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Acute yew (Taxus baccata) intoxication was induced in 7 adult sheep by oral administration of the plant materials at the dose of 2.5g/kg body weight. Blood samples for hematology and serum biochemistry were collected before and after the induced intoxication.Electrocardiography was performed using a base-apex lead. Intoxicated animals displayed lethargy, depression, bradycardia and varying intensity of heart sounds. The ECGs of intoxicated sheep showed a variety of arrhythmias and abnormalities including multifocal ventricular tachycardia, idioventricular and idiojunctinal rhythm and QRS and T widening. Four sheep died within 4-16 hours after yew administration. Significant increase of white blood cells, total neutrophil counts and serum sodium concentration were observed. Post mortem examination and histological evaluation were done with special regards on tissue changes of the myocardium and conduction system. There were hyperemia, focal hemorrhage and multifocal nonsuppurative interstitial myocarditis with mild focal cardiac muscle cell degeneration and necrosis in the affected hearts which appeared more prominent in the right atrium. Histopathological examination of cardiac conduction system also revealed moderate to severe interstitial edema with inflammation of the SA-node, AV-node, the bundle branches and His-bundle in all cases which was most severe in the AV-node and the His-bundle. Oral administration of dried leaves of T. baccata, at the dose of 2.5g/kg can induce acute toxicity, manifested clinically by a variety of cardiac arrhythmia and myocardial lesions including special conduction system in pathology.

Cardiac conduction system enables the heart to pulsate continuously by producing electric impulse and conducting it. As the anatomical information about cardiac conduction system in human being and other mammals needed in various fields of sciences, such as anatomy, histology, physiology, pathology and cardiology, the present review article intends to provide this basic information about an important part of this system; the atrioventricular node. The histological structure and blood supply of the node and also the internodal pathways which conduct impulse from the sinus node to the AV node will be discussed.