Cardiology and techniques to detect cardiac diseases
Esther van Praag Ph.D.
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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 auricles and 2 ventricles separated by inter-auricular 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 auricle 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 auricles by flap valves, which are held in place by tendons. The heart valve between the right auricle and right ventricle (tricuspid valve) possesses only two cusps, and not three as usually observed in other animals.
Right and left auricles, 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 auricle,
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 auricle. Each auricle 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 auricle.
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 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.
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 require 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 sine. 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:
P wave of the electrocardiogram is associated with the atrial contraction,
QRS series of waves is associated with ventricular contraction,
P-Q or P-R interval gives a value for the time taken for the electrical impulse to travel from the auricle to the ventricle.
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 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.
Many thanks to Tom Chlebecek, DVM, (Makai Animal Clinic, Kailua, HI), Frossie Economou, 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 Dr. Tom Chlebecek for his comments.
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