Understand the Electrocardiogram (ECG)
Become acquainted with bioelectrical signals
Understand time and frequency domain signal processing
Become acquainted with biological data acquisition
ElectrocardiographyThe ECG records (indirectly) the electrical activity of the heart. This activity reflects the action of the cardiac muscle as it depolarizes and repolarizes during the cardiac cycle. The ECG represents the temporal and spatial summation of the action potentials of the myocardial fibers typically measured with body-surface electrodes.
ECG's are used to diagnose arrhythmias, abnormal electrolyte (potassium) levels, and conduction abnormalities. They are also used for screening and therapy guidance for heart disease as well as cardiac gating for imaging.
In the laboratory, there will be a software tutorial for you (12Lead) to help understand the ECG signal and supplement this material.
In order to get an electrical signal from the body, suitable electrodes, amplification and appropriate display are required. Some cardiac cells generate action potentials (pacemakers). Once generated, and under physiological conditions, the action potential propagates through the cardiac muscle. The temporal and spatial summation of the monophasic action potentials of the myocardial fibers produces an electrical signal known as the ECG.
Electrical activity of the heart
The sinoatrial (SA) node, is the pacemaker of the heart. These special autorhythmic cells initiate the action potentials that cause contraction. They start the action potential spreading through the atria.
The atrioventricular (AV) node, at the base of the septum, allows the signal started by the SA to spread to the ventricles.
The bundle of His (atrioventricular bundle) starts at the AV, divides into left and right branches which follow the septum and curve around to the outer walls. Purkinje fibers extend from the bundle of His. These help spread the action potential rapidly through the ventricle.
An ECG reflects the sequence of depolarization and repolarization over the contractile chambers of the heart seen using body-surface electrodes. This electrical activity is related to the contraction and relaxation of the heart chambers.Electrodes measure the voltage between points on the body. A depolarization wavefront (or mean electrical vector) moving toward a positive electrode creates a positive deflection on the ECG in the corresponding lead. A depolarization wavefront moving away from a positive electrode creates a negative deflection in the corresponding lead. A depolarization wavefront moving perpendicular to a positive electrode creates an equi-phasic complex: positive as the depolarization wavefront approaches, negative as it passes.
P wave = atrial depolarization
The PR interval corresponds to the time lag from the onset of atrial depolarization to the onset of ventricular depolarization. This time lag allows atrial systole to occur, filling the ventricles before ventricular systole. Most of the delay occurs in the AV node. A long PR interval corresponds to impaired AV conduction.
QRS complex = ventricular depolarization
The QRS interval represents the time it takes for ventricular depolarization. Normal depolarization requires normal function of the right and left bundle branches. A block in either the right or left bundle branch delays depolarization of the ventricle supplied by the blocked bundle, resulting in a prolonged QRS duration.
T wave = ventricular repolarization
The QT interval represents the time of ventricular depolarization and repolarization. It is useful as a measure of repolarization and is influenced by electrolyte balance, drugs, and ischemia. The QT interval is inversely related to heart rate. A QT interval corrected for heart rate can be calculated.
ECG AcquisitionThe electrodes are traditionally placed on arms and legs, making it easy to position the electrodes. These connections are called leads. The standard leads are:
The electrical signal measured from the surface has an amplitude between 10uV and 5 mV and a bandwidth from 0.05 to 100.00 Hz.
The equipment used to measure and visualize the ECG includes:
The cardiac vector indicates the direction of the depolarization in time. The ECG measured from any one pair of the bipolar leads is a time variant, single dimensional projection of the cardiac vector and could be represented using the Einthoven triangle. Willem Einthoven was a Dutch physiologist who pioneered the ECG and won the Nobel prize in Medicine in 1924 for this work. Typically, the QRS segment is represented in the triangle.
Einthoven's triangle and the net electrical heart vector. (Courtesy of Hewlett-Packard)
Einthoven's triangle provides a way to understand the amplitude of the ECG waves. One way to verify that your data is correct is to plot the cardiac vector into the Einthoven triangle. For this:
The perpendicular lines corresponding to the initial point of the vectors will intersect in the center of the triangle; the perpendiculars traced from the end of the vector should also intersect at one point. The line between the two intersections shows the orientation of the heart. This can explain differences in magnitude between your data and the nominal values.
ECG Nominal Data
Nominal range of amplitudes of electrocardiographic waves (mV)
Nominal interval durations. Data from D. A. DeMarre, D. Michales, Bioelectronic measurements, Prentice-Hall, Inc., Englewood Cliff, N.J. 1983
Schematics of ECG Abnormalities:
From Human Physiology, L. Sherwood, Wadsworth Inc.