A 35 y.o. male patient suffers from chronic glomerulonephritis and has been treated with hemodialysis for the last 3 years. He has got irregularities in the heart activity, hypotension, progressive weakness, dyspnea. On ECG: bradycardia, 1st degree atrioventricular block, high sharpened T-waves. Some time before he had seriously broken the water consumption and dietary pattern. What is the most likely cause of these changes?