A 12‐lead ECG machine has 10 cables, which are connected to electrodes fixed to the body. The machine detects and amplifies the electrical impulses that occur at each heart beat and record the waveforms onto graphed paper (using a stylus) or onto a computer. All ECG machines should be tested to ensure accurate data are recorded. Calibration is usually undertaken by qualified engineers; however, this should be verified in the clinical setting to confirm that when a voltage of (usually) 1 mV over 0.2 seconds is put through the machine, the machine produces the expected deflection of 10 mm and the paper moves at the correct speed: 25 mm per second (Aehlert [3], SCST [183]). This calibration verification should be printed at the beginning or end of each line of the ECG as a box shape with a 5 mm width and a 10 mm height (Figure 14.17) (Wesley [209]).

The 10 electrodes are attached to the patient's body in the positions described above and in Procedure guideline 14.2: Electrocardiogram (ECG). The cables clip onto these electrodes and record 12 views of the heart using different combinations of electrodes to measure various signals from the heart.

ECG graph paper is divided into small squares of 1 mm each, and five small squares make up a large square. The horizontal axis of the ECG paper represents time and the vertical axis represents amplitude. Standard ECG paper speed is 25 mm per second; each small square equals 0.04 seconds and one large square equals 0.2 seconds, so five large squares equal 1 second. ECG machines are calibrated so that the deflection amplitude of cardiac conduction is measured in mm/mV; 10 small squares or two large squares show a deflection of 1 mV (10 mm/mV) (see Figure 14.17) (Aehlert [3], Wesley [209]).

Filters are electronic devices that remove artefacts from the ECG, improving the tracing obtained. ECG machines have internal pre‐programmed filters set by the manufacturer or clinical engineers, which cannot be altered by clinical staff, and also control panel filters, which can be deployed by clinical staff. However, these can distort the ECG waveform, and therefore are not recommended for routine use, but they may be required if all other actions to reduce artefacts have been unsuccessful. If a filter is used, this must be clearly documented on the ECG (SCST [183], Wesley [209]).

All ECG machines should be used and maintained in accordance with the manufacturer's instructions. In addition, as they are often used in emergency situations, they should always be available and in good working order (SCST [183]). After use, the ECG machine should be cleaned, returned to its storage location and plugged into a mains electrical supply to charge the internal battery ready for use in an emergency (SCST [183]). Any malfunctioning machine should be taken out of clinical use until repair or service can be undertaken by the relevant department, in accordance with local policy (Wesley [209]).

The conventional placement of the lateral chest leads (V4, V5 and V6) is beneath the left breast. While there is emerging evidence to support the positioning of these over the breast, it is insufficient to suggest a change of procedure (SCST [183]). It is also worth noting that the fifth intercostal space can only be found by lifting the breast and therefore it is logical to position the electrodes here (SCST [183]).

Dextrocardia is any situation where the heart is located within the right side of the chest rather than the left. This may be associated with the condition situs inversus, believed to occur in 1 out of 8000 people, where all of the patient's organs are in mirror‐image positions (Sharma et al. [186]).

The SCST ([183]) suggests the following approach to ECG recording for patients with suspected or known dextrocardia:

The patient must give their consent for the ECG to be performed, after appropriate explanation of what the procedure consists of, the rationale and the intent (NMC [144], Peate and Wild [157]). The patient's privacy and dignity should be ensured and reassurance should be provided, along with an explanation that an ECG is not a painful procedure (NMC [144], SCST [183]). The patient should be positioned comfortably either lying or sitting, preferably in a semi‐recumbent position (at an angle of 45°) with their head supported (SCST [183], Wesley [209]). It is important that the patient is relaxed and keeps still during the procedure to reduce interference and artefacts and to facilitate recording of a clear and stable ECG trace (Aehlert [3], SCST [183]).