In all living animals, the cardiovascular system has the main role in transporting nutrients. Waste materials, and gases. Any disturbances in structure and function of this system can produce major problem. One of these disturbances, is arrhythmias which effect on the regularity of cardiac function in circulating the blood and the best method of recognizing the arrhythmia is electrocardiography. The study of cardiac arrhythmias in the horses which are used for hard work is very important. The present study was undertaken to obtain information with regard to the prevalence of different arrhythmias in apparently normal horses on Tehran area. In this study ECG was recorded on 8 leads from 120 horses ranging in age from 3 to 25 vf both sexes and various breeds that were used for flat race polo and jumping. Different arrhythmias and the heart rate were evaluated statistically on the base of sex. Breed and the type of work and the findings were as follows: The percentage of arrhythmias in 120 horses were 37.5%, 26.6% in the male and 10.83% in the female, the highest incidence was in crossbred and the lowest in the Arab: and the highest prevalence was observed in horses used for polo. The mean and standard deviation of the heart rate in 120 horses was 46.5±7.07. The mean heart rate of the mares was 54.1±19.13 in comparison to the males which was 43.88±13.13. This was statistically significant (P<0.05). Arrhythmias recorded in this study were; sinus tachycardia (15%) sinus arrhythmia (7.5%), sinus arrhythmia and 2nd degree AVB (5%), 2nd degree atrioventricular block (5%), sinoatrial block, atrial and ventricular premature beats. In this study, AF which is common in the horse, was not observed. This could be due to the fact that horses used in this experiment were clinically normal.
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