High-output cardiac failure is a well-known phenomenon of high-flow fistula in hemodialysis patients. The definition of "high flow" is varied and almost always connected to proximal arteriovenous fistulas (AVF). High flow access is a condition in which hemodynamics is affected by a greater rate of blood flow required for hemodialysis and this can compromise circulatory dynamics, particularly in the elderly in the context of pre-existing heart disease. High access flow is associated with complications like high output heart failure, pulmonary hypertension, massively dilated fistula, central vein stenosis, dialysis associated steal syndrome or distal hypoperfusion ischemic syndrome. Although there is no single agreement about the values of AVF flow volume, nor about the definition of high-flow AVF, there is no doubt that AVF flow should be considered too high if signs of cardiac failure develop. The exact threshold for defining high flow access has not been validated or universally accepted by the guidelines, although a vascular access flow rate of 1 to 1.5 l/min has been suggested. Moreover, even lower values may be indicative of relatively excessive blood flow, depending on the patient's condition. The pathophysiology contributing to this disease process is the shunting of blood from the high-resistance arterial system into the lower resistance venous system, increasing the venous return up to cardiac failure. Accurate and well-timed diagnosis of high flow arteriovenous hemodynamics by monitoring of blood flow of fistula and cardiac function is required in order to stop this process prior to cardiac failure. We present two cases of patients with high flow arteriovenous fistula with a review of the literature.