A morphological relationship was observed between the septomarginal trabecula and the atrioventricular conducting system in 45 calves.
1) The septomarginal trabecular were 0.7 to 10.5 millimeters in diameter, showing remarkable individual differences in size.
2) There was no special relationship between the thicknesses of the septomarginal trabecula and that of the right ventricular wall.
3) In all instances, the main branches of the right limbs of the atrioventricular bundles entered the septomarginal trabeculae to reach the lateral walls of the right ventricles.
4) Histologically, no ordinary cardiac muscle fibers were observed in seven of 11 thin septomarginal trabeculae, which were less than 2 millimeters in diameter.
5) These findings indicated that the septomarginal trabecula served as a pathway for the right limb of the atrioventricular conducting system.

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By the least square method, the mean electrical axes for P, QRS and T wave were measured in seven randomly bred laboratory cats originated from the authors' disease-free cat breeding colonies, under different anesthetic conditions.
1) In cats in right lateral recumbency anesthetized with pentobarbiton sodium, mean voltage (R) and degree of angle (θ) of mean electrical axes for P, QRS and T were as follows : R_P=0.059±0.011 MV, θ_P=81±7° R_QRS=0.067±0.037 MV θ_QRS=50±42°, R_T=0.104±0.046 MV and θ_T=73±20°.
2) In cats anesthetized with ketamin hydrochloride in the same body position as mentioned above, mean electrical axes for P, QRS and T were as follows: R_P=0.049±0.01 MV, θ_P=79±15°, R_QRS=0.054±0.032 MV, θ_QRS=109±40°R_T=0.11±0.003 MV and θ_T=50±22°.
3) In the case of horizontal plane in right lateral recumbency, the influence on the mean electrical axes revealed a significant difference between the two anesthetics (P<0.05), except θP.
4) Analysis was carried out on the variance among four different body positions on three plane. θ_QRS and θ_T only on the horizontal plane revealed a significant difference (P<0.05). Scattering was found on the transeverse plane. Further studies would be required on lead positions in the cat.

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The present study was designed to elucidate the cardiovascular effects of ajmaline in the horse. Five healthy Thoroughbred horses were injected intravenously with a dose of 1 mg of ajmaline per kg of body weight with 500 ml of 5% liquid glucose. After the injection of ajmaline, electrocardiogram, carotid arterial pressure curve and right atrial pressure curve were recorded continuously. Cardiac output was determined by the dye dilution method before and 10 and 40 min after the injection. Electrocardiographic studies demonstrated that no heart rate changed significantly, that no arrhythmias appeared, and that P and QRS durations and QT interval prolonged slightly. Hemodynamic studies revealed no significant changes.

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In order to examine the defibrillatory effect of ajmaline on equine atrial fibrillation (AF), the effect of the drug was investigated electrocardiographically on experimentally induced AF. In 9 light-breed horses, AF was induced successfully with electric stimulation on the right atrium. Ajmaline was given intravenously at a dose of 1 mg/kg of body weight with 500 ml of 5% liquid glucose. After the injection of ajmaline, AF was reverted to sinus rhythm in all the cases. Premedication of the drug depressed the induction of experimental AF significantly.

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The atrioventricular (AV) canal malformation was observed in a 10-year-old male cocker spaniel weighing 12 kg. Auscultation revealed pansystoilic murmur at the apex and fixed splitting of the second heart sound at the pulmonic area. On the electrocardiogram (ECG), complete right bundle branch block (CRBBB) with right ventricular hypertrophy was shown. Chest X-ray demonstrated an enlargement of the right atrium, right ventricle and pulmonary artery.
Autopsy revealed a partial form of AV canal malformation which involved ostium primum atrial septal defect (ASD I), mitral cleft, hypoplasia of the diaphragmatic wall of the left ventricle and septal inserion of the anterior mitral leaflet.

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