Cardiology and techniques to detect cardiac diseases in rabbits

 

 

Esther van Praag Ph.D.

 

 

Warning: this page contains pictures that may be distressing for some persons.

 

Cardiology in pet rabbits is a domain on which little information is available. Therefore, the incidence of cardiac problems is not well known.

Although information is scarce, it is possible to diagnose the problem by means of a complete cardiac study including radiography, electrocardiography and/or ultrasound analysis to treat it the cardiac disorder appropriately.

Akira Yamanouchi

 

Left: Ventral view into the abdominal cavity of a rabbit, with heart, surrounded by fat deposits in the foreground, lungs in the he background. Right: Isolated heart

The heart is located in the thoracic cavity with the apex (tip of the heart) directed backward and slightly to the left; the base is directed forwards. As observed in other small animals, the rabbit heart has 4 chambers: 2 atria and 2 ventricles separated by inter-atricular and inter-ventricular septa. It has, furthermore, some anatomical particularities:

•    Right and left ventricles that form the muscular caudal portion of the rabbit heart. The left ventricle is larger than the right one; they are separated from each other by the interventricular septum. Their walls are raised into muscular ridges. The right ventricle is much thicker than the right atrium and forms the right side of the conical apical portion, but without reaching the apex. It gives off the pulmonary artery in front. Cusp valves separate the ventricular chambers from the pulmonary artery and the aorta.

      The ventricular chambers are separated from the atria by flap valves, which are held in place by tendons. The heart valve between the right atrium and right ventricle (tricuspid valve) possesses only two cusps, and not three as usually observed in other animals.

•    Right and left atria, which are located in the cranial part of the heart. They are small chambers that receive the venous blood from:

•    The cranial and caudal vena cava (one of two large veins returning blood from outer parts of the body to the right chamber of the heart) and the coronary sinus (receiving blood from the heart itself), which are drained into the right atria,

•    The left and right pulmonary veins, that bring oxygenated blood from the lungs, open together into the cavity, on the dorsal side of the left atrium. Each atrium possesses, in addition, small muscular flaps.

 

Anatomy of the rabbit heart

 

The sino-atrial node - or pacemaker from which originates the heartbeat - is located in the wall of the right atrium.

Further physiological points differentiate the rabbit heart from that of other small animals:

•    The aortic nerve has no chemoreceptors, but only baroreceptors. This means that it does not have sensory nerve cells that are activated by chemicals, but only pressure-sensitive nerve endings, that stimulate reflex mechanisms that allow the body to adapt to changes in blood pressure by dilating or constricting the blood vessels.

•    The pulmonary artery and its branches are heavily muscular.

•    The coronary arteries, which supply the cardiac muscle and are given off from the aorta, can easily be compressed, leading to ischemia of the myocardium, due to poor collateral circulation.

Rabbit cardiac parameters

Arterial blood pH

7.2 – 7-5

Arterial blood pressure

Systolic (maximum pressure in an artery at the moment when the heart is beating and pumping blood through the body):   90 – 130 mmHg

Diastolic (lowest pressure in an artery in the moments between beats when the heart is resting): 

80 – 90 mmHg

Pulse rate

112 and 300 per minute

 


Rabbit abdominal radiography

An X-ray of the abdominal region of the rabbit is a commonly performed diagnostic examination, in case of dyspnea (shortness of breath), a bad or persistent cough, a chest injury or on suspicion of pneumonia. It will provide information about the shape and the size of the heart and lungs. It can detect heart failure, emphysema, the possible presence of pulmonary edema, the vascular pattern, the presence of abscesses or neoplasia (e.g. thymoma, lung cancer), and other medical conditions. This technique has its limitations though. Small malignant tumors can be too small to be visible. Pulmonary embolism (blood clots to the lungs) is not seen either, and require additional study.

 

 

Kim Chilson

 

Lateral and dorso-vental view of the rabbit thorax and abdomen, showing the location of the heart, the blood vessels leading to the heart and various other organs

Rabbit ultrasound examination or echocardiography

Most rabbits tolerate well the harmless, non-invasive and widely available method of echocardiography, a procedure that can be used without the use of sedative drugs, which can modify the heart characteristics. The method is furthermore sensitive and precise and the obtained images are of excellent quality. The rapid heartbeat of rabbits and the small size of their hearts nevertheless requires equipment with a high frequency transducer (handheld recording probe) and a high frame rate ultrasound machine.

Echocardiography enables detection of abnormalities in the heart structure (e.g. defective heart valves, congenital defects), heart wall or chamber enlargement (e.g. heart failure, cardiomyopathy), heart-wall motion, and allows the measurement of the blood volume that is pumped from the heart with each beat. It can also identify the accumulation of fluids in the pericardium (pericardial effusion) or the presence of scar tissue throughout the pericardium.

Special techniques, like M- or TM-mode (M = movement, T = Time) ultrasound will provide information for the analysis of wall and valve movements. The B-mode technique (B = brightness) is used for examination of the anatomical relationships (e.g. the heart structure, valves), while (color) Doppler ultrasonography will help determine the direction of the blood flow and/or its velocity and can thus detect turbulent flow due to narrowing or blockage of blood vessels.

 

Tom Chlebecek

“Two chamber view” of a rabbit heart (LV = left ventricule, LA = left atrium), which permits evaluation of the motion of the anterior and inferior left ventricular myocardial segments. The mid-portion of the mitral valve motion is also evident.

Tom Chlebecek

Standard echocardiographic view of a healthy rabbit heart.

Top: right parasternal long axis view of the left ventricle,

Bottom: corresponding M-mode with mitral valve in the beam.

Rabbit electrocardiography (ECG or EKG)

Electrocardiography (ECG) is a commonly used, non-invasive, simple and painless procedure that enables to record electrical changes in the heart, by amplifying electrical impulses that flow through the heart. Electrocardiography is used to evaluate and manage causes of symptoms such as chest pain, dyspnea, palpitations, arrhythmia, or syncope.

The rhythm in a healthy rabbit shows a sinus. It excludes respiratory sinus arrhythmia (RSA), as there is no influence of breathing on the flow of sympathetic and vagus impulses to the sinoatrial node.

The obtained electrocardiogram, which shows a series of waves, will provide information about the pacemaker (part that triggers each heartbeat), about the nerve conduction pathways of the heart, and the rate and rhythm of the heart. The different waves are called named P, Q, R, S, and T and follow in alphabetical order:

•   the P wave of the electrocardiogram is associated with the atrial contraction,

•   the QRS series of waves is associated with ventricular contraction,

•   the P-Q or P-R interval gives a value for the time taken for the electrical impulse to travel from the atria to the ventricle.

•   the T wave comes after the contraction.

Electrocardiogram values for a healthy rabbit:

P wave

amplitude:   0.1 – 0.15 mv

                  0.04 – 0.12

duration:     0.03 – 0.04 sec

                  0.01 – 0.05 sec

QRS interval

duration :    0.015 – 0.04 sec

                  0.02 – 0.06 sec

R-wave amplitude : 0.03 – 0-039 mV

P – R duration

0.05 – 0.1 sec

0.04 – 0.08 sec

Q – T duration

0.08 – 0.16 sec

R wave amplitude

amplitude:   0.03 – 0.039 mV

T wave

amplitude:   0.05 – 0.17 mV

Variation is the values presented in the above table may indicate:

•    Abnormal P wave: right or left atrial hypertrophy, atrial premature beat, hyperkalemia.

•    Abnormal QRS interval: right or left bundle branch block, ventricular rhythm, hyperkalemia, among others.

•    Abnormal Q-T duration: hypocalcemia, hypothyroidism, brain hemorrhages, congenital deformations, myocardial infarction, myocarditis.

•    Abnormal T wave: hyperkalemia, hyperacute myocardial infarction and left bundle branch block in case of a tall T wave; ischemia, age, stress, pericarditis, intraventricular conduction delay, electrolyte disturbance, in case of a small, flattened or inverted T wave.

Rabbit cardiac disorders

Various disorders, including congestive heart failure, cardiac myopathy (e.g. myocardial fibrosis), or congenital heart disease (rare) like atrial or ventricular septal defects, arrhythmia, valvular diseases, or vascular diseases have been observed in rabbits.

Acknowledgement

Many thanks to Tom Chlebecek, DVM, (Makai Animal Clinic, Kailua, HI), to Frossie Economou, to Kim Chilson, and to Akira Yamanouchi, (Veterinary Exotic Information Network, http://vein.ne.jp/), for giving their permission to use the pictures. Thank you also Tom Chlebecek, for your comments.

Further information

M.V. Bray MV, WE. C. Weir EC, D. G. Brownstein, M. L. Delano, (1992) Endometrial venous aneurysms in three New Zealand white rabbits. Lab Anim Sci.; 42(4):360-2.

Farkas, A. J. Batey, S. J. Coker (2004) How to measure electrocardiographic QT interval in the anaesthetized rabbit. J Pharmacol Toxicol Methods.; 50(3):175-85. 

L.C. St John, F. P. Bell (1990) Arterial fatty acid-binding protein activity associated with dietarily-induced and spontaneously occurring atherosclerosis in the rabbit (Oryctolagus cuniculus). Comp Biochem Physiol B.; 97(1):123-7. 

C. Kozma, W. Macklin, L. M. Cummins, R. Mauer (1974) The anatomy, physiology and biochemistry of the rabbit, in The Biology of the Laboratory Rabbit (Weisbroth et al., eds), pp 50-69.

L. I. Kupferwasser, M. R. Yeaman, S. M. Shapiro, C. C. Nast, A. S. Bayer (2002) In vitro susceptibility to thrombin-induced platelet microbicidal protein is associated with reduced disease progression and complication rates in experimental Staphylococcus aureus endocarditis: microbiological, histopathologic, and echocardiographic analyses. Circulation; 105(6):746-52.

C. J. Orcutt (2000) Cardiac and respiratory disease in rabbits. Proceedings of the British veterinary Zooligical Society (Autumn meeting)

K. E. Quesenberry, J. W. Carpenter, P. Quesenberry (2004) Ferrets, Rabbits and Rodents: Clinical Medicine and Surgery Includes Sugar Gliders and Hedgehogs, Elsevier Health, pp 211-216

R. S. Simons (1996) Lung morphology of cursorial and non-cursorial mammals: lagomorphs as a case study for a pneumatic stabilization hypothesis. J Morphol. 1996; 230(3):299-316.

F. Harcourt-Brown, Textbook of Rabbit Medicine, UK: Butterworth-Heinemann, 2001.

 

 

 

 

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