ECG Basics

An ECG wave is a deflection that is caused by a change in voltage, measured by an electrode on the skin.

Just like ocean waves, ECG waves come in many different shapes and sizes: sometimes they look like little bumps or ripples, but other times they are big, sharp and spiky.

ECG waves can be triggered by an electrical event called depolarisation, which is the first event of the cardiac action potential. Heart muscle cells are all electrically linked by intercalated discs, which allows action potentials to travel from one cell to another.

After depolarisation, the process of resetting the cells to their resting state is called repolarisation. This can also generate a wave on the ECG.

Because the ECG is recorded from surface electrodes on the skin, ECG waves can also be caused by artefacts, for example interference from electrical devices, movement, or poor contact with the skin.

Let's start by describing ECG waves as positive, negative or biphasic.

Positive waves are above the baseline of the ECG. Positive ECG waves occur when depolarisation spreads towards a positive electrode.

Negative waves are below the baseline (upside down). Negative waves occur when depolarisation spreads towards a negative electrode.

If a wave is both positive and negative, it can be called biphasic.

ECG waves are named using letters of the alphabet.

The first wave of each beat is called the P wave. The P wave is usually small and hard to see.

After the P wave there is a tight group of waves called the QRS complex. The first wave of the QRS complex is named a Q wave, if it is negative. Q waves are always negative. A positive wave in the QRS complex is called an R wave. It is often tall and sharp. If there is a negative wave after an R wave, it is called an S wave. S waves are always negative.

The T wave occurs after the QRS complex.

An ECG complex is a tight group of waves.

The QRS complex is the main ECG complex. It is named by the Q, R and/or S waves that make it up (they are not always all present). For example, if there is no S wave the complex can be named a QR complex. Or, if there is no R wave it can be named a QS complex.

Just like waves, QRS complexes can also be described as positive or negative. This will be useful for working out features like the cardiac axis later on.

Positive complexes are overall taller above the baseline than below.

Negative complexes are overall more below the baseline than above.

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.

For example:

The PR segment is the gap between the end of the P wave and the start of the QRS complex.

The ST segment is the gap between the end of the QRS complex and the start of the T wave.

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 QT interval includes everything from the start of the QRS complex to the end of the T wave.

The RR interval includes everything from one R wave up to the next R wave.

The cardiac conduction system shows us how depolarisation spreads through the heart.

1. The Sinoatrial (SA) Node: a cluster of specialised cardiac cells (not nerves) that spontaneously depolarise at a rate between 60-100 beats per minute. It is found near the entrance of the superior vena cava in the right atrium. It is also known as the Sinus Node.
2. Internodal tracts: also known as interatrial pathways, there are multiple routes that the wave of depolarisation can travel from the SA node to the AV node.
3. The Atrioventricular (AV) Node: a cluster of cells found at the interatrial septum. Because the atria are electrically isolated from the ventricles, the AV node is the only bridge between these two regions of the heart. The AV node spontaneously depolarises at a rate of 40-60 beats per minute.
4. The Bundle of His: the main conduction highway from the AV node down through the ventricles. It is about 2cm long in adults.
5. Bundle Branches: the Bundle of His divides into left and right branches.
6. Fascicles: the Left Bundle Branch is so large (because the left ventricle is so large) that it further divides into Anterior and Posterior Fascicles.
7. Purkinje fibres: these specialised fibres spread conduction through the heart muscle.

Each wave, segment and interval represents different events of the cardiac conduction cycle. In the normal heart, each beat starts at the SA node. This node is electrocardiographically silent: it is so small that it doesn't make an ECG wave by itself, but when the impulse leaves the SA node and travels through the atria it makes the P wave appear. In this way the P wave represents atrial depolarisation.

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 total time taken within the atria from the SA node to leaving the AV node is the PR interval.

The QRS complex represents ventricular depolarisation. Once the impulse has left the AV node it enters the ventricles via the 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.

After the ventricles are depolarised they will contract, then repolarise (reset) ready for the next beat. On the ECG this normally produces a flat ST segment then a T wave. The total ventricular time is the QT interval.

ECG interpretation can be easier if you understand the principles behind common variations in waves, segments and intervals. Here are some examples:

Short depolarisation waves can suggest less cardiac muscle, or a barrier between the heart and the surface ECG electrodes (e.g. pericardial effusion).

Narrow waves suggest fast (normal) conduction. This is especially important in the ventricles where narrow QRS complexes suggest that the rhythm originates somewhere above the ventricles (supraventricular) because the heart is using the normal conduction highways to travel down 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 because the rhythm starts in the ventricles (ventricular arrhythmia) or it might have started normally but encountered a conduction block along the way (e.g. bundle branch 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 conduction delays.

Did you know that people often use the term lead to mean two different things?

First, there's the 12 leads of the ECG, which are like different channels or views of the heart. Each channel records electrical activity in a single direction. Having 12 different views lets us know more about how electricity moves through the heart in 3 dimensions. Common lead names include I, II, III, aVR, aVL, aVF, V1, V2, V3, V4, V5 and V6.

Then there's the 10 electrodes that are placed on the skin to record the ECG. These are also called leads by some people. Each trace is formed by using two or more electrodes. Common electrode names include LA, RA, LL, RL, V1, V2, V3, V4, V5 and V6.

To prevent ongoing confusion, this page will just use the term lead as a channel of ECG recording. This means that a standard 12-lead ECG is recorded using 10 electrodes.

ECG electrodes must be placed on the skin in very specific locations.

The first 4 electrodes are placed on the limbs. They are named after each limb, including Left Arm (LA), Right Arm (RA), Left Leg (LL) and Right Leg (RL).

There are 6 other electrodes that are placed on the anterior chest wall (also known as the precordium). They are named V1-V6.

• V1 is placed in the 4th intercostal space at the right sternal border.
• V2 is placed at the same level on the left sternal border.
• V4 is then placed in the 5th intercostal space at the mid-clavicular line.
• V3 is half way between V2 and V4.
• V5 is on the same level as V4 but at the anterior axillary line.
• V6 is also at this level but in the mid-axillary line.

The V4 electrode is often placed before V3, because it helps to locate the site for V3.

The limb electrodes are used to record 6 limb leads on a 12-lead ECG. These limb leads are named I, II, III, aVR, aVL and aVF. They each view the heart from a different direction in a frontal plane.

• Lead I looks at the heart horizontally from right to left. It compares the RA (-) electrode to LA (+).
• Lead II looks at the heart diagonally from the right shoulder to left foot. It compares the RA (-) electrode to LL (+).
• Lead III looks at the heart diagonally from the left shoulder to left foot. It compares the LA (-) electrode to LL (+).

The next 3 leads are called augmented leads. They are calculated from leads I, II and III with a mathematical transformation to make them large enough to see clearly.

• Lead aVR looks from the center of the heart up to the right shoulder.
• Lead aVL looks from the center of the heart up to the left shoulder.
• Lead aVF looks from the center of the heart down to the feet.

The first 3 limb leads (I, II and III) are also known as bipolar leads because they measure voltage directly between two electrodes. The augmented leads are unipolar because they use a single positive electrode and their negative reference is a combination of the other electrodes.

Chest leads look at the heart in approximately a horizontal plane. They are unipolar leads that use the average of all the limb electrodes as their negative (-) electrode. Each chest lead records electrical activity from the centre of the heart out towards one positive electrode.

• Lead V1 looks from the center of the heart to the V1 electrode, which is good for seeing right ventricular or septal problems.
• Lead V2 looks from the center of the heart to the V2 electrode, which is also good for right ventricular or septal problems.
• Lead V3 looks from the center of the heart to the V3 electrode, which is good for seeing anterior left ventricular problems.
• Lead V4 looks from the center of the heart to the V4 electrode, which is also good for anterior left ventricular problems.
• Lead V5 looks from the center of the heart to the V5 electrode, which is good for lateral left ventricular problems.
• Lead V6 looks from the center of the heart to the V6 electrode, which is also good for lateral left ventricular problems.

When you are trying to localise an infarct, it is sometimes useful to record extra leads by moving the electrodes to the right or posterior surface of the heart.

Right sided leads can be used to identify a right ventricular infarction. They are recorded by swapping one or more of the chest electrodes to the same position on the right side of the chest. The most common right sided lead is V4R.

Posterior leads can be used to identify a posterior ventricular infarction, by moving chest electrodes into new positions in the same horizontal plane as V6. These leads include V7, V8 and V9.

The rate measures the speed of the heart beats. It can be described as normal, tachycardia or bradycardia.

In adults, a normal heart rate is usually 60-100 beats per minute. At this rate and using standard ECG recording settings, there should be 3-5 large grid squares between each beat.

Tachycardia is a rate that is faster than normal. In adults, a fast heart rate is usually over 100 beats per minute. At this rate there should be fewer than 3 large grid squares between each beat.

Bradycardia is a rate that is slower than normal. In adults, a slow heart rate is usually under 60 beats per minute, which should be more than 5 large grid squares between each beat.

ECGs are normally printed on two different sized grid squares. They are recorded using a standard calibration to make comparison easier. These standard settings include a 'paper speed' of 25 mm/sec and 10 mm/mV. This means that the grids they are printed on will represent a consistent time and voltage .

• The large grid squares represent 0.2 seconds each, so there will be 5 large squares per second of recording or 300 large squares per minute.
• The small grid squares represent 0.04 seconds (or 40 milliseconds) each, so there will be 25 small squares per second of recording.

One of the fastest methods of calculating the rate is to count the number of large grid squares between two beats. If the ECG beats are regularly spaced apart and it was recorded at standard settings, you can find the rate by dividing 300 by this number.

Be careful: this method only calculates an approximate beat-to-beat rate. It can be misleading if the beats you choose are not representative of the overall rhythm, or if the beats are not regularly spaced apart.

The rhythm describes the origin of each beat, in terms of whether it starts in the SA node (sinus), atria, AV node (junction) or ventricles.

Any rhythm can be described by 3 basic features: rate, regularity and QRS width.

The cardiac axis describes the overall direction of ventricular depolarsiation. Think of it like painting a single arrow on the front of the chest, or a compass needle pointing in one overall direction. It can be described as normal, left, right or extreme axis deviation.

The hexaxial references system is a model that can help to calculate the axis. It includes the direction of each of the 6 limb leads (hex = 6) in the frontal plane, measured in degrees clockwise from a horizontal line to the left.

• Lead I looks at the heart horizontally (0°)
• Lead II looks diagonally down to the left foot (+60°)
• Lead III is diagonal down to the right foot (+120°)
• Lead aVR looks up towards the right shoulder (-150°)
• Lead aVL looks up towards the left shoulder(-30°)
• Lead aVF looks vertically down to the feet (+90°)

There is a range of degrees for each axis variation.

• Normal axis: -30 to +90°.
• Right axis deviation: +90 to +180°.
• Left axis deviation: -30 to -90°.
• Extreme axis deviation: -90 to -180°.

Once we know the directions of each limb lead, we can deduce the axis by using two simple rules:

• The axis is somewhere towards a positive lead.
• The axis is somewhere away from a negative lead.

The quadrants method uses lead I and aVF to calculate the axis because these two leads make convenient horizontal and vertical axes. However, because a normal axis range is slightly larger than one quadrant, we sometimes also need to look at lead II.

To use the quadrants method:

1. Draw your axes, with lead I horizontal to the left and aVF vertical down to the feet.
2. Check lead I:
   • If it is positive, the axis is leftwards (towards I)
   • If it is negative, the axis is rightwards (away from I)
   Draw this on the chart by crossing out whichever half you know the axis is NOT in. Remember you are looking at the patient so your left is their right side.
3. Check lead aVF:
   • If it is positive, the axis is downwards (towards aVF)
   • If it is negative, the axis is upwards (away from aVF)
   Cross out whichever half you know the axis is NOT in.
4. You should have only one quadrant remaining. If it is normal, right or extreme axis deviation you are finished. If it is somewhere in the left upper quadrant you will need to look at lead II to decide if it is normal or left axis deviation:
   • If II is positive, the axis is somewhere towards lead II (+60), which makes it normal overall.
   • If it is negative, the axis is somewhere away from lead II, which makes it left axis deviation overall.

The isoelectric method is a quick method of finding the axis if you can find an isoelectric lead. Look for a limb lead with QRS complexes that are equally positive and negative (isoelectric). The axis will be found perpendicular to the isoelectric lead in either a clockwise or anti-clockwise direction.

To use the isoelectric method:

1. Draw all 6 limb lead axes on a chart
2. Find the isoelectric lead. The axis will be 90 degrees from this lead in either a clockwise or anti-clockwise direction.
3. Use your chart to find the perpendicular lead
4. Check the perpendicular lead:
   • If it is positive, the axis is directly towards this lead
   • If it is negative, the axis is directly opposite this lead

ECG quality problems are so common that a quick quality check is essential for each ECG interpretation. This should include checking the patient's identity, ensuring the calibration is standard, looking for baseline artefacts and electrode misplacements.

ECGs can only be compared easily if they are recorded at the same settings. The most common standard settings include a paper speed of 25 mm/sec and 10 mm/mV. These settings may be printed on the edge of the page, or there may be a calibration signal at the start of the trace that is a rectangle 5mm wide and 10mm tall (or some ECG machines record a very narrow calibration signal but write "25 mm/sec" separately).

Nonstandard calibration settings include double speed (50 mm/sec), where the trace will appear to be twice as spread out as normal.

Half height (5 mm/mV) is another setting that can make it easier to fit all the complexes on the page if they are very tall, but will not change the overall rate because the paper speed stays the same.

Artefacts are caused by anything other than the heart's electrical activity. They include movement artefacts (e.g. muscles shivering, tremors, or tapping the electrode on purpose), other electrical devices (e.g. nerve stimulators, electrical beds) or electrodes being in poor contact with the skin.

One of the most important artefacts to be aware of is a chest compression artefact. It looks a bit like a very wide complex rhythm but it actually completely obscures the true underlying rhythm. This artefact prevents rhythm analysis while CPR is in progress.

One important clue to whether an abnormal ECG rhythm change is due to an artefact comes from looking across all of the leads. If the appearance is only in some of the leads, it is more likely to be an artefact as a true rhythm change should affect all simultaneously-recorded leads together.

Unfortunately most automated ECG interpretation algorithms are frequently inaccurate. They are best treated with extreme caution, or disregarded. There are too many errors to count, but common ones include:

• Rate errors: counting tall T waves as extra QRS complexes, or not counting ectopic (extra) beats
• Rhythm errors: attributing many different rhythms to Atrial Fibrillation (not all that is irregular is AF), missing pacemaker spikes, mistaking artefacts for VT / VF, mistaking electrodes falling off for asystole.
• Interval errors: QT measurement can be inaccurate, especially with baseline artefact or U waves
• Baseline errors: any interpretation becomes dubious in the presence of baseline artefact
• Ischemia errors: automated interpretation will frequently miss signs of myocardial infarction and ischemia, or mislabel secondary ST or T wave changes as an infarct.

In general, the computer interpretation is better at calculating rate, intervals and axis, but worse at rhythm and ischemia interpretation. It also does not correlate the trace to the clinical context.