EFFECTS OF SEDATION, ANESTHESIA, AND ENDOTRACHEAL INTUBATION ON RESPIRATORY MECHANICS IN ADULT HORSES

Objective-To determine the effects of endotracheal intubation on respi ratory mechanics during xylazine sedation a nd xylazine-diazepam-ketam ine anesthesia in adult horses. Animals-5 healthy adult horses. Proced ure-Measurements were derived from recordings of respiratory gas flow, and transpulmonary and transtracheal pressures. Total pulmonary resis tance (R-T) was partitioned into upper airway resistance (extrathoraci c portion of trachea, larynx, pharynx, nasal cavity, nares; R-UA) and lower airway resistance (intrathoracic portion of trachea, bronchi, br onchioles). Baseline measurements were obtained in unsedated horses, a fter xylazine administration, and following nasotracheal intubation (I D, 18 mm). Measurements were obtained following induction of xylazine- diazepam-ketamine anesthesia and subsequent to endotracheal intubation s (ID, 22, 20, and 16 mm). During recovery, horses were nasotracheally intubated (ID, 18 mm). Measurements were obtained upon standing, and repeated after extubation. Data were examined by use of ANOVA with rep eated measures. Results-Significant increases in mean work of breathin g (W), R-T, and R-UA observed with xylazine sedation were variably att enuated by nasotracheal intubation. During xylazine-diazepam-ketamine anesthesia, the highest mean values for W, R-T, R-UA, transpulmonary a nd transtracheal pressures developed during non-intubation periods. Th e magnitudes of resistance and pressure values were inversely proporti onal to the internal diameter of the endotracheal tube. At recovery, v alues of the W and all measurements of resistances and pressures were significantly increased, compared with presedation values. Extubation resulted in further increases in these measurements. Conclusions-Work of breathing in horses is substantially increased when R-UA is increas ed during xylazine sedation and xylazine-diazepam-ketamine anesthesia. Endotracheal intubation reduces W by reducing R-UA.