Abstract

The overall aim of this investigation was to study the effects of different drug combinations for premedication and dissociative anaesthesia, to examine their suitability for field conditions and their ability to maintain cardiorespiratory function and provide sufficient analgesia for common, but challenging procedures such as castration. Haemodynamic parameters, pulmonary ventilation-perfusion relationships, and clinical effects were studied during sedation and dissociative anaesthesia. The effects of additional premedication and prolongation of dissociative anaesthesia and response to surgery were evaluated. The cardiorespiratory effects of romifidine and tiletamine-zolazepam anaesthesia did not differ significantly from those of prolonged romifidine and ketamine anaesthesia. Prolongation of anaesthesia with ketamine alone after romifidine/ketamine resulted in a poor quality of anaesthesia. There was a decrease in arterial oxygenation during sedation with a2-agonists, which was mainly attributed to a reduced cardiac output and increased ventilation-perfusion mismatch. During dissociative anaesthesia the cardiac output did normalise, but arterial oxygenation was further impaired as a result of increased intrapulmonary shunt and increased ventilation-perfusion mismatch. Administration of acepromazine before sedation with romifidine and butorphanol resulted in better maintenance of circulation and partly prevented the anaesthesia-induced ventilation-perfusion disturbances and fall in arterial oxygen tension. Although the arterial oxygenation was further impaired during anaesthesia and recumbency compared to that during sedation, the oxygen delivery did not decrease further. On the contrary, the arterial-mixed venous oxygen content difference and mixed venous oxygen tension remained closer to standing unsedated values during anaesthesia than in the sedated horse. Breathing high oxygen concentrations (>95% oxygen) during dissociative anaesthesia improved arterial oxygenation compared to air breathing (21% oxygen), but concomitantly increased intrapulmonary shunt and introduced hypoventilation. The intrapulmonary shunt created during anaesthesia with high oxygen concentrations persisted when the horses returned to air breathing, possibly indicating that resorption atelectasis produced during high oxygen breathing subsequently persisted during anaesthesia and recumbency. Tiletamine-zolazepam anaesthesia, after premedication with acepromazine, romifidine and butorphanol, produced anaesthesia and analgesia sufficient for castration of colts under field conditions. When the same regimen was used in the animal hospital there was a need for supplementary anaesthesia in some cases to complete surgery. The induction, anaesthesia and recovery were calm and without excitation in all colts both under hospital and field conditions. Cardiorespiratory changes during air breathing were within acceptable limits in these clinically healthy horses.