RESUMEN
Pregnancy can cause several physiologic changes in mares, including modifications in cardiac output and heart positioning. Therefore, these changes could influence electrocardiographic parameters in various stages of pregnancy. This study aimed to evaluate the electrocardiogram of Crioulo mares in different pregnancy stages. Nine non pregnant mares (G0), 11 mares at three months (G3), 11 mares at six months (G6) and 11 mares at 10 months of pregnancy (G10) were submitted to electrocardiographic evaluation. There was higher (P= 0,002) heart rate of G10 (60.45±12.16bpm) when compared to other groups (G0=45.67±10.75bpm; G3=44.91±7.91bpm; G6=46.91±9.12bpm), probably because of an increase in cardiac output to uterine perfusion at the end of pregnancy. Normal sinus rhythm was predominant in most of mares of groups G0, G3 and G6, and sinus tachycardia in most of G10 mares. In conclusion, changes in hemodynamics and in cardiac position can influence on the electrocardiogram of pregnant Crioulo mares, but do not cause relevant rhythm or conduction disturbs.(AU)