CHRONIC ISOLATED TRACHEAL SEGMENT TO STUDY AIRWAY REFLEXES IN CONSCIOUS DOGS

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Abstract
Airway reflexes are difficult to study in conscious animals because associated changes in ventilation alter intrathoracic airway dimensions. By studying an isolated segment of extrathoracic trachea, this problem was overcome. In 2 dogs an isolated tracheal segment was surgically created just below the larynx, sealed at one end and tapered at the other to a 3 mm opening via a skin fistula. A chronic tracheostomy was created near the thoracic outlet. Intraluminal pressure (Pseg) of the isolated segment ws monitored to reflect changes in smooth muscle tone. During anesthesia with pentobarbital, gentle mechanical stimulation of the carina lung deflation and asphyxia for 7 min increased Pseg (+9 to +16 cm H2O). Lung inflation and alveolar hyperventilation decreased Pseg (-9 to -16 cm H2O). Five breaths of 2% histamine aerosol increased Pseg (+5 cm H2O) when resting tone was normal. The lumen of the isolated segment was coated with tantalum powder and changes in the size of the segment that reflected changes in smooth muscle tone were documented roentgenologically; constriction and dilation in response to asphyxia and lung inflation, respectively, were demonstrated directly by this technique. In conscious dogs lung inflation decreased Pseg and carinal stimulation increased Pseg. Instillation of lidocaine hydrochloride (Xylocaine) into the isolated tracheal segment blocked cough caused by mechanical stimulation of the segment, but carinal stimulation still caused constriction of the segment under these conditions, which indicated that afferent, but not efferent, parasympathetic innervation of the segment had been blocked selectively. Conversely, instillation of atropine sulfate into the isolated tracheal segment blocked constriction of the segment caused by carinal stimulation, but mechanical stimulation of the segment still caused cough under these conditions, which indicated that efferent, but not afferent, parasympathetic innervation of the segment had been blocked selectively. An innervated extrathoracic tracheal segment constricts and dilates via cholinergic pathways and is suitable for the study of airway reflexes in conscious dogs.