Comparison of respiratory function during TIVA (romifidine, ketamine, midazolam) and isoflurane anaesthesia in spontaneously breathing ponies Part I: blood gas analysis and cardiorespiratory variables

Objective To compare pulmonary function and gas exchange in ponies during maintenance of anaesthesia with isoflurane or by a total intravenous anaesthesia (TIVA) technique. Study design Experimental, cross–over study. Animals Six healthy ponies weighing mean 286 (range 233–388) ± SD 61 kg, age 13 (9-16) ± 3 years. Methods The ponies were anaesthetized twice, a minimum of two weeks apart. Following sedation with romifidine [80 μg kg −1 intravenously (IV)], anaesthesia was induced IV with midazolam (0.06 mg kg −1 ) and ketamine (2.5 mg kg −1 ), then maintained either with inhaled isoflurane (F e ’Iso = 1.1 vol%) (T-ISO) or an IV infusion of romifidine (120 μg kg −1 hour −1 ), midazolam (0.09 mg kg −1 hour −1 IV) and ketamine (3.3 mg kg −1 hour −1 ) (T-TIVA). Ponies were placed in lateral recumbency. Breathing was spontaneous and Fi’O 2 60%. After an instrumentation/stabilisation period of 30 minutes, arterial and mixed venous blood samples were taken simultaneously every 10 minutes for 60 minutes and analysed immediately. Oxygen extraction ratio (O 2 ER) and venous admixture were calculated. Tidal volume (TV), minute volume (MV), respiratory rate (f R ), packed cell volume (PCV), arterial blood pressure and heart rate (HR) were measured and recorded. Data were analysed with mixed model anova (a = 0.05). Treatments were compared overall and at two selected time points (T30 and T60) using Bonferroni correction. Results Arterial and mixed venous partial pressures of O 2 and CO 2 , and TV were significantly lower and MV and f R were higher in T-TIVA compared to T-ISO. Venous admixture did not differ between treatments. O 2 ER was significantly higher in T-TIVA. Mean arterial pressure was higher and HR was lower in T-TIVA compared to T-ISO. Conclusions and clinical relevance Whilst arterial CO 2 was within an acceptable range during both protocols, the impairment of oxygenation was more pronounced with the T-TIVA evidenced by lower arterial and venous oxygen partial pressures.