Cardiovascular and respiratory effects of detomidine in isoflurane-anaesthetised horses

Anaesthesia was induced in horses (n = 6) with a mixture of thiopentone and guaiphenesin and maintained by mechanical ventilation with a mixture of oxygen, air and isoflurane. Inspiratory and end-expiratory concentrations of oxygen, carbon dioxide and isoflurane were measured. Electrocardiography was used to evaluate heart rate and rhythm. Mean systemic arterial pressure (SAPm), pulmonary artery pressure (PAPm), right atrial pressure (RAPm) and pulmonary capillary wedge pressure (PCWP) were measured directly. Cardiac output was determined using the thermodilution 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 anaesthesia and were statistically compared to values obtained after an intravenous bolus of detomidine (10 micrograms/kg) had been injected at Time 0. Samples were then collected at 2, 10, 20, 30, 40 and 50 min. End-expiratory concentration of isoflurane was maintained at 1.8% during the experimental period described above. Thereafter, the isoflurane concentration 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 (O2TR) and a significant elevation of the SAPm, systemic vascular resistance (SVR) and coefficient of utilisation of oxygen (O2CU) at 2 min. A significant reduction in HR, CI, SAPm, Hb, content of oxygen in arterial blood (CaO2) and O2TR and a significant elevation of O2CU were observed 10-50 min after the injection of detomidine. Reduction of the isoflurane end-expiratory concentration to 1.3% was followed by a progressive improvement of the SAPm, CI and muscle perfusion towards the baseline values. Further reduction of Hb and CaO2 as compared to the baseline values was observed during the period. The data suggest that detomidine potentiates some of the cardiovascular effects of isoflurane in horses.