EFFECTS OF PARTIAL PARALYSIS ON THE SWALLOWING REFLEX IN CONSCIOUS HUMANS

The ability to swallow may be affected by administration of a small dose of muscle relaxant. To test the hypothesis that a subparalyzing dose of a muscle relaxant can impair swallowing, effects of partial paralysis produced by pancuronium on the swallowing reflex were investigated in eight conscious subjects. The swallowing reflex was induced by a bolus injection or a continuous infusion of distilled water into the mesopharynx. The swallowing function was assessed by electromyogram of suprahyoid muscles (EMG(SH)), mesopharyngeal pressure (P(meso)), and hypopharyngeal pressure (Ph(hypo)). Peripheral muscle activity was simultaneously determined by train of four ratio (TOFR) of hypothenar muscles to electrical stimulation of ulnar nerve and by hand grip strength (HGS). Following control measurements, measurements during partial paralysis and after recovery from partial paralysis were performed after intravenous administration of pancuronium 0.02 mg/kg. Partial paralysis significantly depressed EMG(SH) (bolus injection 44.1 +/- 10.0%, continuous infusion 55.9 +/- 10.2% of control value, P < 0.01). P(meso) also significantly decreased (bolus injection 64.9 +/- 6.7 to 47.8 +/- 5.8 mmHg, P < 0.01; continuous infusion 63.4 +/- 7.7 to 52.5 +/- 5.8 mmHg, P < 0.05). The TOFR of peripheral muscles decreased to 81.4 +/- 6.7% of control value (P < 0.01), and HGS was reduced from 44.6 +/- 1.9 to 39.4 +/- 2.0 kg (P < 0.01). In the bolus injection study, the decrease in P(meso) was significantly greater than the decrease in HGS (73.7% vs. 88.3% of control value, P < 0.05), and the decrease in EMG(SH) was significantly greater than that in TOFR (44.1% vs. 81.4% of control value. P < 0.05). After recovery of peripheral muscle activity, both EMG(SH) and P(meso) had completely recovered. Partial paralysis did not affect the latency of response and the pattern of swallowing after bolus injection, and the frequency of swallows elicited by continuous infusion was also unchanged. Our results suggest that 1) upper airway muscles are more sensitive than peripheral muscles to the effects of pancuronium; 2) both the elevation of the larynx and the propelling force during swallowing are affected by the administration of a small dose of pancuronium; and 3) partial paralysis does not affect the neural pathway of the swallowing reflex in the conscious state.