CARDIOVASCULAR AND RESPIRATORY EFFECTS OF DETOMIDINE IN ISOFLURANE-ANESTHETIZED HORSES

Anaesthesia was induced in horses (n = 6) with a mixture of thiopenton e and guaiphenesin and maintained by mechanical ventilation with a mix ture of oxygen, air and isoflurane. Inspiratory and end-expiratory con centrations of oxygen, carbon dioxide and isoflurane were measured. El ectrocardiography was used to evaluate heart rate and rhythm. Mean sys temic arterial pressure (SAPm), pulmonary artery pressure (PAPm), righ t atrial pressure (RAPm) and pulmonary capillary wedge pressure (PCWP) were measured directly. Cardiac output was determined using the therm odilution method. Microcirculation in the upper triceps brachii muscle was estimated using laser Doppler flowmetry. Blood gas values, pH and haemoglobin concentration (Hb) were determined in arterial and mixed venous blood. Baseline values were taken after stabilisation of anaest hesia and were statistically compared to values obtained after an intr avenous bolus of detomidine (10 mu g/kg) had been injected at Time 0. Samples were then collected at 2, 10, 20, 30, 40 and 50 min. End-expir atory concentration of isoflurane was maintained at 1.8 70 during the experimental period described above. Thereafter, the isoflurane concen tration was reduced to 1.3 % and samples were collected at 60, 70 and 80 min. Detomidine caused a significant reduction of heart rate (HR), cardiac index (CI), oxygen transport (O(2)TR) and a significant elevat ion of the SAPm, systemic vascular resistance (SVR) and coefficient of utilisation of oxygen (O2CU) at 2 min. A significant reduction in HR, Cl, SAPm, Hb, content of oxygen in arterial blood (CaO2) and O(2)TR a nd a significant elevation of O2CU were observed 10-50 min after the i njection of detomidine, Reduction of the isoflurane end-expiratory con centration to 1.3 % was followed by a progressive improvement of the S APm, CI and muscle perfusion towards the baseline values. Further redu ction of Hb and CaO2 as compared to the baseline values was observed d uring the period. The data suggest that detomidine potentiates some of the cardiovascular effects of isoflurane in horses.