ECG Reading

Following a systematic method to reading an ECG is the best way to ensure that any and all pathology is identified. Focusing on the obvious abnormality may cause you to miss something else.

Step 1 –  Rate

If you remember, 300 large squares on an ECG is representative of 1 minute. As heart rate is usually expressed in beats per minute (BPM), we can use this number to work out the heart rate. First take the rhythm strip at the base of the ECG print out, it is typically lead 2. Then count the number of large squares between two R waves (R-R interval). Then, divide this number by 300, and this will give you the heart rate. In the example below, the R-R interval is 5 large square, 300/5= 60, therefore the heart rate is 60bpm. Be mindful that in certain conditions, the rate may be irregular, so counting over a longer period may be more accurate.

A normal heart rate is 60 – 100 bpm. Less than 60 is classed as bradycardia, and over 100 is classed as tachycardia.

Image result for r-r interval ecg

Step 2 – Rhythm

The rhythm of an ECG can be described as regular of irregular, and if the rhythm is irregular it can be classed as regularly irregular or irregularly irregular. To work out which is which, count the number of squares in the R-R interval in a number of complexes, if they are all completely different the rhythm is irregularly irregular as is the case in atrial fibrillation. If the intervals vary, but have a pattern, they are classed as regularly irregular as is the case in certain types of heart block.

Step 3 – Axis

The cardiac axis the way that the electrical activity spreads across the heart, and in a normal heart should start at the SAN and spread to the AVN, then along the bundles of his and purkunje fibres. We can determine the cardiac axis by looking at leads 1, 2 and 3. In right axis deviation, lead 3 will be taller than leads 1 and 2, and 1 will be negative. In left axis deviation, lead 1 will be taller and 2 and 3 will be negative.

Step 4 – P Waves

Can you see P waves? Remember P waves are caused by the wave of electrical activity spreading across the atria and causing it to contract. If P waves are present and there is a corresponding QRS complex, this is termed ‘sinus rhythm’, absence may indicate Atrial Fibrillation in-conjunction with an irregularly irregular rhythm.  How long is the duration of the P wave? It should be no longer than 3 small squares, longer than that may be indicative of atrial enlargement. What it the shape of the P wave? A saw-tooth pattern without a corresponding QRS complex may indicate atrial flutter.

Image result for sawtooth p wave

Step 5 – PR interval

By measure the PR interval, we can work out whether there is a clear corresponding relationship between the P wave and the QRS complex as their should be. The PR interval should be between 3-5 small squares (0.12 -0.20 seconds). Longer than this could indicate that there is an antrioventricular block. In First Degree Heart Block there is a fixed prolongation of the PR interval, the rhythm however is unaffected; first degree heart block may not require treatment and patients may be asymptomatic. There are two types of Second Degree Heart Block. Mobitz type 1 involves the PR interval getting progressively larger until finally a QRS complex is dropped. In Mobitz type 2 the PR interval is fixed but eventually it drops a QRS compex. This can be expressed as a ratio of P waves to dropped beats.

Finally the Third Degree or Complete Heart Block, is where the there is no relationship between the P waves and the QRS complexes. Typically these patients will be bradycardic.

Image result for heart block ecg

Equally the PR interval may be shorter than 3 small squares, suggesting the presence of an accessory pathway allowing the electrical impulse to travel to the ventricles quicker. This may also be associated with the presence of a delta wave, typically seen in patients with Wolf-Parkinson White Syndrome, whereby patients have an accessory pathway known as a ‘bundle of kent’.

Step 6 – QRS Complexes

When considering the QRS complexes we consider whether they are broad or narrow, and small or tall. Normally, the QRS should be narrow, a broad complex QRS suggests the depolarization has initiated in the ventricles such as in a ventricular ectopic beat. A broad complex tachycardia indicates Ventricular Tachycardia which is life threatening. A broad complex QRS is also seen in patients with bundle branch block (bundles of his), as the electrical impulse from one bundle must propagate across both ventricles and thus does so slowly. To determine which side is blocked, look at leads V1 and V6 and remember WILLIAM MARROW.

Image result for william marrow


As mentioned we also consider how tall the QRS complex is, tall Q waves indicate high voltages and thus may suggest ventricular hypertrophy. It should be noted however that tall QRS complexes are often seen when performing an ECG on slender patients.

The presence of Q waves should also be observed. They are normal in certain leads, but consider their presence in an entire territory and ask whether it is indicative of a previous MI.

Step  7 – ST Segment

The second to last step in reading the ECG is to look at the ST segment, which can give us valuable diagnostic information in the case of ischemic events. ST elevation in a particular territory may suggest an ST elevation myocardial infarction (STEMI), whereas depression is indicative of ischemia.

To determine which area of the heart is involved, consider which leads are affected:

Image result for which ecg leads look at which part of the heart

Step 8 – T wave

The final step in reading the ECG is to look at the T wave. The T wave demonstrates re-polarization of the ventricles. Tall T waves can be seen in hyperkalemia (high potassium levels) and in the early acute stages of an MI. Inverted T waves can be the result of a number of conditions and a high percentage of generally unwell patients will experience T wave inversion. I can also be demonstrable in patients with Pulmonary Embolism, Ischemia (consider exercise stress test) and bundle branch blocks. The presence of T wave inversion should be considered in the context of the patients condition


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