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From the Department of Anesthesiology, Erlangen-Nuremberg University, Germany.
Present address for correspondence: Dr. Thomas M. Hemmerling, Department of Anesthesiology, University of Montreal, CHUM, Hôtel-Dieu, 3580 rue St-Urbain, Montreal, (Quebec) H2W 1T8, Canada. Phone: 514-843-2611; Fax: 514-843-2690; E-mail: thomashemmerling{at}hotmail.com
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Abstract |
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Methods: In 24 patients undergoing thyroid surgery, evoked responses from the adducting laryngeal muscles and the AP muscle were obtained using surface electromyography (EMG). Skin electrodes were placed paravertebrally near T12/L1 or L1/L2 (novel position; n=12) or conventionally (n=12). After stimulation of the recurrent laryngeal, phrenic and ulnar nerves, the lag, onset time and maximum effect were measured (0.1 Hz, single twitch) as well as the time to reach 25% of T1/T0 (T 25%) using train-of-four stimulation every 20 sec.
Results: A mean maximum block of more than 94% was reached at all sites. Lag, onset time and T 25% at the adducting laryngeal muscles and the diaphragm were significantly (P <0.005) shorter than at the AP muscle and did not differ significantly between the two diaphragmatic monitoring sites (conventional: 64 ± 21 sec, 166 ± 41 sec and 20 ± 3 min vs novel: 60 ± 16 sec, 161 ± 40 sec and 22 ± 2 min respectively).
Conclusion: Onset and duration of action of 0.1 mg•kg–1 cisatracurium was shorter at the larynx and the diaphragm than at the AP muscle. EMG results obtained from the novel, paravertebral site did not differ from the conventional monitoring site at the seventh or eighth intercostal space and suggest this alternative site is appropriate for monitoring of the diaphragm.
MONITORING of the diaphragmatic response using surface skin electrodes was introduced into clinical research in 1986 by Donati et al.1 These authors investigated the seventh or eighth intercostal space between the midclavicular and anterior axillary line as a possible monitoring site. This site has been successfully used in assessment of neuromuscular block (NMB) at the diaphragm for research purposes over the last decade.
Its use, however, is limited by the fact that the electrodes often have to be placed within the surgical field, specially in abdominal or cardiothoracic surgery. Open and closed abdominal surgery could benefit from monitoring of NMB of the diaphragm since diaphragmatic function will influence abdominal pressure.
In this study, a novel site for diaphragmatic surface electromyography (EMG) recording in the paravertebral region was compared with the established site at the seventh or eighth intercostal space. We simultaneously determined the onset and peak effect of 0.1 mg•kg–1 cisatracurium at the adducting laryngeal muscles2,3 and the adductor pollicis (AP) muscle using surface electrodes.
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Materials and methods |
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Two minutes after the start of a remifentanil infusion at 0.5 µg•kg–1•min–1, anesthesia was induced using a target- controlled infusion of propofol (target concentration: 4 µg•ml–1), programmed to reach the target concentration within 30 sec. After induction of anesthesia was completed, the patients were ventilated via face-mask for three minutes and the trachea was intubated using a Woodbridge® tube (Mallinckrodt, UK, size 7.0: female, size 8.0: male) with the surface laryngeal electrode (Magstim company, UK) attached 2 cm above the cuff and placed between the vocal cords for an optimal EMG tracing.3 Two Ag/AgCl-skin-electrodes were placed over the right thenar area for EMG recording of the AP muscle using the Relaxograph® (Datex, Finland), and two Ag/AgCl-skin-electrodes were used to stimulate the ulnar nerve.
Anesthesia was maintained with a target-controlled infusion of propofol (target concentration: 3 µg•ml–1) and remifentanil at 0.25 µg•kg–1•min–1. Mechanical ventilation (30% oxygen in air) was adjusted to achieve an end-tidal CO2 -pressure of 25–40 mmHg.
Patients were randomly assigned to one of two groups. In the "novel" group (n=12), two Ag/AgCl-skin- electrodes were placed on the right of vertebrae T12/L1 or L1/L2 (wherever the maximal response was obtained), 2–3 cm apart, for EMG-monitoring of the right diaphragmatic muscular crux4 (Figure 1
). In the "conventional" group (n=12), two Ag/AgCl-skin-electrodes were placed at the right seventh or eighth intercostal space between the midclavicular and anterior axillary line.1
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The amplitudes of the diaphragmatic and the laryngeal compound action potentials (peak-to-peak) were measured and recorded (Figure 2).
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The time from the end of injection of the muscle relaxant to the first twitch depression, the maximum twitch depression (lag time, onset time) as well as maximum block (% reduction of the maximal neuromuscular response) of NMB were measured. Clinical duration of NMB was defined as the time to reach 25% of T1/T0 (T 25%).
Difference of means ± and standard deviation of onset time at the diaphragm and the AP muscle of a pilot study6 was used to determine group size for a power of more than 0.9 (beta error=10%). The results are expressed as mean standard deviation and range. The pharmacodynamic parameters were compared between larynx, diaphragm and AP muscles using the Kruskal-Wallis test. Differences were then compared using rank sum test, corrected for the number of comparisons. The pharmacodynamic parameters between the two diaphragmatic sites were compared using the paired rank sum test. A P <0.05 was considered to indicate a statistically significant difference.
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Results |
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The pharmacodynamic data are presented in the Table.
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Lag, onset time, maximum effect and clinical duration were not different between the two diaphragmatic monitoring sites.
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Discussion |
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The seventh or eighth intercostal space, between the midclavicular and anterior axillary lines, have been used to monitor diaphragmatic NMB in research for more than 15 yr.1 With the introduction of closed endoscopic techniques in modern surgery, monitoring the degree of NMB of the diaphragm during surgery may be important since diaphragmatic tone greatly influences abdominal pressure. However, using the seventh or eighth intercostal space for intraoperative monitoring in laparoscopic surgery is impossible because the monitoring electrodes would remain within the sterile field. Since the lumbar diaphragm inserts with its muscular crura on the first two to three lumbar vertebrae, it seemed possible to monitor its response to phrenic nerve stimulation at a paravertebral site near vertebrae T12–L3 ( Figure 1
). By starting at T12/L1 and moving the electrodes downwards, we were able to obtain signals in all patients. The signals from the patient's paravertebral region may reflect mainly the posterior portion of the diaphragm, just as the electrodes placed at the conventional site reflect its anterior portion. It is unknown to what extent these regional responses reflect the response of the entire diaphragm or that of the surrounding skeletal muscles.
Transcutaneous stimulation of the phrenic nerve was selected in such a way that no or only minimal concomitant stimulation of the brachial plexus occurred. The concomitant stimulation of the brachial plexus causes interference with monitoring of the diaphragm, especially due to parallel contraction of the shoulder and anterior thoracic muscles. In comparison to diaphragmatic monitoring at the seventh or eighth intercostal space, the "novel" site seems less prone to interferences by concomitant stimulation of these muscles. There is always the possibility of accidentally stimulating the brachial plexus in such a way that the latissimus dorsi muscle could be contracting and this would impair the measurements in the paravertebral region. However, this was not the case in any of our patients. One major setback at present – true for both sites – is the difficulty to stimulate the phrenic nerve transcutaneously at the neck. A stimulation using a needle stimulator produces quick and forceful phrenic nerve stimulation, but it cannot be used for ethical reasons. Transcutaneous stimulation of the phrenic nerve is time-consuming (in some patients up to 15 min was required to find the maximal signal) and great care has to be taken during measurements to ensure that no neck movement occurs. This difficulty to establish a quick, easy and reliable transcutaneous stimulation of the phrenic nerve is one of the major hurdles preventing the introduction of diaphragmatic monitoring into clinical practice.
We did not compare the two sites for diaphragmatic monitoring intra-individually since we were stimulating only one phrenic nerve. Insufficient stimulation and accidental concomitant stimulation of the brachial plexus could have caused a significant bias, making correlation and comparison of the two sites difficult. Bilateral stimulation of the phrenic nerve would have been technically arduous and could pose a risk to the patient by causing bradyarrythmias.
Only two studies objectively measured onset of NMB of cisatracurium at the larynx. Kim et al.7 used the cuff pressure method5 to determine onset times after 0.1 mg•kg–1 cisatracurium at the larynx and at the AP muscle. They measured a mean onset of 2.7 min at the larynx, much shorter than the mean onset time of 3.9 min at the AP. This is in concordance with our results for larynx, diaphragm and AP. In a former study,3 we measured an onset time of 200 sec in ten patients, approximately 25 sec longer than determined in the current study, reflecting the expected interindividual variability. Again, the surface electrodes proved easy to use and reliable. Incidentally, Ag/AgCl-electrodes are routinely placed in the patient's back for electrocardiographic monitoring in our hospital setting; the use of a cushioned operating table avoids pressure induced irritation or damage to the patient's skin.
Our results show that monitoring of the onset of NMB at the AP muscle, a routine clinical practice, insufficiently reflects the onset of respiratory muscle NMB. Although this has been demonstrated for all neuromuscular blocking drugs, a potential hazard has been suggested in a recent study8 in which cisatracurium was used during induction of anesthesia. Waiting for complete onset of NMB of 0.1 mg•kg–1 cisatracurium could create problems when short acting induction drugs such as etomidate, thiopental or propofol9 are used for induction of anesthesia. Given the duration of action of these drugs is shorter than the onset of the NMB at the AP, the patient might have sufficient NMB at the hand but be awake during intubation.8 Knowledge that early paralysis of the respiratory muscles will allow endotracheal intubation should avoid this problem.
In conclusion, we present a novel site for monitoring NMB at the diaphragm using skin electrodes in the lumbar paravertebral region. As expected, determination of the onset of NMB after 0.1 mg•kg–1 cisatracurium showed a much faster onset at the respiratory muscles (mean of 2.5 min) in comparison to almost four minutes at the AP. This novel site of monitoring the diaphragm provides an interesting alternative to assess NMB of the diaphragm, specially when the conventional, anterior site is not accessible for surgical reasons.
Accepted for publication December 6, 2000.
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References |
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2
Hemmerling TM, Schurr C, Walter S, Dern S, Schmidt J, Braun GG. A new method of monitoring the effect of muscle relaxants on laryngeal muscles using surface laryngeal electromyography. Anesth Analg 2000; 90: 494–7.
3
Hemmerling, TM, Schmidt J, Wolf T, Wolf SR, Jacobi KE. Surface vs intramuscular laryngeal electromyography. Can J Anesth 2000; 47: 860–5.
4 Moore KL, Dalley, AF. Clinically Oriented Anatomy. Baltimore: Lippincott Williams & Wilkins Inc., 1999: 291.
5 Donati F, Plaud B, Meistelman C. A method to measure elicited contraction of laryngeal adductor muscles during anesthesia. Anesthesiology1991; 74: 827–32.[Medline]
6 Wolf T, Hemmerling TM, Hanusa C, Schmidt J, Jacobi KE. Cisatracurium onset at larynx, diaphragm using new forms of surface electromyography (EMG). Anesthesiology 2000; 93(suppl 3A): A1031 (abstract).
7
Kim KS, Chung CW, Shin WJ. Cisatracurium neuromuscular block at the adductor pollicis and the laryngeal adductor muscles in humans. Br J Anaesth 1999; 83: 483–4.
8 St. Pierre M, Landsleitner B, Schwilden H. High incidence of conscious awareness as determined by the isolated forearm technique during induction of anesthesia with etomidate and cisatracurium. Anesthesiology 1998; 89(suppl 3A): A105 (abstract).
9 Schuettler J, Neglein J, Bremer F. Checkliste Anaesthesie Hamburg Thieme 2000; 102–4.
This article has been cited by other articles:
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  T. M. Hemmerling and N. Le Brief review: Neuromuscular monitoring: an update for the clinician: [Article de synthese court : Monitorage neuromusculaire : une mise a jour pour le clinicien] Can J Anesth, January 1, 2007; 54(1): 58 - 72. [Abstract] [Full Text] [PDF]
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T. M. Hemmerling and F. Donati Neuromuscular blockade at the larynx, the diaphragm and the corrugator supercilii muscle: a review: [Blocage neuromusculaire du larynx, du diaphragme et du muscle sourcilier : une revue] Can J Anesth, October 1, 2003; 50(8): 779 - 794. [Abstract] [Full Text] [PDF]
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