly different from onset of NMB at the laryngeal muscles, making it a useful monitoring site for laryngeal relaxation. This might be helpful for the determination of the earliest possible good intubating conditions during rapid sequence anesthetic induction.

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