Twelve heartworm-free mongrel dogs (10 males, 2 females) mean weight 22.2 kg (range 17.7-28.2 kg) were sedated and placed in a supine position with the neck extended. A double tracheotomy was performed under sterile conditions. The first tracheotomy tube was inserted 2 cm above the sternum in the direction of the carina using a shortened Silastic American tracheotomy tube (id 10 mm); the second was inserted two cartilage rings proximal to the first and approximately 5 cm distal to the larynx. All animals were kept deeply anesthesized by repeated iv injections of 3-6 mg/kg pentobarbitone sodium so that all reflexes (in particular laryngeal and palatal) were abolished. After control translaryngeal pressure measurements were obtained, the vocal cords were visualized, and steam was introduced onto and through the cords for a mean of 10 sec from above. For 2 hr after this, repeat pressure measurements, as described above, were made. After an overnight stabilizing period, two to three of five randomly chosen air environments were passed over the larynx at 10 liters/min via the proximal tracheotomy, and four half-hourly pressure measurements taken with control periods of at least 2 hr, as described above, separating each experiment. Environments utilized were as follows: i) “cold dry” air, i.e., air at a mean temperature of 9°, obtained by passing compressed air through a coil placed in a solution of alcohol/Dry Ice; ii) “cold moist” air, i.e., air as above, subsequently passed over cold water at a mean temperature of 11°; iii) “warm dry” air, i.e., air at a mean temperature of 36.5°, obtained by passing compressed air through a copper coil, immersed in hot water; iv) “warm moist” air at a mean temperature of 36°, obtained by passing compressed air through a heated Puritan humidifier; v) “ultrasonically produced mist” at room temperature (24°), produced by a De Vilbiss ultrasonic nebulizer 900, set at maximum output.Speculation
Hot or cold dry air may decrease upper airway resistance in children with acute infective croup, although it is possible that laryngeal and/or other reflexes may modify the diameter of the upper airway to a greater extent than any therapeutic regimen.