Electrocardiogram Basics

Study Notes

An ECG is a modified biphasic record of the heart's electrical activity during the cardiac cycle.
The apparatus uses a sensitive galvanometer with a pen recorder that writes on a moving paper.
An amplifier magnifies the potential changes recorded by electrodes attached to the galvanometer.
Recording electrodes use a positive electrode connected to the positive pole of the galvanometer, and a negative electrode connected to the negative pole.
Electrical currents of depolarization and repolarization within the heart travel through surrounding body tissues (thorax acts as a conducting medium).
Electrodes placed on the body surface measure these currents.
ECG waves represent depolarization and repolarization of atria and ventricles.

Leads measure the difference in electrical potential between two points, creating a complete circuit.
These measurements come from two different points on the body (bipolar leads) or a certain point on the body (unipolar leads).

Einthoven, the scientist who first recorded ECGs, detailed lead placement based on this triangle.

One positive and one negative electrode are employed on each limb.
The positive and negative electrodes for each lead are detailed to show the view of the heart for each given lead.

One positive electrode is connected to a part, while the other two limbs act as a negative electrode.

The potential at one electrode (negative pole of the ECG) is set to zero.
Another electrode (the positive exploring electrode) is placed on a limb or chest to record potential differences.

Potential differences are measured at points on the chest with a positive electrode and three other limbs as the negative electrode.
The placements for each of the V leads (V1-V6) are detailed for their individual view of the heart.

The printed graph on paper for ECG measures amplitude (vertical) and time (horizontal).
The details of this graph show how fast the ECG is recorded.
1 small square on the paper represents 0.04 seconds.
A 1 large square represents 0.2 seconds.
Amplitude is determined based on the paper's markings correlating to millivolts.

Detailed representation and labelling of ECG waves, including P, Q, R, S, T, and U waves/segments.
The P wave represents atrial depolarization; QRS complex represents ventricular depolarization; and T wave represents ventricular repolarization.
QT interval, PR interval, and ST segment are also described.

Detailed animation demonstrating the electrical activity of the heart and tracing of ECG waveforms corresponding to each step.

Detailed descriptions of the components of ECG waveforms, including corresponding lead labels, segments, intervals, and their significance.

Method for determining heart rate from the ECG by counting large boxes between R waves and using 300/number of large boxes or 1500 divided by the number of small boxes between R waves.