How to read an Electrocardiogram (ECG). Part One: Basic principles of the ECG. The normal ECG (2024)

Author(s): Dr Dallas Price

Consultant Cardiologist, St Mary's Hospital, Isle of Wight, UK

Introduction

The electrocardiogram (ECG) is one of the simplest andoldest cardiac investigations available, yet it can provide a wealth of usefulinformation and remains an essential part of the assessment of cardiacpatients.

With modern machines, surface ECGs are quick and easy toobtain at the bedside and are based on relatively simple electrophysiologicalconcepts. However junior doctors often find them difficult to interpret.

This is the first in a short series of articles that aim to:

Helpreaders understand and interpret ECG recordings.
Reducesome of the anxiety juniors often experience when faced with an ECG.

Basic principles

What is an ECG?

An ECG is simply a representation of the electrical activityof the heart muscle as it changes with time, usually printed on paper foreasier analysis. Like other muscles, cardiac muscle contracts in response toelectrical depolarisation of themuscle cells. It is the sum of this electrical activity, when amplified andrecorded for just a few seconds that we know as an ECG.

Basic Electrophysiology of the Heart (seeFigure 1)

The normal cardiac cycle begins with spontaneousdepolarisation of the sinus node, an area of specialised tissue situated in thehigh right atrium (RA). A wave of electrical depolarisation then spreadsthrough the RA and across the inter-atrial septum into the left atrium (LA).

Figure 1. Basicelectrophysiology of the heart

Electrical axis and recording lead vectors (seeFigures 2 and 3)

The ECG is measured by placing a series of electrodes on thepatient’s skin – so it is known as the ‘surface’ ECG.

The wave of electrical depolarisation spreads from the atriadown though the IVS to the ventricles. So the direction of this depolarisationis usually from the superior to the inferior aspect of the heart. The directionof the wave of depolarisation is normally towards the left due to the leftwardorientation of the heart in the chest and the greater muscle mass of the leftventricle than the right. This overall direction of travel of the electricaldepolarisation through the heart is known as the electrical axis.

A fundamental principle of ECG recording is that when thewave of depolarisation travels toward a recording lead this results in apositive or upward deflection. When it travels away from a recording lead thisresults in a negative or downward deflection.

The electrical axis is normally downward and to the left butwe can estimate it more accurately in individual patients if we understand fromwhich ‘direction’ each recording lead measures the ECG.

Figure 2. Orientation ofthe limb leads showing the direction from which each lead 'looks' at the heart

By convention, we record the standard surface ECG using 12different recording lead ‘directions,’ though rather confusingly only 10recording electrodes on the skin are required to achieve this. Six of these arerecorded from the chest overlying the heart – the chest or precordial leads. Four are recorded from the limbs – the limb leads. It is essential thateach of the 10 recording electrodes is placed in its correct position,otherwise the appearance of the ECG will be changed significantly, preventingcorrect interpretation.

The limb leads record the ECG in the coronal plane, and socan be used to determine the electrical axis (which is usually measured only inthe coronal plane). The limb leads are called leads I, II, III, AVR, AVL andAVF. Figure 2 shows the relative directions from which they ‘look’ at theheart. A horizontal line through the heart and directed to the left (exactly inthe direction of lead I) is conventionally labelled as the reference point of 0degrees (0 ^o). The directions from which other leads ‘look’ at theheart are described in terms of the angle in degrees from this baseline.

The electrical axis of depolarisation is also expressed indegrees and is normally in the range from -30 ⁰ to + 90 ⁰.A detailed explanation of how to determine the axis is beyond the scope of thisarticle but the principles mentioned here should help readers to understand theconcepts involved.

The chest leads record the ECG in the transverse orhorizontal plane, and are called V1, V2, V3, V4, V5 and V6 (see Figure 3).

Figure 3. Transversesection of the chest showing the orientation of the six chest leads in relationto the heart

Voltage and timing intervals

It is conventional to record the ECG using standard measuresfor amplitude of the electrical signal and for the speed at which the papermoves during the recording. This allows:

Easyappreciation of heart rates and cardiac intervals and
Meaningfulcomparison to be made between ECGs recorded on different occasions or bydifferent ECG machines.

The amplitude, or voltage, of the recorded electrical signalis expressed on an ECG in the vertical dimension and is measured in millivolts(mV). On standard ECG paper 1mV is represented by a deflection of 10 mm. Anincrease in the amount of muscle mass, such as with left ventricular hypertrophy(LVH), usually results in a larger electrical depolarisation signal, and so alarger amplitude of vertical deflection on the ECG.

An essential feature of the ECG is that the electricalactivity of the heart is shown as it varies with time. In other words we canthink of the ECG as a graph, plotting electrical activity on the vertical axisagainst time on the horizontal axis. Standard ECG paper moves at 25 mm persecond during real-time recording. Thismeans that when looking at the printed ECG a distance of 25 mm along thehorizontal axis represents 1 second in time.

ECG paper is marked with a grid of small and large squares.Each small square represents 40 milliseconds (ms) in time along the horizontalaxis and each larger square contains 5 small squares, thus representing 200 ms.Standard paper speeds and square markings allow easy measurement of cardiactiming intervals. This enablescalculation of heart rates and identification of abnormal electrical conductionwithin the heart (see Figure 4).

Figure 4. Sample ofstandard ECG paper showing the scale of voltage, measured on the vertical axis,against time on the horizontal axis

The normal ECG

It will be clear from above that the first structure to bedepolarised during normal sinus rhythm is the right atrium, closely followed bythe left atrium. So the first electrical signal on a normal ECG originates fromthe atria and is known as the P wave.Although there is usually only one P wave in most leads of an ECG, the P waveis in fact the sum of the electrical signals from the two atria, which areusually superimposed.

Thereis then a short, physiological delay as the atrioventricular (AV) node slowsthe electrical depolarisation before it proceeds to the ventricles. This delayis responsible for the PR interval, a short period where no electrical activityis seen on the ECG, represented by a straight horizontal or ‘isoelectric’ line.

Depolarisation of the ventricles results in usually thelargest part of the ECG signal (because of the greater muscle mass in theventricles) and this is known as the QRScomplex.

TheQ wave is the first initial downward or ‘negative’ deflection
TheR wave is then the next upward deflection (provided it crosses the isoelectricline and becomes ‘positive’)
TheS wave is then the next deflection downwards, provided it crosses theisoelectric line to become briefly negative before returning to the isoelectricbaseline.

In the case of the ventricles, there is also an electricalsignal 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 inmost leads is an upright deflection of variable amplitude and duration (seeFigures 5 and 6).

Figure 5. The majorwaves of a single normal ECG pattern

Figure 6. Example of anormal 12 lead ECG; notice the downward deflection of all signals recorded fromlead aVR. This is normal, as the electrical axis is directly away from thatlead

Normal intervals

The recording of an ECG on standard paper allows the timetaken for the various phases of electrical depolarisation to be measured,usually in milliseconds. There is a recognised normal range for such‘intervals’:

PR interval (measured from thebeginning of the P wave to the first deflection of the QRS complex). Normalrange 120 – 200 ms (3 – 5 small squares on ECG paper).
QRS duration (measured from firstdeflection of QRS complex to end of QRS complex at isoelectric line). Normalrange up to 120 ms (3 small squares on ECG paper).
QT interval (measured from firstdeflection of QRS complex to end of T wave at isoelectric line). Normal rangeup to 440 ms (though varies with heart rate and may be slightly longer infemales)

Heart rate estimation from the ECG

Standard ECG paper allows an approximate estimation of theheart rate (HR) from an ECG recording. Each second of time is represented by250 mm (5 large squares) along the horizontal axis. So if the number of largesquares 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.