Cardiac Electrical Signals

Learn the basics of cardiac electrical signals


Electrodes strategically placed on the body measure the magnitude and direction of the heart's electrical potential from viewpoints known as “leads”. Lead measurements are quantified by a graph that displays the electrical activity of the heart over time known as an electrocardiogram (ECG or EKG). The ECG of each unique lead provides insights into heart health, and ECGs from multiple unique leads will deliver a comprehensive picture of cardiac health.

The 12-lead ECG 

The conventional 12-lead ECG measures electrical activity through ten electrodes. Six electrodes are placed on the chest and generate one lead each (the Precordial Leads). These electrodes and leads are referred to as V1, V2, V3, V4, V5, and V6. The remaining four electrodes are placed on the patient’s right arm (RA), left arm (LA), right leg (RL), and left leg (LL). The measurements from these electrodes are added and subtracted from each other to create six Limb (Frontal) Leads: I, II, III, aVR, aVL, and aVF. 

Graph Axes and Scale

The standard ECG is drawn on 1 mm x 1 mm gridded paper, where each small vertical 1 mm box represents 0.1 mV and each small horizontal 1 mm box represents 0.04 sec (40 msec). Thicker, darker lines denote larger 5 mm x 5 mm boxes which have a height of 0.5 mV and width of 0.2 sec (200 ms). Most ECG graphs will also include a standardized reference pulse at the beginning of the tracings with a width of 5 mm and height of 10 mm, which correlates to 0.2 sec and 1 mV, respectively.


During each cardiac cycle (heartbeat) the heart muscle contracts as it depolarizes, and then relaxes as it repolarizes. These electrical shifts are recorded by electrodes, grouped into “leads”, and rendered through an electrocardiogram (ECG). The ECG has three main components: the P wave that shows the depolarization of the atria; the QRS complex which depicts the depolarization of the ventricles; and the T wave, which records the repolarization of the ventricles.  

RR Interval

In addition to the shape of individual ECG waves and complexes, the time intervals between waves and complexes are important. For example, the RR interval reflects heart rate and regularity; normal variation from beat to beat should be less than 120 msec or the patient is diagnosed as having an irregular heartbeat. The PR interval reflects the health of the atrioventricular node. A PR interval greater than 120 msec or a variation in the PR interval from beat to beat indicates disease in the node. 


Each ECG lead captures a different perspective of the heart’s electrical activity and produces an ECG waveform with an unique morphology (shape). The standard 12-lead ECG will produce 12 unique graphs. Despite their differences, each ECG lead still displays the three main ECG components of the heartbeat: P wave, QRS complex, and T wave. For the purposes of this software, Lead II will be used as the standard for assessing heart health. Below is an example of an abnormal 12 lead ECG recording.

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