Effects of dobutamine on cardiovascular function and respiratory gas exchange after enoximone in isoflurane-anaesthetized ponies.

To evaluate the combined effects of enoximone and dobutamine on the cardiovascular system and respiratory gas exchange in isoflurane-anaesthetized ponies. Prospective, randomized, experimental study. Six ponies (286 +/- 52 kg), aged 5.0 +/- 1.6 years. After sedation (romifidine 80 microg kg(-1)), anaesthesia was induced with midazolam (0.06 mg kg(-1)) and ketamine (2.2 mg kg(-1)) and maintained with isoflurane in oxygen. The ponies were ventilated to maintain eucapnia. After 90 minutes (=T0), enoximone alone (0.5 mg kg(-1)) (E) or enoximone, followed by a constant rate infusion of dobutamine (0.5 microg kg(-1) minute(-1)) (ED) for 120 minutes, was administered. Each pony received both treatments in a crossover trial, with at least 2 weeks between treatments. Heart rate (HR), cardiac output (CO), stroke volume (SV), right atrial (RAP), systolic (SAP), diastolic (DAP) and mean arterial pressure (MAP), blood gases, systemic vascular resistance (SVR), oxygen delivery (DO(2)) and several respiratory gas exchange variables were measured before treatment and until T120. Statistical analysis was based on a mixed model with treatment, time and their interaction as fixed categorical effects, pony as random effect, comparing treatments globally (alpha = 0.05) and at specific timepoints (Bonferroni-adjusted alpha = 0.00625). Compared to enoximone alone, ED treatment produced an increase in HR, CO, SV, RAP, SAP, DAP, MAP, packed cell volume (PCV) and DO(2). The difference was significant from T60 to T120 (except at T80) for HR, throughout the observational period for CO, SAP, MAP, PCV and DO(2), from T40 to T120 for DAP, at T10,T60,T80 and T120 for SV and at T10 and T20 for RAP. Overall decreases occurred in SVR and dead space ventilation (V(D)/V(T)). V(D)/V(T) was lower at T20 and from T80 to T120. Venous oxygen saturation was increased from T60 onwards. The results suggest that enoximone and dobutamine have additive cardiovascular effects and reduce V(D)/V(T) in isoflurane-anaesthetized ponies.