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Objective To compare cardiac output ( Q ˙ t ) measurements using lithium dilution (LiDCO) and pulse contour analysis (PulseCO) techniques in isoflurane‐anaesthetized ponies before and during the administration of different inotropic/vasoactive drugs. Study design Prospective randomized experimental cross‐over trial. Animals Six ponies aged 5.0 ± 1.6 (4–6.5) years and weighing 286 ± 53 (212–368) kg. Methods After sedation (romifidine) and induction (midazolam + ketamine), anaesthesia was maintained with isoflurane in oxygen. After 90 minutes (= T0), one of four treatments was administered: saline 0.1 mL kg−1 (S), enoximone 0.5 mg kg−1 IV (E), enoximone followed by dobutamine (0.5 μg kg−1 minute−1 for 120 minutes) (ED) or enoximone followed by a calcium chloride infusion (0.5 mg kg−1 minute−1 for 10 minutes) (EC). Data were recorded for 120 minutes after T0. The PulseCO (recorded from carotid artery) was calibrated before T0, no further recalibrations were performed. Q ˙ t was determined with LiDCO ( Q ˙ t LiDCO) and PulseCO ( Q ˙ t PulseCO) simultaneously at T5, T10, T20, T40, T60, T80, T100 and T120. Systemic vascular resistances (SVRLiDCO and SVRPulseCO) were calculated. Results In the saline group, Q ˙ t PulseCO was 4.9 ± 12.3% lower than LiDCO (p < 0.01), whereas SVRPulseCO was 6.9 ± 14.4% higher than SVRLiDCO (p < 0.01). These differences increased over time (mean ± SEM), Q ˙ t by 0.06 ± 0.03% minute−1 (p = 0.042) and SVR by 0.08 ± 0.03% minute−1 (p = 0.018). Q ˙ t PulseCO was higher than Q ˙ t LiDCO in the EC group (1.8 ± 23.3%), but lower than Q ˙ t LiDCO in groups E (−11.7 ± 20.4%) and ED (−10.0 ± 25.9%) (significant difference between treatments, p Conclusions and clinical relevance Pulse contour analysis values deviated significantly from LiDCO measurements in isoflurane‐anaesthetized ponies. This difference was influenced by inotropic/vasoactive drugs.

Objective To evaluate the combined effects of enoximone and dobutamine on the cardiovascular system and respiratory gas exchange in isoflurane-anaesthetized ponies. Study design Prospective, randomized, experimental study. Animals Six ponies (286 ± 52 kg), aged 5.0 ± 1.6 years. Methods After sedation (romifidine 80 μg 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 μg 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 (D⌽O 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 (α = 0.05) and at specific timepoints (Bonferroni-adjusted α = 0.00625). Results Compared to enoximone alone, ED treatment produced an increase in HR, CO, SV, RAP, SAP, DAP, MAP, packed cell volume (PCV) and D⌽O 2 . The difference was significant from T60 to T120 (except at T80) for HR, throughout the observational period for CO, SAP, MAP, PCV and D⌽O 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. Conclusions and clinical relevance The results suggest that enoximone and dobutamine have additive cardiovascular effects and reduce V D / V T in isoflurane-anaesthetized ponies.

Objective Enoximone is a phosphodiesterase III inhibitor frequently used to improve cardiac output (CO) in man. As the use of enoximone has not been reported in horses, the effects of this inodilator were examined in isoflurane anaesthetized ponies. Study design Prospective, randomised, experimental study. Animals Six healthy ponies, weighing 286 (212–367) ± 52 kg, aged 5.0 ± 1.6 years (4–6.5). Methods After sedation with romifidine [80 μ g kg −1 intravenously (IV)], general anaesthesia was induced with midazolam (0.06 mg kg −1 IV) and ketamine (2.2 mg kg −1 IV) and maintained with isoflurane in oxygen (Et Iso 1.7%). The ponies were ventilated to maintain eucapnia (PaCO 2 4.66–6.00 kPa). Each pony was anaesthetized twice with an interval of 3 weeks; receiving enoximone 0.5 mg kg −1 IV (E) or saline (S) 90 minutes post-induction. Heart rate (HR), arterial (AP) and right atrial pressure (RAP) were measured before treatment, every 5 minutes between T0 (treatment) and T30 and then every 10 minutes until T120. Cardiac output measurements (lithium dilution technique) and blood gas analysis (arterial and central venous samples) were performed before T0 and at T5, T10, T20, T40, T60, T80, T100 and T120. Stroke volume (SV), systemic vascular resistance (SVR), venous admixture ( Q ˙ s/ Q ˙ t) and oxygen delivery ( D ˙ O 2 ) were calculated. Results Enoximone induced significant increases in HR, CO, SV, Q ˙ s/ Q ˙ t and D ˙ O 2 and a significant decrease in RAP. No significant differences were detected for AP, SVR and blood gases. No cardiac arrhythmias or other side effects were observed. Conclusions and Clinical relevance The present results suggest that in isoflurane anaesthetized ponies, enoximone has beneficial effects on CO and SV without producing significant changes in blood pressure. Despite an increase in Q ˙ s/ Q ˙ t, D ˙ O 2 to the tissues was improved.