Of the many advancements in diagnostics, one which has stayed so conservative for a long time is the humble ECG. It just measures the electrical activity of the heart and scribbles it in a piece of paper. But every little detail of the scribbling is so precious, that it gives information like, heart rate, rhythm, conduction defects, enlarged chambers, ischaemia, infarct, etc aiding to arrive at a diagnosis. Another good thing about knowing how to read an ECG is that, You can show off..!
Lets discuss how to enlist the findings in an ECG just by glancing at it like a pro..!
Quick Basics Recap:
Conduction System of Heart
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ECG:Waves, Segments and Intervals |
1) Calibration:
The first thing to do while looking at an ECG is make sure the machine is not at fault and is calibrated correctly. The machine automatically performs a calibration every time before a recording, which is usually found somewhere along the left hand side of the ECG. Check if its 2 large boxes tall and 1 large box wide like shown in the image
This confirms, 1mm =0.1mV and and the paper speed is 25mm/sec which is the most commonly used calibration setting.
In the horizontal x-axis, time is calibrated and, Each small box is 1mm wide, and 5 small boxes make up a large box(5mm).
The machine records for a period of 10 seconds at a rate of 25mm/sec.
So each small box is 0.04seconds or 40ms.
Each large box is 0.2seconds or 200ms.
In the vertical y-axis, each small box is 0.1mV and hence a large box makes up 0.5mV,and 2 large boxes makes up 1mV.
Lead Misplacement:
The right and left arm electrodes are most prone for misplacement. It is manifested by
- Inverted Lead I ( Inverted P wave, QRS complex, T wave)
- Upright aVR
Rate and Rhythm:
The next thing to look at an ECG is the heart rate. You can find it in the automatic analysis easily. But what if it shows something like this as shown in the image..!
Wait wait the patient is not dead. Machines use algorithms to give automatic analysis and it may sometimes be erroneous.
So dont depend on the automatic analysis, But always have a look at it, as it may point something you missed.
To calculate the heart rate look at the rhythm strip which is usually Lead II given at the bottom of ECG. First check if the distance between two consecutive R waves i.e R-R interval is equal.
If the R-R interval is equal, it means the rhythm is regular, now count the number of large boxes between two consecutive R waves. Divide 300 by the number of large boxes.
Example: If 4 large boxes are present between two R waves, 300/4 =75bpm.
If the R-R interval is not equal, It means the rhythm is irregular and You cannot use the above method. Count the number of QRS complex over a period of 10seconds/50 large boxes and multiply it by 6 which will give the heart rate in bpm
Example: 12QRS complex is found in the rhythm strip. So heart rate is 12 x 6= 72bpm.
- <60- Bradycardia
- >60- Tachycardia
If P wave is followed by QRS complex, it is called sinus rhythm. Confirm if it is regular/ irregular.
For more on Arrhythmias,
Axis deviation:
Remember the diagram about conduction system of the heart. If incase, one ventricle is hypertrophied, the extra tissue will generate more electrical signal resulting in deviation of the axis towards that side.
Axis is determined by 3 leads I,II,III.
Normal axis:
If the axis is normal, QRS in all these leads I, II, III will be predominantly POSITIVE
Right Axis deviation:
In RAD, Lead I is NEGATIVE and Lead III is positive. Lead II may be positive or negative.
RAD may be seen in conditions causing Right Ventricular Hypertrophy and also as a normal variant in very tall individuals.
Left Axis deviation:
In LAD, Lead I is POSITIVE and Leads II, III are NEGATIVE.
LAD is seen mostly in conduction defects and also rarely in conditions causing Left Ventricular Hypertrophy.
A part of ur diagnosis is now ready:
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