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‘Patients who sing need to be relaxed’-neuromuscular blockade as a solution for air-leaking during intermittent positive pressure ventilation using LMA

Université de Montréal, Montréal, Canada, E-mail: thomashemmerling_2000{at}yahoo.com]

To the Editor:

Laryngeal mask airways (LMA) are used with intermittent positive pressure ventilation (IPPV).¹ We present a series of 12 patients where problems with IPPV occurred. Twelve patients (four women, eight men, mean age: 48 ± 14 yr, mean weight 80 ± 14 kg, ASA classification 1 and 2) underwent surgery under general anesthesia using a flexible LMA (FLMA; Laryngeal Mask Co. Ltd, St. Helier, Jersey, UK), size 4 for women, size 5 for men, blocked with 30 or 35 mL of air.

LMA insertion was facilitated by rocuronium 0.2 mg·kg^–1 iv. Bispectral index monitoring (BIS; 2000 monitoring system, Aspect Medical Systems, Newton, MN, USA) was applied with a target of 40 to 55. Neuromuscular monitoring consisted of phonomyographic monitoring at the corrugator supercilii muscle and adductor pollicis muscle. Intermittent positive pressure ventilation was set at 15·min^–1, the tidal volume adjusted to PETCO₂ between 30 to 35 mmHg. During surgery, peak inspiratory pressures (PIP) were below 15 mmHg. In no patient was any initial leakage heard. Remifentanil was infused at a rate between 0.1 and 0.25 µg·kg^–1·min^–1, and sevoflurane was adjusted according to BIS.

After 55 ± 25 min of anesthesia these patients began to emit snoring or other high pitched sounds. Within ten minutes in all patients, the PIP exceeded 20 mmHg, tidal volumes decreased, and IPPV became difficult. Neuromuscular monitoring showed a train-of-four (TOF) ratio of more than 0.9 at the corrugator supercilii muscle, and no BIS value exceeded 58. Manual manipulation of the FLMA did not decrease the stridor. Intra-cuff pressures were below 80 mmHg in all patients; intermittent application of 1 mL of air into the cuff did not change the phenomenon. In all patients, a bolus of 1 mg·kg^–1 iv propofol was administered and remifentanil was increased to 0.5 mg·kg^–1·min^–1 to increase depth of anesthesia (BIS < 35). The difficult IPPV situation (high PIP, low tidal volumes, air leakage) did not change. In accordance with the principle that neuromuscular blockade might change the geometry of the larynx and facilitate IPPV via the FLMA, rocuronium 0.15 mg·kg^–1 iv was given. Within three minutes, ‘singing’ stopped in all subjects, tidal volumes and PIP returned to initial levels, and anesthesia continued without further disturbance.

We would recommend use of a small dose of neuromuscular blocking agent whenever patients start to ‘sing’ during IPPV via LMA to avoid ventilation problems.

Reference

1 Sidaras G, Hunter JM. Is it safe to artificially ventilate a paralysed patient through the laryngeal mask? The jury is still out (Editorial). Br J Anaesth 2001; 86: 749–53.[Free Full Text]

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