Though the cardiac conduction system of sheep has been frequently used for electrophysiological experiments, yet a satisfactory morphological comparison between that of sheep and human had not been made. To clarify the difference, young adult male hearts of sheep and human were histopathologically studied for their conduction systems using Lev's serial sectioning method.
The sinus node of sheep is rounder in shape at cross section, the nodal cells are more concentrically arranged and it contains tighter connective tissue than that of human.
The atrioventricular nodes of sheep and human are similar in structure but the former has a fatty pad between the node and the central fibrous body (CFB), and the latter has fatty tissue covering its surface to the atrial muscle.
His' bundle in sheep consists of Purkinje type large cells and penetrates the CFB more centrally than that in human which consists of smaller cells. The special channels which are occupied with fatty tissue peretrate the CFB of sheep, and they resemble the dual His' bundles rerely observed in human congenital heart anomalies. At the top of the ventricular septum, a conal muscle cap over the bundle is observed in sheep but this phenomena is very rare in human except in case of Pokkuri disease.
The left bundle branch of sheep is a solitary narrow bundle consisting of Purkinje type cells, and contains tight connective tissue in and around the branch, whereas that of human is a fan-shaped broad bundle consisting of small cells and loose connective tissue. The right bundle branch in sheep is shorter, the branch cells are larger, and it has tighter connective tissue than that in human. In human, evidence that both the His' bundle and the bundle branch cells show signs pre- and post-blockade hypertrophy, suggests the possibility of transforming of the small conducting cells to Purkinje cells in pathologic situation.

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There are significant differences in the configuration of QRS-T waves on electrocardiograms (ECGs) among species such as man, dog and rat. The waveform of dog ECGs, which is similar to that of man, has an “Rs” type in the QRS complex, and a prominent ST segment followed by T wave. On the other hand, the waveform of rat ECGs has an “Rs” type in the QRS complex, but no ST segment with a short QT interval. These characteristics of the ECG waveforms relate to the configuration of action potentials (APs) in the ventricular myocytes in each species. Compared to dog and man, the APs in the rat ventricular myocyte has a shorter plateau phase (phase 2) and a rapid earlier repolarization phase (phase 3). 90%-action potential durations (APD₉₀) are 300-400msec in man, 200-250 msec in the dog, and 30-40 msec in the rat. The APD₉₀ is closely related to QT interval on ECGs in each species. Ionic currents in the ventricular myocyte are responsible for the configuration of APs. Ina. f (Na channel ) is related to the depolarization phase (phases 0 and 1), Isi (Ca channel) maintains the depolarization (phase 2). Ikr and Ito (K channels) reduce the depolarization (phase 2) and produce the repolarizaiton (phase 3). Iki is related to Ek (phase 4). Compared to man and dogs, the rat ventricular myocyte has a much larger Ito current (K channel) (Ikr, partially) with a much smaller Isi current (Ca channel). These characteristics of ionic currents in the myocyte are one of the major sources of the species differences in QRS-T waveforms on ECG.

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Postmortem coronary angiography was performed in ten thoroughbred racehorses, and the origin and distribution of the sinoatrial or sinus node (SAN) and atrioventricular node (AVN) arteries (SAN art, AVN art) were subsequently investigated by means of xeroradiography and histological examination. The SAN art received a dual blood supply from the atrial branches of the coronary arteries: one origination in the circumflex branch of the left coronary artery, and another in the right coronary artery. Interarterial coronary anastomoses, where these branches communicated with one another, were located in the SAN with the anterolateral free wall of the right atrium. In addition, the atrial branches comprising the SAN art gave off fine branches into the AVN area. The AVN also appeared to be perfused by arterial branches of both the left and right coronary arteries: The AVN art consisted of one of the anterior septal branches of the interventricular paraconal branch of the left coronary artery, and the other two of the posterior septal branches of the circumflex and interventricular subsinusal branches of the right coronary artery, respectively. Including the atrial branch arteries comprising the SAN art, the AVN art with such a triple blood supply seemed to have intercoronar y anastomoses in the AVN. Although no such specific conducting fibers as an internodal tract have been found anywhere, the features of the arteries running through the atria that supply blood to the SAN in the thoroughbred racehorse, seem to resemble those of the internodal conduction system in man.

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To evaluate mitral regurgitation (MR) in Maltese dogs which have a high prevalence ratio of MR, an ausculation, thoracic radiograph, electrocardiography and echocardiography were taken in 18 Maltese dogs with chronic MR as the MR (+) group (n=13 ; 7 males and 6 females) and without MR as the MR (-) group (n=5 ; 3 males and 2 females). Age and body weight of the MR (+) group and MR (-) group were 11.2±1.6 and 10.0±1.6 years and 3.4±0.7 and 3.6±1.3kg, respectively. Our results suggested that mitral valve thickness and prolapse, left atrial diameter, left ventricular end-diastolic diameter, left ventricular end-systolic diameter and left ventricular end-systolic posterior wall thickness were useful in evaluating the heart function of Maltese with chronic MR.

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Postnatal changes in heart rate, duration or interval, amplitude, mean electrical axis of QRS complex and ST-segment of the standard limb-lead ECG in voles and Mongolian gerbils were studied. 1) The heart rate in both animal species increased until the 21st day of the postnatal stage and thereafter decreased gradually. 2) The shortest PR and QT intervals were observed at the 14th or 21st day in both species. The QRS durations of voles were also shortened toward the 7th day and were then prolonged, whereas in Mongolian gerbils they were almost completely stable until the 60th day. There were significant positive correlations between the RR intervals and PR or QT intervals in both species. 3) The amplitude of the P and R waves was maximum at the 14th (vole) or 21st (Mongolian gerbil) day. The amplitude of the T wave increased from the 7th to the 21st day and decreased thereafter, while the S wave was maximum at the 14th (vole) and 21st days (Mongolian gerbil). 4) The mean electrical axis of the QRS complex was seem predominantly in the right and caudal directions and shifted to the left and caudal directions with maturation. 5) The ST-segment was recognized in the earliest stage but became obscure as age advanced. In some voles the ST-junction disappeared in mature stages, whereas in all Mongolian gerbils the ST-junction was present.

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A female mongrel dog with 14 days of ageand a weight of 600g presented severe pulmonary edema with a congestive heart failure. She was periodically examined by ECG, PCG, X-ray, and UCG studies, and at one year and three months old diagnosed as the R-L shunt PDA accompanied by the severe pulmonary hypertension. This was confirmed by autopsy after she died at two years and two months of age.

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