An animal model simulating acute infective upper airway obstruction of childhood and its use in the investigation of croup therapy.

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 the 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 degrees, 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 degrees; iii) "warm dry" air, i.e., air at a mean temperature of 36.5 degrees, obtained by passing compressed air through a copper coil, immersed in hot water; iv) "warm moist" air at a mean temperature of 36 degrees, obtained by passing compressed air through a heated Puritan humidifier; v) "ultrasonically produced mist" at room temperature (24 degrees), produced by a De Vilbiss ultrasonic nebulizer 900, set at maximum output. Six 2-hr experimental periods were utilized for each regimen of treatment (i--v). The mean of the combined experimental resistance calculations for each environment were compared statistically with those of its similarly acquired control data, expressed as a positive or negative percentage change, and displayed as a histogram. In addition, the data from each experimental environment were compared with the others, and mean percentage changes from control against time were calculated. Results indicate no statistically significant change in resistance from control for ultrasonic mist or warm moist treatment regimens. Warm dry, cold dry, and cold moist treatments, however, all produced significant reductions in translaryngeal resistance (74%, 60%, and 54%, respectively), there being no difference between cold dry and cold moist and warm dry and cold dry. Significant differences between warm dry and cold moist treatments were obtained, the former decreasing translaryngeal resistance the greatest amount...