EKG - Diagnostic Tool for Heart Disease
Electrocardiogram (ECG) in Heart Health, Heart Attack, Heart Failure
Nov 16, 2009
An electrocardiogram or ECG is a measure of the electrical activity of the heart, recorded on a graph. An ECG is a non-invasive diagnostic test that can be easily and quickly performed to help identify and monitor a wide variety of heart diseases like heart attack and heart failure.
Electrical impulses initiate the alternate contraction and relaxation of the heart necessary for it to pump blood to the rest of the body. These impulses begin in the Sinuatrial Node (SAN) and travel along a conducting system to reach and stimulate various parts of the cardiac muscle.
The electrical impulses originating in the heart spread to adjacent tissues of the chest wall and reach the surface of the body where they can be measured and recorded by the placement of electrodes surrounding the heart.
How Does an ECG work – ECG Leads and ECG Machines
An ECG is recorded by placing electrodes (called ECG leads) on the chest wall and limbs and hooking them to an ECG machine. Different types of electrodes may be used to record electrocardiographic signals. These are:
- Bipolar leads
- Unipolar leads.
Bipolar ECG Leads
Bipolar leads are three in number and are placed as follows:
- One on the left leg
- The remaining two, one on each arm.
This placement forms a sort of equilateral triangle around the heart called the Einthoven’s triangle, with a current source in the centre.
Unipolar ECG Leads
Unipolar leads are nine in number and are placed as follows:
- Six unipolar chest leads, called precordial leads, numbered V1 through V6
- Three unipolar augmented limb leads, labeled aVR, aVL and aVF.
ECG Interpretation
On the graph that is recorded the ECG appears as a series of waves. These waves are named in the following order:
- P wave
- QRS wave complex comprising of Q, R and S waves
- T wave.
The P wave is generated when the atria of the heart is just about to contract. The QRS wave complex is generated when the ventricles are stimulated and are just about to contract. The T wave is generated when the ventricles recover from the contracted state. Therefore, the P and QRS waves are called depolarization waves while the T wave is called a repolarization wave. The wave for atrial recovery from the contracted state (repolarization wave) is hidden in the QRS complex and is not seen as a separate wave.
These waves become distorted or show changes when the part of the heart from which they originate is diseased. Therefore, in order to interpret an ECG, a physician will look carefully at the morphology of each wave generated by each lead.
The parameters studied in an electrocardiogram are:
- Shape of the individual waves
- Size of the waves
- Intervals between the waves.
Uses or Applications of ECG
The ECG is typically used to detect various physiological and abnormal conditions of the heart.
Conditions that can be diagnosed using ECG are:
- Sinus bradycardia
- Tachycardia
- Sinus arrhythmias
- Heart blocks – condition where the electrical conduction through the heart muscle is interrupted
- Myocardial infarction, commonly called the cardiac arrest or heart attack
- Changes in the composition of electrolytes like potassium and calcium in the blood as these affect the cardiac function.
The ECG is also used to continuously monitor cardiac function during general anesthesia for surgery or when a person is hooked to a ventilator.
The electrocardiogram or ECG, used to diagnose a variety of heart diseases, is a simple and easy procedure and can even be performed in emergency situations like a heart attack.
Sources:
Ganong, William F. Review of Medical Physiology (McGraw Hill; 22nd edition, 2005)
Read more about and watch a movie on the heart's electrical system.
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