email meBIOPAC - Fundamentals of an ECG Recording
For nearly all ECG measurements, use an electrode and patch attach it to the input using a connector cable. Position the subject about 12 feet away to allow for some movement. Finally use an ampliphier to give a recordable signal to process.
The heart as a potential source
We learned that the heart pumps the blood under command from an electrical stimulus in the electroconduction system.
Action potentials create electrical currents that spread from the heart throughout the body. The spreading electrical currents create differences in electrical potentials between various locations in the body, and these potentials can be detected and recorded through surface electrodes attached to the skin.
The ECG Waveform
The bipolar limb leads (I, II, III) form what is called the Einthoven Triangle.
An electrocardiogram (EKG) is a representation of of the electrical activity of the heart as measured at the surface of the body. It allows observation of the sequential events of excitation and recovery of the chambers of the heart. An EKG allows identification of normal and abnormal electrical activity and rhythms, including tissue damage, atrial or ventricular hypertrophy (enlargement), congenital defects and the effect of drugs on cardiac muscle.
A typical EKG pattern arises from the wave of depolarization and repolarization passing through the chambers of the heart. When the entire heart myocardium is in the resting state (polarized), a flat baseline (isoelectric line) is produced. The first deflection is the relative slow and weak depolarization of the atria (P wave), occuring just before the atria contract. The next series of deflections (QRS complex) occurs just before the ventricles contract. As the impulse spreads through the ventricles, the potential rises rapidly to a sharp peak, falling to zero as the ventricular myocardial cells depolarize. The T wave rises to a round peak as the ventricular cells repolarize, falling back to zero when all cells are repolarized. The atrial repolarization is obscured by the QRS complex.
Amplifiers are an important part of instrumentation signals. Such measurements involve voltages that often are at low levels, have high source impedances or both. Amplifiers are required to increase signal strength while maintaining high fidelity. Biopoential amplifiers are those specifically designed for this type of processing of biopotentials.
The beating heart generates an electrical signal that can be used as a diagnostic tool for examining some of the functions of the heart. This electrical activity of the heart can be approximately represented as a vector quantity. Thus we need to know the location at which signals are detected, as well as the time dependance of the amplitude of the signals. A simple model to represent the electrical activity of the heart has been developed. In this model, the heart consists of an electrical dipole located in the partially conducting medium of the thorax.
This particular field and the dipole that produces it represent the electrical activity of the heart at a specific instant. At the next instant the dipole can change its magnitude and its orientation, thereby causing a change in the electrical field.