Objective To study pulmonary gas exchange and cardiovascular responses to sedation achieved with romifidine and butorphanol (RB) alone, or combined with acepromazine, and during subsequent tiletamine–zolazepam anaesthesia in horses.
Animals Six (four males and two females) healthy Standardbred trotters aged 3–12 years; mass 423–520 kg.
Study design Randomized, cross-over, experimental study.
Materials and methods Horses were anaesthetized on two occasions (with a minimum interval of 1 week) with intravenous (IV) tiletamine–zolazepam (Z; 1.4 mg kg−1) after pre-anaesthetic medication with IV romifidine (R; 0.1 mg kg−1) and butorphanol (B; 25 µg kg−1 IV). At the first trial, horses were randomly allocated to receive (protocol ARBZ) or not to receive (protocol RBZ) acepromazine (A; 35 µg kg−1) intramuscularly (IM) 35 minutes before induction of anaesthesia. Each horse was placed in left lateral recumbency and, after tracheal intubation, allowed to breathe room air spontaneously. Respiratory and haemodynamic variables and ventilation–perfusion (; multiple inert gas elimination technique) ratios were determined in the conscious horse, after sedation and during anaesthesia. One- and two-way repeated-measures anova were used to identify within- and between-technique differences, respectively.
Results During sedation with RB, arterial oxygen tension (PaO2) decreased compared to baseline and increased mismatch was evident; there was no O2 diffusion limitation or increase in intrapulmonary shunt fraction identified. With ARB, PaO2 and remained unaffected. During anaesthesia, intrapulmonary shunt occurred to the same extent in both protocols, and mismatching increased. This was less in the ARBZ group. Arterial O2 tension decreased in both protocols, but was lower at 25 and 35 minutes of anaesthesia in RBZ than in ARBZ. During sedation, heart rate (HR) and cardiac output (Q̇t) were lower while arterial-mixed venous oxygen content differences and haemoglobin concentrations were higher in RBZ compared with ARBZ. Total systemic vascular resistance, mean systemic, and mean pulmonary arterial pressures were higher during anaesthesia with RBZ compared to ARBZ.
Conclusions and clinical relevance Acepromazine added to RB generally improved haemodynamic variables and arterial oxygenation during sedation and anaesthesia. Arterial oxygenation was impaired as a result of increased shunt and mismatch during anaesthesia, although acepromazine treatment reduced disturbances and falls in PaO2 to some extent. Haemodynamic variables were closer to baseline during sedation and anaesthesia when horses received acepromazine. Acepromazine may confer advantages in healthy normovolaemic horses.