12-lead ECGs are usually displayed on special gridded paper (Fig. 2.1). The 12 leads I, II, III, aVR, aVL, aVF and V₁–V₆ are displayed on the paper under their respective headings. The limb leads are found on the left hand side and the chest leads V/C 1–6 on the right. The leads are separated by lead divider markers which resemble an elongated punctuation colon (:). The gridded area of the paper is split into larger boxes with smaller boxes inside them. Each large box measures 5 mm² (containing 25 smaller 1 mm² boxes). Time is measured along the x-axis (horizontal) in seconds. Each large box represents 0.2 s of time, with each smaller box measuring 0.04 s. Each lead represents around 3 s of time. Most 12-lead ECGs also include a rhythm strip, shown below the other leads. This strip is one of the existing leads displayed above, shown for around 12 s, usually lead II or V₁. This allows interpreters to look for patterns and features that might not otherwise be visible in shorter time periods. Some ECGs have more than one rhythm strip included. The y-axis (vertical) represents the amplitude of the ECG waveforms, measured in millivolts. One large square represents 0.5 mV, with a small square measuring 0.1 mV.

There are several details that should be checked prior to analysing the waveforms and formulating an interpretation. These checks include:

Is any artificial disturbance that negatively impacts on the quality of the ECG. There are many forms of interference that can affect the quality. There are however several types of commonly encountered interference that can be easily recognised and prevented or reduced in most cases. Table 2.1 shows the most common forms of interference, including possible causes and solutions.

Standard ECGs in the UK, USA and many other parts of the world are recorded at a speed of 25 mm/s and a voltage of 10 mm/mV. This is usually displayed on the ECG somewhere (often at the bottom). This information is also displayed graphically in the form of calibration markers, sometimes called calibration signal boxes. These markers look like rectangles (Fig. 2.2) and are usually seen on the left hand side of the ECG preceding the leads.

When the ECG is set up to record at the standard 25 mm/s and 10 mm/mV, the calibration markers should measure 1 cm in height (2 large boxes) by 0.5 cm in width (1 large box). The authors recommend that practitioners recording and interpreting ECGs always check that the ECG was recorded in the standard calibration before attempting interpretation. Speed and amplitude settings can be altered on most ECG machines, it is therefore possible that someone may either deliberately or accidentally alter these settings. If the recording speed was altered to 50 mm/s, it would have the effect of elongating the waveforms, and can make it appear that the patient has an extremely low heart rate and a long QT interval, even though their other observations could otherwise be normal. Sometimes altering these settings is done deliberately. A patient may have a very rapid heart rate making it difficult to see P waves. This can sometimes be overcome by changing the recording speed and elongating the waveforms to see features that would otherwise be missed. Figure 2.3 shows a standard calibration marker and one that is set to 50 mm/s. As shown in the image the second marker is twice as wide as normal, encompassing two large boxes.

Similar changes can be made to the amplitude of the ECG. Certain conditions, such as left ventricular hypertrophy can cause the height/depth of the waveforms to be very large and they may overlap each other, making the ECG difficult to view. In these cases some practitioners may request the ECG is recorded at half normal voltage. Conversely if a normal ECG is viewed at half voltage the waveforms can appear very small, which can lead to a false interpretation, as it can display false features of conditions like pericardial effusion, which features small voltage deflections. Figure 2.4 shows a calibration marker at half voltage.

Some machines will also allow the voltage to be reduced just for the limb or chest leads, leaving the other leads at normal voltage. This is represented by a calibration marker with a step in it. If the step is on the left it represents a reduction in the limb lead voltage only, whereas a step on the right hand side represents a reduction in the voltage of the chest leads only (Fig. 2.5).

Finally the voltage can be decreased and the recording speed changed simultaneously. This is represented by a small and wide calibration marker (5 mm in height and 10 mm in width). Practitioners should be familiar with both normal and adjusted calibration markers, what they represent and what impact changing these settings may have on the interpretation of the ECG. Even though the calibration markers will reflect any changes it is still good practice to document on the ECG any changes to the normal settings made when recording to draw other practitioners attention to these changes explicitly.

R wave progression refers to the deflection changes that occur in the chest leads (V₁–V₆) as they move from a predominantly negative to a predominantly positive defection (Fig. 2.6.).

If there are any sudden changes in deflection it is possible that one or more of the chest leads have been placed in the wrong position.

Incorrect lead placement in the limb leads can be identified by a positively deflected lead aVR, especially if the patients previous ECG(s) have a negatively deflected aVR. Lead aVR is nearly always negatively deflected. If it is positive the limb leads may have been applied the wrong way around. This can mimic a condition called dextrocardia, where heart is situated on the right hand side of the chest as oppose to the left. With dextrocardia there is often right axis deviation, positively deflected complexes in lead aVR, negatively deflected complexes in lead I and an absence of R wave progression in the precordial leads. The principle difference between true and technical dextrocardia is that in the later there are no changes seen in the precordial leads.

Basic quality control checks should be carried out by the recorder of the ECG whilst the leads are still in situ. This makes it easier to identify issues such as incorrect lead placement, identification of artifact and any issues with recording settings. Not only is it easier to correct problems at this stage before interpretation is attempted but it also reduces the potential for subsequent interpretation errors.

The normal heart rhythm is very rarely exactly regular. The human heart rate has a fractal quality. A fractal is a mathematical term to describe patterns that look similar at progressively smaller scales. Other examples from nature include snowflakes and fern leaves. If you were to zoom in on a snowflake again and again the pattern you would see would resemble the larger pattern. An example of this self similarity can be seen in Fig. 2.7. The Sierpinski’s triangle or gasket as it is sometimes referred is a basic example of self similar sets.

The heart rate itself has a fractal variance making it ever so slightly variable. This is extremely important because if this was not the case the stress on the heart muscle each beat would occur at exactly the same point. This slight variance allows wear and tear of the heart to be dramatically reduced.

For pragmatic clinical purposes however rhythms tend to be defined as either regular or irregular. There are a couple of practical ways rhythms can be checked for irregularity. One of the easiest is to take a piece of paper and place it just under the tip of the R waves on the rhythm strip. Next mark a line directly under two consecutive R waves on the piece of paper. Now move the paper along the rhythm strip and check that the two lines on the paper line up with the preceding R waves (Fig. 2.8). Some practitioners do this with a set of calipers (like a compass for drawing circles but with two needles). The calipers are set to mark the distance between two consecutive R waves then swung between the preceding ones (Fig. 2.8).

When checking the rhythm practitioners should consider the following; is the rhythm regular or irregular? if irregular is there a pattern to the irregularity or not? and are PQRST waves present for each beat?

Is a normal variation that produces an irregularity in the heart rate. This is caused by the heart rate increasing as the individual breaths in and slowing down when they breath out. Sinus arrhythmia does not cause any symptoms and PQRST waves are all present. Sinus arrhythmia is discussed in more detail in Chap. 6.

The normal adult heart rate is between 60 and 100 BPM. When lying still in bed the normal rate is between 60 and 80 BPM. Normal childrens heart rates can be much higher than adults, for more information on children see Chap. 9, which discusses the pediatric ECG in more detail. With adults anything below 60 BPM is classified as a bradycardia. Anything above 100 BPM is defined as a tachycardia (Table 2.2). Although the ECG can be used to determine the heart rate, a lot of useful information can also be determined from manual palpation of the patients pulse. To feel a pulse slight compression should be made to an artery located against a bone. The common pulse points are shown in Fig. 2.9. Manual inspection of the pulse should not be underestimated. For example; pulseless electrical activity (PEA) is an arrest rhythm where electrical activity is present on the ECG, because the conduction system is working normally, however there is no effective mechanical pumping action taking place and no cardiac output.

There are several different methods for determining the heart rate on the ECG. These calculations vary in accuracy and complexity. Some of the methods only work when the heart rate is regular. Several of these methods are described here in the hope that the reader will select the one(s) that are of the most use to their individual circumstances, and that they will develop an awareness of the existence of other methods. The same ECG is used to demonstrate the different methods.