The cardiovascular and respiratory effects of medetomidine and thiopentone anaesthesia in dogs breathing at an altitude of 1486 m.

The purpose of this study was to evaluate the cardio-respiratory effects of the combination of medetomidine and thiopentone followed by reversal with atipamezole as a combination for anaesthesia in 10 healthy German Shepherd dogs breathing spontaneously in a room at an altitude of 1486 m above sea level with an ambient air pressure of 651 mmHg. After the placement of intravenous and intra-arterial catheters, baseline samples were collected. Medetomidine (0.010 mg/kg) was administered intravenously and blood pressure and heart rate were recorded every minute for 5 minutes. Thiopentone was then slowly administered until intubation conditions were ideal. An endotracheal tube was placed and the dogs breathed room air spontaneously. Blood pressure, pulse oximetry, respiratory and heart rate, capnography, blood gas analysis and arterial lactate were performed or recorded every 10 minutes for the duration of the trial. Thiopentone was administered to maintain anaesthesia. After 60 minutes, atipamezole (0.025 mg/kg) was given intramuscularly. Data were recorded for the next 30 minutes. A dose of 8.7 mg/kg of thiopentone was required to anaesthetise the dogs after the administration of 0.010 mg/kg of medetomidine. Heart rate decreased from 96.7 at baseline to 38.5 5 minutes after the administration of medetomidine (P < 0.05). Heart rate then increased with the administration of thiopentone to 103.2 (P < 0.05). Blood pressure increased from 169.4/86.2 mmHg to 253.2/143.0 mmHg 5 minutes after the administration of medetomidine (P < 0.05). Blood pressure then slowly returned towards normal. Heart rate and blood pressure returned to baseline values after the administration of atipamezole. Arterial oxygen tension decreased from baseline levels (84.1 mmHg) to 57.8 mmHg after the administration of medetomidine and thiopentone (P < 0.05). This was accompanied by arterial desaturation from 94.7 to 79.7% (P < 0.05). A decrease in respiratory rate from 71.8 bpm to 12.2 bpm was seen during the same period. Respiratory rates slowly increased over the next hour to 27.0 bpm and a further increases 51.4 bpm after the administration of atipamezole was seen (P < 0.05). This was maintained until the end of the observation period. Arterial oxygen tension slowly returned towards normal over the observation period. No significant changes in blood lactate were seen. No correlation was found between arterial saturation as determined by blood gas analysis and pulse oximetry. Recovery after the administration of atipamezole was rapid (5.9 minutes). In healthy dogs, anaesthesia can be maintained with a combination of medetomidine and thiopentone, significant anaesthetic sparing effects have been noted and recovery from anaesthesia is not unduly delayed. Hypoxaemia may be problematic. Appropriate monitoring should be done and oxygen supplementation and ventilatory support should be available. A poor correlation between SpO2 and SaO2 and ETCO2 and PaCO2 was found.