How to Read an ECG in Six Easy Steps

This is a brief guideline for you to help you understand your heart by reading an ECG!

Table of Contents

Part I: What is an ECG?

The heart is largely made up of a type of muscle tissue called cardiac muscle. The cardiac muscle is a group of strong muscles which sends out electrical signals when contract. During its operation, it will continuously produce rhythmic bioelectric signals. As early as 1903, Dutch physiologist Willem Einthoven designed and invented the first cardiac motor. Its working mechanism is still in use today. He found out that through improved leads, doctors can measure weak bioelectrical signals. A large part of this signal is the bioelectrical rhythm generated when the heart beats. By measuring the electrical signals in different directions, the electrical signals generated when the heart beats can be calculated.

^{Figure2: The capillary electrometer created by Willem Einthoven}

Part II: Understanding of ECG

P waves

The first smaller wave appears, the atrial depolarization wave.

The normal P-waves is between 0.06~0.12s

PR segment

The atria begin to depolarize to the ventricles begin to depolarize.

PR interval

The PR interval should be between 0.12~0.2s.

QRS complex

The whole process of left and right ventricular depolarization.

The normal Q wave time is less than 0.04 seconds, and the QRS complex time is 0.06~0.1s

ST segment

A straight line from the end of the QRS complex to the beginning of the T wave represents the process of slow ventricular repolarization.

The normal ST segment should be at the horizontal baseline, with a downward deviation of no more than 0.05mv and an upward deviation of no more than 0.1mv in any lead.

T wave

The process of rapid ventricular repolarization.

The normal time is 0.1-0.25 seconds, and the voltage is 0.1-0.8mv.

QT interval

QT interval (measured from first deflection of QRS complex to end of T wave at isoelectric line). Normal range up to 0.36-0.44 ms (though varies with heart rate and may be slightly longer in females).

Part III: The Normal ECG

It will be clear that the first structure to be depolarized during normal sinus rhythm is the right atrium, closely followed by the left atrium.

So the first electrical signal on a normal ECG originates from the atria and is known as the P wave.
The Q wave is the first initial downward or ‘negative’ deflection
The R wave is then the next upward deflection (provided it crosses the isoelectric line and becomes ‘positive’)
The S wave is then the next deflection downwards, provided it crosses the isoelectric line to become briefly negative before returning to the isoelectric baseline.

In the case of the ventricles, there is also an electrical signal reflecting repolarisation of the myocardium. This is shown as the ST segment and the T wave. The ST segment is normally isoelectric, and the T wave in most leads is an upright deflection of variable amplitude and duration (see Figures 4).

^{Figure 4: The major waves of a single normal ECG pattern}

To understand the electrocardiogram is very simple, firstly remember the appearance of the normal waveform, and then follow the steps below to read the ECG step by step! The above picture is a healthy ECG waveform. Keep in mind that the upper few sections are used to view the shape, and the lower section is used to view the heart rhythm.

Part IV: How to Read an ECG

Firstly, is the P wave of I, II, aVF upward, and is aVR downward? If they are, make sure the sinus node is normal. Pay attention to whether the P wave pattern is normal (the top is blunt and smooth, and it is upright)? Are all P waves the same size and type?

Secondly, is the direction and time of the QRS complex in leads I and III normal? Are all QRS complexes the same size and type? Is there a QRS complex after every P wave?

Thirdly, is the T wave upward? Refer to the legend and the content of the second part to determine whether the shape and time of the remaining waveforms are normal.

But If there is arrhythmia, premature beats, missed beats, etc., then basically every lead will have problems, but the performance is different in different leads. If you encounter this kind of problem, please seek professional medical advices.