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Recording a 12lead ECG requires the connection of two groups of electrodes to the patient.

Limb leads (I, II, III, aVR, aVL, aVF).

Chest or precordial leads (generally V1 to V6) (Wilson et al. 1944, Noble et al. 2010).

The 12lead ECG is the gold standard for diagnostic purposes and the standard electrode placement is shown in Figure 12.7 and described in Procedure guideline 12.2. However, it should be noted that an alternative positioning of the limb leads on the torso, known as the MasonLikar 12lead system, can also be used. The MasonLikar arrangement is more suitable in acute areas where continuous 12lead ECG monitoring is required as the waveforms are easily viewed without interference from limb movement (Pelter 2008).

Evidencebased approaches

Rationale

An ECG is performed electively prior to various interventions such as surgery and anticancer chemotherapy. An ECG should also be considered to aid diagnosis during an acute situation, particularly in the presence of chest pain, haemodynamic disturbance or cardiac rhythm changes. Twentyfour hour ambulatory ECG monitoring, also known as 24hour tape, may be used to record and analyse the patient’s heart rhythm. This is useful to capture abnormalities that might be missed with a standard 12lead ECG and is a specialist test. If required, up to 72 hours of ambulatory ECG monitoring may be recorded.

Legal and professional issues

Competencies

Any healthcare professional who has been trained and assessed as competent can take a 12lead ECG in line with local hospital policy. However, its analysis and diagnosis are usually undertaken by medical staff or specialist nurses (Sharman 2007).

Preprocedural considerations

Equipment

12lead ECG machine: consisting of 12 wires which are connected to electrodes fixed to the body. The machine detects and amplifies the electrical impulses that occur at each heart beat and records them on to a paper or computer.

ECG paper (25 mm/s style): waveforms produced by the heart’s electrical current are recorded on graphed ECG paper by a stylus. ECG paper consists of horizontal and vertical lines forming a grid. A piece of ECG paper is called an ECG strip or tracing. The horizontal axis of the strip represents time. Each small block equals 0.04 second, and five small blocks form a large block, which equals 0.2 second. Five large blocks equal 1 second (5 × 0.2). The ECG strip’s vertical axis measures amplitude in millimetres (mm) or electrical volts in millivolts (mV) (Levine et al. 2008) (see Figure 12.6).

Filter selection: modern ECG monitors offer multiple filters for signal processing. The most common settings are monitor mode and diagnostic mode. In monitor mode, the lowfrequency filter is set at either 0.5 Hz or 1 Hz and the highfrequency filter is set at 40 Hz. This limits artefact for routine cardiac rhythm monitoring. The highpass filter helps reduce wandering baseline and the lowpass filter helps reduce 50 or 60 Hz power line noise. In diagnostic mode, the highpass filter is set at 0.05 Hz, which allows accurate ST segments to be recorded. The lowpass filter is set to 40, 100 or 150 Hz. Consequently, the monitor mode ECG display is more filtered than diagnostic mode, because its passband is narrower.

12 leads: in electrocardiography, the word lead may refer to either the electrodes attached to the patient or the voltage between two electrodes. The electrodes are attached to the patient’s body, usually with sticky circles of thick tapelike material (the electrode is embedded in the centre of this circle), onto which ECG leads clip, using different combinations of electrodes to measure various signals from the heart.

Abrasive strips: can be used to aid electrode contact by gently rubbing over the skin in the position of the electrodes. Gentle friction creates a roughened skin surface without causing cuts, which is helpful in creating a good contact between the electrode and the skin.

24–72hour ambulatory ECG recorder: a continuous recording of an ECG over a 24–72hour period, which enables a patient to continue