Reptile cardiology is an underdeveloped specialty of reptile medicine. During the 19th century, a significant amount of research was done to elucidate the cardiac physiology of reptiles. Although a great deal is known regarding the function of the reptile heart, its application to clinical veterinary medicine has been limited.
In snakes, the heart is found along the midline axis and its longitudinal position varies with species. Thus, in marine and freshwater snakes, it is positioned near the middle of the body (25 to 45% of total body length). In non-tree dwelling snakes, it is found at about 25% of total body length, and about 15% (i.e. more cranially) in arboreal species. These variations in heart positions can be understood when considering the different physical forces to which the heart is subjected to in snakes occupying different biotypes. A cranially positioned heart reduces the hydrostatic pressure above the heart and so stabilizes variations in cephalic blood pressure.
Some studies have shown that those species of snakes that spend time raising their heads (arboreal and other climbing snakes) possess relatively shorter vasculature between the heart and head than do terrestrial species. The reverse is true in aquatic species, which experience less effects of gravity in water. Because of the lack of diaphragm, the snake’s heart is mobile within the coelomic cavity, which probably facilitates the movement of large whole prey in the esophagus. In the coelom, the heart is found adjacent to the caudal tracheal rings, caudal to the thyroid, cranial to the bronchial bifurcation, close to the cranial pole of the lung(s) and slightly cranial to the liver. Externally, the position of the heart is indicated by percutaneous visualization of the ventral precordial tap by placing the animal in dorsal recumbency and stretching it.
Unlike mammals, reptiles do not have a specialized cardiac conduction system such as pacemaker nodes and Purkinje fibers. Instead, the contractions are initiated by cardiac muscle fibers in the sinus venosus of the right atrium, which spread in sequential coordination, first to the left and caudally. The ventricle is depolarized starting at its base and then proceeding to the left. Repolarization starts from the base and spreads equally to the right and left towards the apex. The heart receives innervations from both parasympathetic and sympathetic fibers. The parasympathetic fibers run in the vagus nerve and provide cholinergic (inhibitory) control. The less well-developed sympathetic fibers cause positive chronotropism via adrenergic innervation. Heart rates (HR) are in general slower in reptiles compared to mammals or birds.
Heart rate is dependent on numerous factors:
1. Body temperature (increasing during basking and lowering during cooling, myocardial efficiency being optimum when the reptile is within its preferred optimal temperature zone)
2. Body size (heart rate is inversely proportional to the animal’s size)
3. Activity (heart rate is proportional to metabolic level)
4. Respiratory rate bradycardia is observed during apnea as pulmonary resistance increases and blood flow to the lungs decreases)
5. Hypovolemia (reptiles that experience blood loss as a result of surgery or trauma can become tachycardic to ensure that the tissues remain oxygenated)
6. Digestion, gravidity and sensory (stimulations such as
handling, postural and gravitational stress.
7. Heart rate also plays a significant role in
thermoregulation: it increases when the animal lies in
the sun, and vice versa. Thus, tachycardia and peripheral
vasodilatation increase heat loss from cutaneous
vascularization when ambient temperature increases,
whereas bradycardia and peripheral vasoconstriction
decrease heat loss when ambient temperature drops. This
mechanism, which often anticipates internal temperature
variations, is regulated by cutaneous thermoreceptor.
Electrocardiography (ECG) can greatly enhance the diagnosis
of cardiac disease in reptiles and is also beneficial for monitoring
patients under anesthesia. The main challenge associated with
the use of ECG in reptiles are the low electric amplitudes (usually
<1.0mv) which do not always provide readings of diagnostic
quality. The electrodes can be self-adhering skin electrodes
(designed for human use), stainless steel hypodermic needles,
stainless steel suture material or alligator clips. Placement of
the electrodes varies according to species but is usually inspired
from the traditional four limb lead placement. In snakes they
should be placed two heart lengths cranial and caudal to the
heart (Figure 1).
Moreover, standard parameters are not established for
many species, thus interval and segments values may not always
be of much use. Performing routine ECGs on healthy patients
can help to build up baseline values for future comparison.
Interpretations of ECGs in reptiles are very similar to those in
mammals, with P, QRS and T complexes. An SV wave represented
by the depolarization of the sinus venosus (and the portocaval
vein) may be measured just before the P wave. The SV wave is
followed by sinal contraction, the P wave is followed by atrial
contraction and the R wave is followed by ventricular contraction.
The T wave indicates ventricular repolarization. It should be
noted that correct placement of the probe is primordial as bad
positioning of the electrodes can influence ECG readings.
Currently, the interpretation of the ECG can be difficult because
of limited reference material for comparison. In addition, the
ECG can be influenced by various environmental factors.
For example, the heart rate is dependent on the body
temperature, and the intervals such as the P-R and Q-T segments
are influenced by the heart rate. To reduce the likelihood of
misclassifying the results of an ECG, it is important to perform
these tests under optimal conditions. In addition, performing
routine ECGs on clinically normal reptiles can be helpful in
establishing a baseline for comparison. The ECG should be used
in addition to other diagnostic tests to confirm the presence of
Reptilian clinical cardiology is in its infancy. In the near
future, it will be important for veterinarians to continue to
collect and publish baseline reference material for the different
diagnostic tests considered in this article, including ECG and
echocardiology. Once this information is readily available, the
practical usage of these tests for reptile cardiac case management