The electrocardiogram (ECG) is one of the simplest and oldest cardiac investigations available.
However, it can provide a wealth of useful information and remains an essential part of the evaluation of cardiac patients.
With modern machines, surface ECGs are fast and easy to obtain and are based on relatively simple electrophysiological concepts. However, young doctors may find it difficult to interpret.
An ECG performs a representation of the electrical activity of the heart muscle, as it changes over time, usually printed on paper to facilitate analysis.
Like other muscles, the heart muscle contracts in response to the electrical depolarization of muscle cells. It is the sum of this electrical activity, when it is amplified and recorded for a few seconds that we know as an ECG.
Basic electrophysiology of the heart
The normal cardiac cycle begins with spontaneous depolarization of the sinus node, an area of specialized tissue located in the upper right atrium. A wave of electrical depolarization extends through the right upper atrium and through the interatrial septum to the left atrium.
The atria are separated from the ventricles by an electrically inert fibrous ring, so that in the normal heart the only way of transmission of electrical depolarization from the atrium to the ventricles is through the atrioventricular node.
The node delays the electrical signal for a short time, and then the depolarization wave extends through the interventricular septum, through the bundle of His and the branches of the right and left bundle.
Therefore, with normal conduction, both ventricles contract simultaneously, which is important to maximize cardiac efficiency.
After complete depolarization of the heart, the myocardium must re-polarize, before it can be ready to depolarize again for the next cardiac cycle.
Electric shaft and main recording vectors
The ECG is measured by placing a series of electrodes on the patient’s skin, which is why it is known as a “surface” ECG.
The wave of electrical depolarization extends from the atrium down to the ventricles. So, the direction of this depolarization is generally from the superior to the inferior aspect of the heart.
The direction of the depolarization wave is usually to the left due to the leftward orientation of the heart in the thorax and the greater muscle mass of the left ventricle than the right.
This general direction of displacement of electrical depolarization through the heart is known as the electrical axis.
A fundamental principle of the ECG record is that when the depolarization wave travels towards a recording conductor, this results in a positive or ascending deflection. When moving away from a recording conductor, this produces a negative or descending deflection.
The electrical axis is usually downward and to the left, but we can estimate it more accurately in individual patients if we understand from which direction each record score measures the ECG.
By convention, we record the standard surface ECG using 12 different recording ‘lead’ addresses, although in a very confusing way only 10 recording electrodes are required on the skin to achieve this. Six of these are recorded from the chest that covers the heart, thorax and precordial leads.
Four are recorded from the extremities. It is essential that each of the 10 recording electrodes is placed in its correct position; otherwise, the appearance of the ECG will change significantly, which will impede correct interpretation.
The limb leads record the ECG in the coronal plane, so they can be used to determine the electrical axis (which is usually measured only in the coronal plane).
The electrical axis of depolarization is also expressed in degrees and is usually in the range of -30 0 to + 90 0.
A detailed explanation of how to determine the axis is beyond the scope of this article, but the principles mentioned here should help readers understand the concepts involved.
The thoracic leads record the ECG in the transverse or horizontal plane, and are called V1, V2, V3, V4, V5 and V6).
Voltage and timing intervals
It is conventional to record the ECG using standard measurements for the amplitude of the electrical signal and for the speed at which the paper moves during recording. This allows:
- Easy appreciation of heart rates and cardiac intervals.
- Make a meaningful comparison between the ECGs recorded on different occasions or by different ECG machines.
The amplitude or voltage of the recorded electrical signal is expressed on an ECG in the vertical dimension and measured in millivolts (mV).
In the standard ECG paper, 1 mV is represented with a deflection of 10 mm. An increase in the amount of muscle mass, such as left ventricular hypertrophy (LVH), usually results in a greater electrical depolarization signal and, therefore, a greater amplitude of vertical deviation in the ECG.
An essential characteristic of the ECG is that the electrical activity of the heart is shown as it varies with time. In other words, we can think of the ECG as a graph, plotting the electrical activity on the vertical axis versus time on the horizontal axis.
The standard ECG paper moves at 25 mm per second during real-time recording. This means that when looking at the printed ECG, a distance of 25 mm along the horizontal axis represents 1 second in time.
The ECG paper is marked with a grid of small and large squares. Each small square represents 40 milliseconds (ms) in time along the horizontal axis and each larger square contains 5 small squares, which represents 200 ms.
Standard paper speeds and square marks allow easy measurement of cardiac timing intervals. This allows the calculation of heart rates and the identification of abnormal electrical conduction within the heart.
It will be clear that the first structure that will depolarize during normal sinus rhythm is the right atrium, closely followed by the left atrium.
So the first electrical signal in a normal ECG originates from the atria and is known as the P wave. Although there is usually only one P wave in most of the leads of an ECG, the P wave is, in fact, the sum of the electrical signals of the two atria, which are usually superimposed.
Then there is a brief physiological delay since the atrioventricular node slows down electrical depolarization before it proceeds to the ventricles.
This delay is responsible for the interval, a short period in which no electrical activity is observed on the ECG, represented by a horizontal or ‘isoelectric’ straight line.
Depolarization of the ventricles generally produces most of the ECG signal (due to the greater muscle mass in the ventricles) and this is known as the QRS complex:
- Wave Q is the first initial deflection down or “negative”.
- The R wave is then the next upward deflection (as long as it crosses the isoelectric line and becomes ‘positive’).
- The S wave is then the next downward deflection, as long as it crosses the isoelectric line to become briefly negative before returning to the isoelectric base line.
In the case of the ventricles, there is also an electrical signal that reflects the repolarization of the myocardium. This is shown as the ST segment and the T wave.
The ST segment is usually isoelectric, and the T wave in most of the leads is a vertical deflection of varying amplitude and duration.
The recording of an ECG on standard paper allows measuring the time necessary for the various phases of electrical depolarization, usually in milliseconds.
There is a recognized normal range for such ‘intervals’:
- PR interval (measured from the beginning of the P wave to the first deviation of the QRS complex). Normal range 120 – 200 ms (3 – 5 small squares on ECG paper).
- QRS duration (measured from the first deviation of the QRS complex to the end of the QRS complex in the isoelectric line). Normal range up to 120 ms (3 small squares on ECG paper).
- QT interval (measured from the first deviation of the QRS complex until the end of the T wave in the isoelectric line). Normal range up to 440 ms (although it varies with the heart rate and may be a bit longer in women)
ECG heart rate estimation
The standard ECG paper allows a rough estimate of the heart rate (HR) from an ECG recording.
Each second of time is represented by 250 mm along the horizontal axis. So, if the number of large squares between each QRS complex is:
- 5 – the HR is 60 beats per minute.
- 3 – the HR is 100 per minute.
- 2 – the HR is 150 per minute.