ECG INTERPRETATIONS
An ElectroCardioGram or ECG (more frequently as EKG in which K replaces C which represents the first letter of the German word 'Kardio') is the testing of electrical conductions through the heart muscles (Myocardium) which describes the regular 'SINUS RHYTHMs' and the heartbeats in the form of line waves which can be recorded graphically.
In this article we can study all the basics of how to interpret the ECG.
All ECGs are recorded in graph papers that contain large and small squares. Every large square is composed of five small squares. These squares represent seconds.
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| Fig-2 |
Each small squares are 1 m.m in width horizontally by which we can calculate the width of the large squares as 5 mm horizontally.
The horizontal axis (the X-axis) represents time. Every 1mm square in the X-axis(the time axis) represents 0.04 seconds and every large square represents 0.04 x 5⇔0.20 seconds.
The above calculations are based on the ECG machine speed which is set up to move the ECG paper by 25 mm per second.
The vertical Y-axis represents the electrical conductions that can be recorded in millivolts and which are in the form of positive and negative deflections.
The positive deflection is the one which deflects upward and above the isoelectric line-the X-axis.
On the contrary the negative deflection is the one which goes down below the isoelectric line.
An isoelectric line is the one in which there are no deflections or electric conductions and that is a brief resting position in between two conduction cycles and that is usually represented by the T-P interval which we will understand in due course.
Again the vertical Y-axis contains the dependent variable the cardiac electric deflections in millivolts which can be calculated as follows.
On a standard ECG paper a deflection of the pointer above or below the isoelectric line with 10 small squares represents + or - 1 mV provided that each small square has a width of 1mm.
That is 10mm above or below deflection =+ or - 1mV.
From a normal ECG we can study so many important health parameters including arrhythmias and other heart problems such as atrial fibrillation, atrial flutter, ventricular fibrillation, myocardial infarction(MI), heart rate, normal sinus rhythm, sinus bradycardia, and sinus tachycardia etc.etc.
A normal adult heartbeat is 80 to 100 bpm. (beats per minute)
The normal range is from 70 to 100 bpm.
For a newborn baby it may be between 90 to 160.
A beat or rhythm is a full ventricular depolarization or contraction, which is the systole represented by the QRS complex in the ECG graph. The QRS is followed by a brief gap to another small bump deflected above the isoelectric line which is the T-wave.The T- wave is the end of one full sinus rhythm.T-wave represents the ventricular repolarization or relaxation.
T-P interval is the resting interval in which no conduction which represents the isoelectric line. The isoelectric line goes until the next cycle begins at P-wave which represents the atrial depolarization, the diastole.
To calculate the heart beats per minute you have to do the followings:-
We already know that the speed of the ECG is 25mm per sec. That means,
1 sec = 25 mm
60 sec = 25X60=1500 mm
Hence 1 min =1500 mm or 1500 small squares
We know that 5 small squares constitute one large square and hence in one minute 300 large squares are moved in the ECG. Count the number of R-waves which represent the heart rate per minute by this 300 sq.method.This 300 is the magic number that we can use to find out our heart rate in the standard large square method. There is another method known as a 6-sec method which we will see later.
Before we perform any of the above two methods we must confirm that the ECG is normal with regular sinus rhythms. To do this first you take an empty white strip of a paper. overlap the strip exactly to coincide with the ECG and mark with a pen any one of the completed sinus rhythms or any of the two consecutive R-R intervals to find out all R-R gaps are equal to each other. This can be done by overlap the mark upon one by one of the following R-R intervals. If the mark fits exactly on all the R-R intervals then we can say the ECG is normal with regular sinus rhythm.
Now we can proceed with the large square method to determine the heart or pulse rate.
Method-1:-The Large Square Method:-
Spread your ECG on the table.
Count the number of large squares between any two consecutive R-R intervals.
Divide 300 by this number,
The result is your heart beats per minute.
Method-2-The Six seconds Method:-
In this method the ECG must be taken in a 6-second strip paper. Note that there are three lines on the upper part of the strip for every 6 seconds. Count the number of R-waves within these 3 lines (6 sec) and multiply the number by ten which gives the heartbeats per minute.
This method is useful even if your ECG is not normal.
The 6-second gap can also be calculated as follows:-
Assuming that the speed of the ECG is 25mm/sec.
Accordingly we already noticed that in one minute 300 large squares have passed.
hence 1 min = 300 large sq.
(ie) 60 sec= 300 large sq
then for 6 sec = 30 large sq
Hence mark down the gap which includes 30 large squires.
Now count the number of R-waves within this gap and multiply it by 10 which will yield the heart rate in beats per minute.
The vertical Y-axis represents the electrical conductions that can be recorded in millivolts and which are in the form of positive and negative deflections.
The positive deflection is the one which deflects upward and above the isoelectric line-the X-axis.
On the contrary the negative deflection is the one which goes down below the isoelectric line.
An isoelectric line is the one in which there are no deflections or electric conductions and that is a brief resting position in between two conduction cycles and that is usually represented by the T-P interval which we will understand in due course.
Again the vertical Y-axis contains the dependent variable the cardiac electric deflections in millivolts which can be calculated as follows.
On a standard ECG paper a deflection of the pointer above or below the isoelectric line with 10 small squares represents + or - 1 mV provided that each small square has a width of 1mm.
That is 10mm above or below deflection =+ or - 1mV.
From a normal ECG we can study so many important health parameters including arrhythmias and other heart problems such as atrial fibrillation, atrial flutter, ventricular fibrillation, myocardial infarction(MI), heart rate, normal sinus rhythm, sinus bradycardia, and sinus tachycardia etc.etc.
A normal adult heartbeat is 80 to 100 bpm. (beats per minute)
The normal range is from 70 to 100 bpm.
For a newborn baby it may be between 90 to 160.
A beat or rhythm is a full ventricular depolarization or contraction, which is the systole represented by the QRS complex in the ECG graph. The QRS is followed by a brief gap to another small bump deflected above the isoelectric line which is the T-wave.The T- wave is the end of one full sinus rhythm.T-wave represents the ventricular repolarization or relaxation.
T-P interval is the resting interval in which no conduction which represents the isoelectric line. The isoelectric line goes until the next cycle begins at P-wave which represents the atrial depolarization, the diastole.
To calculate the heart beats per minute you have to do the followings:-
We already know that the speed of the ECG is 25mm per sec. That means,
1 sec = 25 mm
60 sec = 25X60=1500 mm
Hence 1 min =1500 mm or 1500 small squares
We know that 5 small squares constitute one large square and hence in one minute 300 large squares are moved in the ECG. Count the number of R-waves which represent the heart rate per minute by this 300 sq.method.This 300 is the magic number that we can use to find out our heart rate in the standard large square method. There is another method known as a 6-sec method which we will see later.
Before we perform any of the above two methods we must confirm that the ECG is normal with regular sinus rhythms. To do this first you take an empty white strip of a paper. overlap the strip exactly to coincide with the ECG and mark with a pen any one of the completed sinus rhythms or any of the two consecutive R-R intervals to find out all R-R gaps are equal to each other. This can be done by overlap the mark upon one by one of the following R-R intervals. If the mark fits exactly on all the R-R intervals then we can say the ECG is normal with regular sinus rhythm.
Now we can proceed with the large square method to determine the heart or pulse rate.
Method-1:-The Large Square Method:-
Spread your ECG on the table.
Count the number of large squares between any two consecutive R-R intervals.
Divide 300 by this number,
The result is your heart beats per minute.
Method-2-The Six seconds Method:-
In this method the ECG must be taken in a 6-second strip paper. Note that there are three lines on the upper part of the strip for every 6 seconds. Count the number of R-waves within these 3 lines (6 sec) and multiply the number by ten which gives the heartbeats per minute.
This method is useful even if your ECG is not normal.
The 6-second gap can also be calculated as follows:-
Assuming that the speed of the ECG is 25mm/sec.
Accordingly we already noticed that in one minute 300 large squares have passed.
hence 1 min = 300 large sq.
(ie) 60 sec= 300 large sq
then for 6 sec = 30 large sq
Hence mark down the gap which includes 30 large squires.
Now count the number of R-waves within this gap and multiply it by 10 which will yield the heart rate in beats per minute.
General Nomenclature of an ECG:-
In this section we can find the naming of the ECG parts.
Normally an ECG is appearing like this.
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| Fig-3 |
In the above diagram we can observe various conductions and their relationships with a normal ECG.
The electrical responses of the various parts of the heart muscle can be classified briefly as follows:-
0-Phase:-In this phase sodium ion flows inwards rapidly into the myocardial cell and depolarizes it the muscle contracted which is represented by elevation in the graph (see diagram)
1-Phase: -A short but powerful drop-in which all sodium ion gates closed and there is a powerful outflow of potassium ion from the cell with the exchange for the chloride ion which flows into the cell and the myocardium repolarizes and relaxes (note in the above diagram this phase is absent in SA and AV nodal conductions.)This phase is present only in the Purkinje fibers that are the ventricular response. This means the ventricles after a shoot up contraction undergo a brief relaxation which can be evidently seen in the ECG as S-wave at the QRS complex.
2-Phase:-This is the phase very prominent and important in the electrophysiology of the heart. In this there is a continuous potassium ion efflux and calcium ion influx to form a plateau in the cardiogram. In the above diagram you can notify this plateau as the S-T interval in ECG.
3-Phase:-In this phase the calcium ion gates are closed and continuous efflux of potassium ion. This is the rapid repolarization or relaxation of the ventricles which can be seen in the ECG as T-waves.
4-Phase:-This is diastolic depolarization in which the ventricles are ready for another cycle. The resting phase is maintained by continuous potassium efflux and slow sodium and calcium ion influx. This is represented in the ECG as the isoelectric line.
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| FIG-4 |
At the beginning of the heartbeat first the conduction starts at the SA (Sinoatrial)- node which is situated at the top right corner of the right atrium (right upper chamber) of the heart. Sinoatrial node is the pacemaker to start the heart rhythm. It spontaneously depolarizes with a slow response and spread the electrical conduction to the left atrium and down to the ventricles.
The diaphragm which separates the atria from the ventricles is totally inert and will not respond to electrical conduction. Hence the conduction from the SA node goes to the AV node directly.
But before it reaches the ventricles the conduction is paused for a while by the AV (atrioventricular) node which briefly takes time to decides to send the conduction down to the lower chambers are not. This is represented in the ECG as the P-R or P-Q interval. The AV node is situated at the left bottom of the right atrium and just above and very near to the center of the two ventricles.
Up to the AV node the muscle responses to the conduction are slow in order to give enough time to the AV node to decide.
Once the AV-node comes decided it responds quickly and passes the conduction to the Bundle of His, which is situated at the top-bottom junction of the two ventricles or at the inferior end of the internal septum to the ventricles. (See the figure-4 above)
The bundle of His quickly responds to the conduction and passes it to its branches on either side. The branches then pass it quickly and finally to the Purkinje Fibers which holds the two ventricles. The conduction passes to the ventricular muscles which depolarize quickly and powerfully contract with a sound 'dup'.The ventricular contraction is the powerful one that overshadows the atrial repolarization or relaxation. The ventricular contraction is represented by the QRS complex which masks the atrial relaxation in the ECG. You can note from the above figure the QRS complex is the main part of an ECG which exactly at the overshoot region of the Purkinje fiber (ventricular) response drawn above the ECG (See Fig-3 above)
Now we will go back to the P-wave the first part of the ECG.
From the above figure (Fig-3) the P-wave is the region represented by the atrial contraction(depolarization) which is slow and deflect to a small part just above the isoelectric line (X-axis or Time-axis)
We have already seen that the atrial relaxation (repolarization) is masked in the QRS complex.
Now come to the end portion which is the T-wave in the ECG. The T-wave represents ventricular relaxation (repolarization).
We can summarise these waves as follows:-
1.P-wave-Atrial contraction
2.P-Q interval-A brief pause at the AV-node
3.Q-wave-The brief preparation of the ventricular muscles to overshoot
4.R-Wave-Ventricular Contraction, the overshoot phase.
5.S-wave-after the systole brief repolarization followed by a diastolic depolarization to reach the isoelectric line.
6.S-T-interval-The resting phase before the ventricle relaxes.
7.T-wave:-Repolarization of the ventricles followed by the diastolic depolarization of the atria for the next P-wave cycle.
8.T-P interval-The The isoelectric line.
A brief Terminology:
1.Depolarization-means contraction of the muscles
2.Repolarization-means relaxation of the muscle.
3.Systole-Ventricular contraction (atrial relaxation)-R-wave (QRS -Complex)
4.Diastole-Atrial contraction -P-wave
5.Ventricular relaxation-T- wave
Continued in the next
A brief explanation regarding to 'Depolarization' and 'Repolarization'.
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