Guaifenesin: cardiopulmonary effects and plasma concentrations in horses

We determined the cardiopulmonary changes in horses given guaifenesin alone and in the same horses given xylazine (IV) immediately before administration of guaifenesin. In addition, plasma guaifenesin concentrations were determined in horses and 2 pregnant mares and compared with previously published values in ponies. The dose of guaifenesin necessary to produce lateral recumbency in adult horses was 134 +/- 34 mg/kg (mean +/- SD). The administration of guaifenesin caused insignificant (P less than 0.05) changes in heart rate, respiratory rate, right atrial pressure, pulmonary arterial pressure, and cardiac output. Systolic, diastolic, and mean arterial blood pressures were significantly (P less than 0.05) decreased. Arterial partial pressure of oxygen (PaO2) was transiently, but significantly (P less than 0.05), reduced. The administration of xylazine (IV) to the horses before they were given guaifenesin reduced the dose of guaifenesin necessary to produce lateral recumbency to 88 +/- 10 mg/kg. In these horses, there were significant transient decreases in heart rate, respiratory rate, cardiac output, and PaO2. Central venous pressure remained significantly (P less than 0.05) increased, whereas systolic, diastolic, and mean arterial blood pressures were significantly (P less than 0.05) decreased at all times. In horses given guaifenesin alone, plasma concentrations at the time of lateral recumbency were 313 +/- 108 microgram/ml, and in those given xylazine-guaifenesin, the concentrations of the drug were 277 +/- 83 microgram/ml. Neonatal concentrations of guaifenesin were approximately 30% of that of the mare immediately after delivery, indicating that significant amounts of guaifenesin cross the placental barrier in the mare.