Learning to interpret ECGs takes time and practice. Let's start out by introducing you to the basic parts of an ECG trace, including the waves, complexes, segments and intervals.
Wondering whether to skip ahead? Here are some of the questions you should be able to answer by the end of this topic:
An electrocardiograph (ECG) is a quick and fairly simple tool that records the electrical activity of the heart. It uses electrodes placed on the surface of the skin. The ECG recording is basically a graph of voltage over time. Because it is only a surface recording it is non-invasive and painless. It can also be affected by artefact such as skeletal muscle contractions and nearby electronic devices, so getting a good clear recording can sometimes be tricky.
An ECG can be recorded for many different reasons. It is often used when a patient has symptoms such as chest pain, palpitations, syncope or loss of consciousness. It can also be used when the patient is asymptomatic, e.g. looking for evidence of Left Ventricular Hypertrophy in patients with chronic hypertension. It can diagnose a range of cardiac arrhythmias, and can suggest a range of electrolyte, pulmonary/pericardial or toxicological problems. It is often abnormal in myocardial infarction, but it can also be normal so the ECG cannot rule out an infarction on its own. It may also show signs of hypertrophy or chamber enlargement, but an echocardiogram is a better tool for this.
The cardiac conduction system shows us how depolarisation spreads through the heart.
The conduction system includes the following components:
An ECG wave is a deflection (i.e. a bump, spike or ripple) that is recorded by the ECG machine when the surface voltage changes. Just like ocean waves, ECG waves come in many different shapes and sizes: they can be sharp or smooth, tall or short, wide or narrow.
Let's start by describing ECG waves as positive, negative or biphasic.
Positive waves are above the baseline of the ECG. They occur when depolarisation spreads towards a positive electrode of the ECG.
Negative waves are below the baseline (upside down). They occur when depolarisation spreads away from a positive electrode.
If a wave is both positive and negative, it can be called biphasic.
The first wave of each beat is called the P wave. The P wave is usually small and hard to see. It represents atrial depolarisation.
In the normal heart, each beat starts at the SinoAtrial (SA) node. When the SA node is depolarised there is no change on the ECG (it is electrocardiographically silent). When the impulse leaves the SA node and travels through the atria it makes the P wave appear on the ECG.
After the P wave there is a tight group of waves called the QRS complex. The QRS complex represents ventricular depolarisation.
Once the impulse has left the AV node it enters the ventricles and their main conduction highway, the Bundle of His. The impulse races down this bundle and into the left and right bundle branches, through the fascicles, to the Purkinje fibres and through the myocardium. As it speeds down these highways, it makes the QRS complex appear on the ECG.
The T wave occurs after the QRS complex. The T wave represents ventricular repolarisation. This repolarisation starts from the apex and works back up the heart from there.
The first wave was named P by Einthoven, most likely to signify a point on a curve (similar to the mathematician Descartes), and possibly to allow for future discovery of waves either side of PQRST in the alphabet. Prior to this the first detectable ECG wave was indeed named A. You can read more about the history of these terms here.
An ECG complex is a tight group of waves. The QRS complex is the main ECG complex and it represents ventricular depolarisation. It is named by the Q, R and/or S waves that make it up.
The Q, R and S waves are not always present. Only the waves that are present should be used to name the complex. For example, if there is no S wave the complex may be a QR complex. A deep broad negative QS complex may have no R wave at all.
Some people also name the waves within the complex as upper or lower case letters depending on whether they are the largest or smallest waves in the complex. This is optional.
Just like waves, QRS complexes can also be described as positive or negative. This is useful for working out the cardiac axis and other features like the R wave progression.
Positive complexes are overall taller above the baseline than below. They occur when depolarisation spreads towards a positive electrode of the ECG.
Negative complexes are overall more below the baseline than above. They occur when depolarisation spreads away from a positive electrode.
If a complex is equally positive and negative, it can be called isoelectric or equiphasic.
ECG segments are the gaps between two waves. They are generally named by the waves on either side of them.
The PR segment is the gap between the end of the P wave and the start of the QRS complex. When the impulse reaches the AV node, there is a delay before it enters the ventricles. This delay allows atrial contraction to finish and it makes up part of the PR segment on the ECG.
The ST segment is the gap between the end of the QRS complex and the start of the T wave. After the ventricles are fully depolarised, they normally will contract before beginning to be repolarised (reset) ready for the next beat. On the ECG this normally produces a flat ST segment.
ECG intervals are periods of time that include at least one wave and segment. They are generally named by the waves at either end.
The PR interval includes everything from the start of the P wave to the start of the QRS complex. The PR interval represents the total time between the impulse leaving the SA node, travelling through the atria, depolarising the AV node, pausing, and then leaving the AV node to enter the ventricles.
The QT interval includes everything from the start of the QRS complex to the end of the T wave. It represents all of the ventricular events (ventricular depolarisation and repolarisation).
The RR interval includes everything from one R wave up to the next R wave.
Tall depolarisation waves can suggest larger amounts of heart muscle to depolarise (i.e. atrial or ventricular hypertrophy).
Short depolarisation waves can suggest a barrier between the heart and the surface ECG electrodes, e.g. pericardial effusion.
Narrow waves suggest fast (normal) conduction. For example, narrow QRS complexes suggest that the rhythm origin is supraventricular and the heart is using the normal conduction highways through the ventricles.
Wide waves suggest that conduction is slower than normal. For example. wide QRS complexes suggest that depolarisation is NOT spreading through the ventricles via the normal conduction highways. This could be due to a ventricular arrhythmia or a conduction block.
Segments can be described as isoelectric (flat), elevated or depressed. Isoelectric means flat, elevated is above the baseline and depressed is below the baseline.
Intervals can be normal, short or long. Short intervals can suggest conduction shortcuts such as accessory pathways or genetic ion channel disorders. Long intervals suggest delays in depolarisation and/or repolarisation.