Cardiovascular effects of epidural administration of methadone, ropivacaine 0.75% and their combination in isoflurane anaesthetized dogs.

Objective: To compare the cardiovascular effects of four epidural treatments in isoflurane anaesthetised dogs.

Study Design: Prospective, randomized. experimental study.

Animals: Six female, neutered Beagle dogs (13.3±1.0 kg), aged 3.6±0.1 years.

Methods: Anaesthesia was induced with propofol (8.3±1.1 mg kg(-1)) and maintained with isoflurane in a mixture of oxygen and air [inspiratory fraction of oxygen (FiO(2))=40%], using intermittent positive pressure ventilation. Using a cross-over model, NaCl 0.9% (P); methadone 1% 0.1 mg kg(-1) (M); ropivacaine 0.75% 1.65 mg kg(-1) (R) or methadone 1% 0.1 mg kg(-1) + ropivacaine 0.75% 1.65 mg kg(-1) (RM) in equal volumes (0.23 mL kg(-1)) using NaCl 0.9%, was administered epidurally at the level of the lumbosacral space. Treatment P was administered to five dogs only. Cardiovascular and respiratory variables, blood gases, and oesophageal temperature were recorded at T-15 and for 60 minutes after epidural injection (T0).

Results: Mean overall heart rate (HR in beats minute(-1)) was significantly lower after treatment M (119±16) (p=0.0019), R (110±18) (p< 0.0001) and RM (109±13) (p<0.0001), compared to treatment P (135±21). Additionally, a significant difference in HR between treatments RM and M was found (p=0.04). After both ropivacaine treatments, systemic arterial pressures (sAP) were significantly lower compared to other treatments. No significant overall differences between treatments were present for central venous pressure, cardiac output, stroke volume, systemic vascular resistance, oxygen delivery and arterial oxygen content (CaO(2)). Heart rate and sAP significantly increased after treatment P and M compared to baseline (T-15). With all treatments significant reductions from baseline were observed in oesophageal temperature, packed cell volume and CaO(2) . A transient unilateral Horner's syndrome occurred in one dog after treatment R.

Conclusions And Clinical Relevance: Clinically important low sAPs were observed after the ropivacaine epidural treatments in isoflurane anaesthetised dogs. Systemic arterial pressures were clinically acceptable when using epidural methadone.