Electrical gradients in the myocardium can be detected on the body surface ECG. (a) An illustrative example of a single cardiac cycle detected as spatial and temporal electrical gradients on the ECG. The P wave is generated by the spread of excitation through the atria. The QRS complex represents ventricular activation and is followed by the T wave reflecting ventricular repolarization gradients. (b) Schematic representation of cellular electrical activity underlying the ECG (see text for details). Where downward arrows represent inward current and upward arrows represent outward current.