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.
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.
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
Rabbit abdominal radiographyAn 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.
Rabbit ultrasound examination or echocardiographyMost 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.
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:
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 disordersVarious 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.;
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W. Macklin, L. M. Cummins, R. Mauer (1974) The
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Bayer (2002) In vitro susceptibility to thrombin-induced platelet microbicidal protein is associated with reduced disease
progression and complication rates in experimental Staphylococcus aureus
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(2000) Cardiac and respiratory disease in rabbits. Proceedings of the British
veterinary Zooligical Society (Autumn meeting) K. E. Quesenberry,
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Rabbits and Rodents: Clinical Medicine and Surgery Includes Sugar Gliders and
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of Rabbit Medicine, UK: Butterworth-Heinemann, 2001. |
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