New methods of neuromuscular blockade monitoring

Thomas Fuchs-Buder, MD

Department of Anaesthesia and Critical Care

University of Saarland, Homburg/Saar, Germany

The interest in monitoring neuromuscular function during anaesthesia has been growing over the past few years: New short- and intermediate-acting nondepolarizing neuromuscular blocking drugs have become available and awareness of the problems of postoperative residual neuromuscular blockade has been increasing (1). In the following new methods of neuromuscular blockade monitoring will be discussed.

Although intubating conditions are affected by the depth of anaesthesia, excellent intubating conditions might not be obtained unless complete blockade of the respiratory muscles (2). However, monitoring of the laryngeal adductor muscles or the diaphragm is invasive and difficult to set up. Some studies suggest that the orbicularis occuli behaves like laryngeal muscles, whilst others could not confirm this findings (3,4). As recently demonstrated the discrepancies reported about the muscle relaxant effects registered around the eye may be related to the site of measurement. It has been suggested that the corrugator supercilii and not the orbicularis oculi has a sensitivity similar to that of the laryngeal adductors (5). By consequence, the corrugator supercilii may be the most appropriated muscle to assess the time course of neuromuscular blockade for endotracheal intubation.

Recovery of adaequate neuromuscular function postoperatively is mandatory to assure that patients are able to sustain adequate ventilation and cough, and maintain their airway open. For many years a train-of-four ratio of 0.7, measured at the adductor pollicis, was considered synonymous with adaequate ventilatory function postoperatively. This was based mainly on the assumption that sustained maximum inspiratory force and minute ventilation indicated safe recovery from neuromuscular blockade. However, new insights into the pathophysiological consequences of residual neuromuscular blockade required more rigorous criteria for determining the adequacy of neuromuscular recovery (6). Thus, a TOF ratio of 0.7 can no longer be accepted as a index of sufficient recovery of neuromuscular blockade; it is now generally accepted that at least a TOF ratio of 0.9 is required. By consequence tactile or visual evaluation of the TOF ratio - probably the technique most commonly used to assess neuromuscular recovery - is no longer sufficient because it allows only to detect residual paralysis corresponding to a TOF ratio of 0.4 to 0.5 (7). Only objective monitoring such as accelerography allows to determine neuromuscular recovery with TOF stimulation (8). However, when objective monitoring is not available tetanic stimulation with either 50 or 100 Hz may be the most appropriated stimulation pattern to assess discrete residual paralysis (9).

Recently, Dascalu and co-workers evaluated the performance of a low-frequency microphone in monitoring intraoperative muscular function. The rational for their investigation was the finding that contraction of skeletal muscle generates intrinsic low-frequency sounds and these acoustic waves propagate to the skin, generating pressure waves which can be recorded by low-frequency microphone. The amplitude of an acoustic signal has been shown to be proportional the degree of muscle contraction and thus it can be used as a non-invasive technique to quantify the development of force in human muscle. The authors reported that acoustic monitoring of intraoperative neuromuscular block is clinically feasible and correlated closely with established methods s.a. mechanomyography, electromyography or accelerography (10). To determine the clinical relevance of this method, however, further exploration is indicated.

Literature

1) Viby-Mogensen J. Neuromuscular Monitoring. In: Miller RD (ed.) Anesthesia, Philadelphia , Curchill Livingstone 2000; 1351 - 1366.

2) Schlaich et al. Remifentanil and propofol without muscle relaxants or different doses of rocuronium for tracheal intubation in outpatient anaesthesia. Acta Anaesthesiol Scand 2000; 44:720-6.

3) Donati F et al. Vecuronium neuromuscular blockade at the diaphragm, the orbicularis oculi, and the adductor pollicis muscles. Anesthesiology 1990; 73:870-5.

4) Rimaniol JM et al. A comparison of the neuromuscular blocking effects of atracurium, mivacurium, and vecuronium on the adductor pollicis and the orbicularis oculi muscle in human. Anesth Analg 1996; 83:808-13.

5) Plaud B et al. The corrugator supercilii, not the orbicularis oculi, reflects rocuronium neuromuscular blockade at the laryngeal adductor muscles. Anesthesiology 2001; 95:96-101

6) Eriksson LI. The effects of residual neuromuscular blockade and volatile anaesthetics on the control of ventilation. Anesth Analg 1999; 89:243-51.

7) Viby-Mogensen J et al. Tactile and visual evaluation of the response to train-of-four stimulation. Anesthesiology 1985; 63: 440-3.

8) Mortensen CR et al. Perioperative monitoring of neuromuscular transmission using accelerography prevents residual neuromuscular block following pancuronium. Acta Anaesthesiol Scand 1995; 39: 797-803.

9) Baurain MJ et al. Visual evaluation of residual curarization in anaesthetised patients using one hundred hertz, five second tetanic stimulation at the adductor pollicis muscle. Anesth Analg 1998; 87:185-9.

10) Dascalu A et al. Acoustic monitoring of intraoperative neuromuscular block. Br J Anaesth 1999; 83:405-9.