REN and acute kidney injury: Very high mortality rates in CRT-D recipients, who developed CI-AKI, might be the result of an interaction of many clinical and biological factors such as follows: predisposition to fluid overload leading to higher rates of hospitalization due to heart failure, proneness to electrolyte disturbances that may lead to arrhythmic events and sudden deaths, natural course of renal failure with its all biological consequences like abnormal vascular biology and endothelial dysfunction, hyperactivation of the renin-angiotensin system, anemia, changes in lipids, and disturbances in coagulation.