Cardiac arrhythmias are one of the most frequent complications in the immediate post-operatory period of cardiac surgery with extracorporeal circulation, this occurring in between 35% and 50% of the patients subjected to this therapeutic intervention. Among the causal factors, electrolytic alterations, direct surgical aggression on the heart tissue and alterations secondary to ischemic cell damage that is produced in the myocardial during extracorporeal circulation are found. Due to the frequency of appearance of arrhythmias, we decided to study them and know them in depth. Thus, the objective of our study was to know the incidence and prevalence of arrhythmias during the immediate post-operatory period of cardiovascular surgery. Out of all those admitted to the unit, all the patients who had undergone valvular and coronary surgery with a stay in the intensive care unit of the University Hospital Marques de Valdecilla that did not exceed 7 days were selected. This means 258 patients, the most frequent alterations during the post-operative period being right branch blockage (16%), atrial fibrillation (12%), ventricular tachycardy (7%), atrial ventricular block (4%), among other rhythm and conduction alterations. As conclusions, we stress that the most frequent alterations during the immediate post-operative period were branch blocks, arrhythmia being more prevalent than atrial fibrillation. A total of 85% of the patients undergoing valvular replacement and 89% of those operated on for coronary derivation plus valvular replacement had more arrhythmia episodes than the rest.