M-mode, Two-dimensional, Spectral Doppler and Tissue Doppler Echocardiographic Findings in 40 Healthy Pet Rabbits

Next to dogs and cats, rabbits are popular companion animals in the United Kingdom with 1.7 million kept as pets. Almost 60% of these rabbits are registered with a veterinary practice. The number of reported or anecdotal reports describing congenital or acquired cardiac diseases is increasing, which results in a growing demand to perform echocardiography in pet rabbits as clinical cases. Reports of normal echocardiographic findings for comparison with potentially diseased rabbits are very scarce, and most involve the description of selected echocardiographic parameters in small cohorts of young rabbits in a research setting and under the influence of anaesthetic agents. The aim of the present study was to report the findings of m-mode, two-dimensional, spectral Doppler and tissue Doppler echocardiography in a large cohort of healthy, manually restrained, adult pet rabbits. The feasibility of performing echocardiography was evaluated and normal values were obtained. The correlation of the different echocardiographic measurements with variables such as weight, heart rate and age was analysed. Furthermore, statistical differences of echocardiographic variables between different breeds and sexes were assessed. Ultimately, we aimed to report the prevalence of trivial valvular regurgitation in the same cohort of healthy rabbits. Forty healthy pet rabbits of breeds commonly found as pets (22 Dwarf Lops, 14 French Lops and 4 Alaskan) underwent a full physical examination and conscious mmode, two-dimensional, spectral Doppler and tissue Doppler echocardiography. The median age of the rabbits was 21.5 months and the median weight was 2.9 kg (Dwarf Lops 2.4 kg/ Alaskans 4.35 kg/ French Lops 6.0 kg). Echocardiography with ECG monitoring was feasible in all rabbits and adequate standard echocardiographic and radial colour views and traces could be obtained. However, pulsed-wave tissue Doppler traces of satisfactory quality were only obtained in 85% of cases. Left atrial and ventricular dimensions were significantly larger in French Lops in comparison to Dwarf Lops and an overall positive correlation with weight was present. No significant differences between breeds were identified for flow velocities. Comparison between breeds was not possible for diastolic waves both for spectral Doppler and tissue Doppler because of low numbers due to division into two groups: separated and summated diastolic waves. Separation of mitral early and late diastolic waves was observed in 40% of cases using conventional spectral Doppler and in > 60% of cases using pulsed-wave tissue Doppler. Most systolic tissue Doppler parameters were significantly higher in French Lops than in Dwarf Lops and/or were positively correlated with weight. Trivial regurgitations were detected at the aortic valve in 7/40 (17.5%) rabbits, at the tricuspid valve in 5/40 (12.5%) and at the pulmonic valve in 1/40 (2.5%) rabbits. Performing ECG-gated m-mode, two-dimensional, spectral Doppler and tissue Doppler echocardiography is feasible in healthy manually restrained pet rabbits. The results of this study can be used for comparison with echocardiography obtained from clinical cases. Breed specific values should be used when measuring left atrial and ventricular sizes and radial tissue Doppler systolic parameters in French Lops and Dwarf Lops. Moreover, these breed-specific values might also be applicable for rabbits of similar weight. Most other parameters were independent of breed, weight, age, sex and heart rate and can therefore be used for the general pet rabbit population. Pulsed-wave tissue Doppler might be useful for the assessment of diastolic function in rabbits, as a separation of mitral tissue Doppler early and late diastolic waves persist at heart rates resulting in summation of mitral inflow E and A waves. Trivial tricuspid, aortic and pulmonic valvular regurgitations were detected in healthy pet rabbits, but were uncommon. ACKNOWLEDGEMENT The present work was carried out at the Bristol Veterinary School with support from the Exotic Practice and the Department of Statistics at the same institution. I would like to express my sincere gratitude to: Dr Sonja Fonfara, my supervisor, for the “out of this world” and invaluable help, support and mentoring; both during the completion of this research and during the diploma training. Dr William Browne for his help and supervision with the statistics, and Richard Saunders and Sarah Elliott for their help and advice in any aspect of “the rabbit world” during the completion of this research. Nat Harran, Zoe Iles, Rowena Killick, Sarah Gosden, Colin Blakey and Elisa Mazzotta for their occasional help in handling some of the rabbits of this study and/or for their help organising some of the echocardiographic examinations. CONTENTS ABSTRACT ACKNOWLEDGEMENTS ABBREVIATIONS ....................................................................................................1