Cardiopulmonary effects of narcotic agonists and a partial agonist in horses

The cardiopulmonary effects of the narcotic agonists morphine, meperidine, oxymorphone, and methadone and of the partial agonist pentazocine were examined in the pain-free adult horse. The drugs produced dysphoric followed by euphoric effects. Increases in heart rate, arterial blood pressure, and cardiac output were observed in all horses with all drugs. Arterial blood pressure remained increased even after heart rate and cardiac output had returned to base-line values. Respiratory rate generally remained unchanged or increased shortly after drug administration and then decreased insignificantly (P less than 0.05) below base-line values at later times. Arterial pH and blood gas values (Paco2, Pao2) did not change.