This document discusses the effects of changes in electrolyte concentrations on the heart. It summarizes that: 1. Distribution of electrolytes like sodium, potassium, and calcium are responsible for the electrical activity of tissues including the heart. Changes in the concentration of these electrolytes can alter cardiac electrical activity and the electrocardiogram (ECG). 2. Hypokalemia and hyperkalemia affect the heart's resting membrane potential and excitability, causing changes to the ECG like tall/tented T waves or merger of QRS and T waves respectively. 3. Hypocalcemia reduces cardiac excitability and prolongs the ST segment and QT interval on ECG, while