Chapter 14: Observations
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Evidence‐based approaches
12‐lead ECG
A 12‐lead ECG (see Figure 14.11) provides 12 views (also termed ‘leads’) of the electrical current of the heart. It consists of the following:
- Three bipolar leads (I, II and III) are also called ‘standard limb leads’ because they are obtained from the electrodes placed on the right arm, left arm and left foot. They measure the electric potential between a positive and a negative electrode and create a triangle around the heart called the Einthoven triangle (Figure 14.14) (Wesley [209]).
- Three augmented unipolar leads (aVR, aVL and aVF) and six unipolar chest leads or precordial leads (V1–V6). Contrary to the bipolar leads, the unipolar leads have only one positive electrode (Wesley [209]).
- One neutral lead is connected to an electrode placed on the right leg; this reduces interference (Aehlert [3]).
Figure 14.14 Einthoven triangle.
Electrode placement
Table 14.3 describes the anatomical locations of each chest electrode and Figure 14.15 shows the standard positions of the six chest electrodes. The limb electrodes should be placed just proximally to the wrist and ankle bones unless there is a clinical reason, such as amputation, burns or surgical wounds, to do otherwise; ECGs recorded using any other limb position must be clearly labelled as such to account for any changes that might affect interpretation (SCST [183]). It does not matter if the electrode is positioned on the inside or outside of the limb – position the patient comfortably and use the most accessible aspect for electrode placement (Blows [21]).
Table 14.3 Three‐ and five‐lead ECG positioning
| Electrode | Position |
|---|---|
| Three‐lead placement | |
| Right arm limb lead (RA, red) | Right clavicle proximal to right shoulder |
| Left arm limb lead (LA, yellow) | Left clavicle proximal to left shoulder |
| Left leg limb lead (LL, green) | Lower edge of left ribcage, below pectoral muscles |
| Five‐lead placement | |
| Right arm limb lead (RA, red) | Right clavicle proximal to right shoulder |
| Left arm limb lead (LA, yellow) | Left clavicle proximal to left shoulder |
| Left leg limb lead (LL, green) | Lower edge of left ribcage, below pectoral muscles |
| V (white electrode) | Fourth intercostal space at the right sternal edge |
| RL (black electrode) | Lower edge of the right ribcage, on a non‐muscular area. |
Figure 14.15 Position of chest electrodes for a 12‐lead electrocardiogram (ECG). Source: SCST ([183]) – The Society for Cardiological Science and Technology (www.scst.org.uk).
The 12 leads (or viewpoints) look at the heart from different directions. Each lead records a positive or negative wave depending on which direction the impulse is travelling in relation to the observing lead; a positive wave will be recorded if the impulse is travelling towards the observing lead, whereas a negative wave will be recorded if the impulse is travelling away from the observing lead (SCST [183]). Correct electrode placement is essential to ensure an accurate ECG recording is obtained. Incorrect electrode placement can lead to morphology changes or alter the amplitude of waves, meaning that ECG changes may be caused by artefacts rather than physiological abnormalities, which may lead to misdiagnosis (Aehlert [3], Wesley [209]).
An alternative positioning of the limb electrodes on the torso, known as the Mason–Likar 12‐lead ECG system (Figure 14.16), can be used when continuous 12‐lead ECG monitoring is required, as it enables the waveforms to be easily viewed without interference from limb movement (Khan [93]). However, it should be noted that the QRS complexes are slightly different in amplitude and axis when repositioned on the torso in the Mason–Likar position and so any deviation from the standard 12‐lead electrode placement should be clearly documented on the ECG trace (SCST [183], Wesley [209]).
Figure 14.16 Mason–Likar 12‐lead electrocardiogram (ECG) system.
Three‐lead and five‐lead ECGs
The three‐lead ECG is mostly used in acute clinical areas where continuous cardiac monitoring is required (Adam et al. [2]). The three limb electrodes can be placed according to Figure 14.12 and Table 14.3. The monitor should be set to display lead II as this runs from the right arm to the left foot and so is normally positive, showing the greatest deflection of all the limb leads in a normal heart (Marini and Dries [112]).
A five‐lead ECG offers the features of a three‐lead ECG with the addition of two extra leads, offering more detailed views of the heart.
The 12‐lead ECG is, however, the gold standard for diagnostic purposes and is described in Procedure guideline 14.2: Electrocardiogram (ECG).
Rationale
Indications
A 12‐lead ECG may be performed electively or to aid diagnosis following any acute deterioration or after any cardiac event. For example, it may be performed in the following circumstances:
- to provide a baseline prior to surgery or a course of medical treatment
- sudden onset of chest pain
- shortness of breath
- haemodynamic disturbance
- cardiac rhythm or rate changes
- suspected acute coronary syndrome
- suspected or confirmed myocardial infarction
- cardiac surgery
- percutaneous coronary intervention
- after successful cardiopulmonary resuscitation (RCUK [173]).
Serial 12‐lead ECGs may be required in patients known to have cardiac toxicity, developing myocardial ischaemia or infarction (Bunce and Ray [33]).
Ambulatory ECG monitoring, also known as a ‘24‐hour tape’, may be used to record and analyse a patient's heart rhythm during normal daily activities (Wesley [209]). Usually applied and interpreted by specialist cardiac services, it is useful to capture abnormalities that might be missed with a standard 12‐lead ECG. It is typically recorded continuously over a period of 24–48 hours (Aehlert [3]).
Contraindications
There are no absolute contraindications to performing a 12‐lead ECG, but obtaining the ECG should not compromise or delay immediate care – for example, if a patient presents with an arrhythmia requiring immediate shock or in cardiac arrest (RCUK [173]).
