What is Electrocardiogram?
Electrocardiogram is a non-invasive test conducted to find out problems with electrical activity of your heart. It is also referred to as a 12 lead electrocardiogram because it collects information on 12 different areas of your heart. 1, 2, 3
Why is Electrocardiogram done?
Electrocardiogram test is done to:
- Look for electrical activity of your heart
- Determine the cause of mysterious chest pain and pressure
- Determine the size or thickness of the walls of your heart chambers
- Find out if any medicines you are using are working properly. 1, 3
How is Electrocardiogram done?
You can have an electrocardiogram test in the hospital during your official appointment with a health professional.
You can also have this anywhere because the electrocardiogram is portable. There are specific cells known as pacemakers in the upper right chamber of your heart. These cells produce electrical impulses which trigger your heartbeat.
An electrocardiogram records information about these signals as they pass through your heart. During an electrocardiogram test, particular areas of your chest, arms and legs will be cleaned using soap and water.
Your hair is also shaved by using scissors because hair can interrupt the electrical contact.Also, your health professional may use a dry cloth or rough patch to rub those sites to remove dead skin cells and enhance electrical contact.
How to Observe the Cardiac Rhythm?
Once cleaning is complete, your health professional attaches some moist electrodes on your chest, each arm and leg.
These electrodes are then traced onto a monitor that displays electrical activity of your heart in form of a waveform on a screen. The monitor will automatically produce a rhythm strip when the alarm goes off.
The rhythm strip produced may contain the following information: patient’s name, date and time the test was conducted, identification number and rhythm interpretation.
Your health professional will keep the rhythm strip in your medical record file for future reference.
How to Read an Electrocardiogram strip?
Your heart’s electrical current produces waveforms which are recorded on an electrocardiogram paper using a stylus. The electrocardiogram paper has vertical and horizontal lines which form a grid.
The wave form is usually labeled P, Q, R, S, T and U.
During the normal sinus rhythm, the right atrium is depolarized first followed by the left. P wave is the first signal from the atria.
PR interval is a short period with no electrical activity because of delay which slowed depolarization of the signal before it goes to the ventricles. QRS complex is the largest portion of the ECG signal which occurs after depolarization of the ventricles.
- Q wave is first deflection downwards
- R wave is the next deflection upwards if it passes the isoelectric line and change to positive
- S wave is the next deflection downwards
- T wave indicate repolarization of the ventricles.
- QT interval determines repolarization and depolarization of the ventricles
- ST segment indicates the end of ventricle depolarization and the start of ventricle recovery.
ECG rhythm script
A small square equals to 0.04 seconds.
The horizontal axis of an electrocardiogram strip usually represents time. Each small square is 0.04 seconds and five small squares form a large square of 0.2 seconds.
The vertical axis of an electrocardiogram strip is used to measure the electrical voltage in millivolts (mV). Each small square is 0.1mV or 1.00mm and each larger square is 0.5mv or 5.00mm.
You can determine the amplitude of a wave, interval or segment by simply counting the small squares from the reference point (baseline) to the highest or lowest point of the wave, interval or segment.
How to Interpret a Rhythm strip?
You can employ different approaches to interpret a rhythm strip as long as you are consistent. The following is an eight step method to help you interpret a rhythm strip:
Determine the rhythm
You can use caliper or paper-and-pencil methods to determine ventricular or atria rhythm of your heart.
Paper-and-pencil method
Put an electrocardiogram strip on a flat surface and then place the edge of a piece of paper on the baseline of the strip.
Move the paper up slightly such that the edge of the paper is close to the highest point of wave R. Use the pencil to mark the point of R waves and consecutive QRS complexes. This distance is known as R-R interval.
Then move the paper across the strip and align the two marks with R-R interval. Measure the distance between each mark and the interval R-R. If the distance is the same, the ventricular rhythm is regular and when the distance varies, the rhythm is irregular.
Use the same procedure to measure the distance between two P waves (P-P interval). This will help you determine whether the artria rhythm is normal or not.
Repeat all this procedure in several cardiac cycles and distances R-R and P-p. If the variation is not more than 0.04 seconds, it is a regular rhythm.
Assess P waves
Assess P waves in a rhythm strip. If you observe that every QRS has a P wave, which has similar size and shape; the amplitude of 2 to 3mm high and duration of 0.006 to 0.12 seconds, then P wave is regular.
Determine the heart rate
If the heart rate is regular, you can use a 1,500 approach to determine the rate. In this method, 1,500 small squares equals to 1 minute. Count the number of small squares between similar points on consecutive P waves.
Then divide 1,500 by the number of those squares to find the artria rate. Use the same procedure to determine the ventricular rate by using R waves.
Measure the duration of PR interval
This is done by counting the number of small squares between the beginning of P waves and start of QRS complex deflection and the result multiplied by 0.04 seconds. If the result is between 0.12-0.2 seconds, then PR interval is normal.
Determine the duration of QRS complex
In this step, measure the distance from PR to the end of S wave. To compute time, count the number of small squares from the first deflection of QRS and end of QRS complex and multiply the result by 0.04 seconds.
If the duration of QRS is between 0.006 to 0.12 seconds, then it is a narrow QRS. It is a wide QRS if the value is more than 0.12 seconds.
Assess T waves
Assess the rhythm strip for T waves. If you observe that T wave is round and smooth, has amplitude of 0.5 to 10mm and an upright deflection, then it is regular wave.
Measure the duration of QT interval
To determine the duration of QT interval, count the number of small squares between the start of deflection of QRS complex and the end of T wave (the point where T wave returns to the baseline).
Then multiply the answer by 0.04 seconds. If the duration is not greater than 0.44 seconds then the rhythm is regular.
Assess the strip for any other component
Look for any abnormalities, ectopic pulses, and presence of U waves. Then name the rhythm strip according to the source of the rhythm, abnormalities and rate.
What may Affect Electrocardiogram Test results?
You may get incorrect electrocardiogram test results because of the following:
- You performed a physical exercise before the test
- Your health professional did not attach the electrodes on your body properly hence interfered with electrical contact
- You talked or moved during the test
- You were very anxious during the test
- You had a very deep breath during the test.
Reference List
- ECG Interpretation. http://www.theheartcheck.com/documents/ECG%20Interpretation%20Made%20Incredibly%20Easy!%20(5th%20edition).pdf
- http://www.mayoclinic.org/tests-procedures/ekg/home/ovc-20302144
- Electrocardiogram. http://www.webmd.com/heart-disease/electrocardiogram#3
- How to Read an ECG Strip. http://www.rnceus.com/ekg/ekghowto.html