Regulation of airway smooth muscle tone in sleeping dogs

We examined the influence of sleep state on airway smooth muscle tone in 4 unanesthetized dogs that were trained to sleep in the laboratory. The dogs had been prepared with a permanent side-hole tracheostomy and bilateral cervical vagal loops. During the studies, the dogs breathed through a cuffed endotracheal tube inserted through the tracheostomy. To monitor changes in tracheal smooth muscle tone, we measured the pressure in the water-filled cuff of the endotracheal tube. The technique was validated by examining changes in cuff pressure after administration to the dogs of a series of chemical agents and physiologic stimuli known to constrict or relax tracheobronchial smooth muscle. Sleep state of the dogs was determined by behavioral, electroencephalographic, and electromyographic criteria. During quiet wakefulness, tracheal smooth muscle tone was stable. With the onset and progression of sleep through the nonrapid-eye movement stages, airway smooth muscle tone relaxed (decrease in cuff pressure of 20 to 40 cm H2O), reaching a new steady level during slow-wave sleep. In contrast, during rapid-eye-movement sleep, tracheal smooth muscle tone fluctuated markedly and erratically, as reflected by changes in cuff pressure as large as 90 cm H2O. Partial blockade of the vagus nerves, by cooling the exteriorized cervical vagal loops, decreased or abolished the fluctuations in tracheal smooth muscle tone during rapid-eye-movement sleep at temperatures that did not abolish resting tone, demonstrating that the changes in tone during rapid-eye-movement sleep were related to variability in neural control of airway smooth muscle.