Chapter 14: Observations
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Definition
Muscle contraction is associated with electrical activity in the heart called depolarization, which can be monitored, amplified and recorded with a simple test called an electrocardiogram (ECG) (Marieb and Hoehn [110]). An ECG provides a graphical representation of the myocardium's electrical conduction and excitation (Marieb and Hoehn [110]), and illustrates a three‐dimensional event in a two‐dimensional graph recorded over a period of time (Blows [21]). It is a non‐invasive, routine clinical examination that can be performed by a range of healthcare professionals by applying electrodes on the surface of the patient's body (Peate and Wild [157]).
When used correctly and in the context of the patient's clinical history, it is a valuable tool that can be used to ascertain information about the electrophysiology of the heart and provide guidance relevant to diagnosis, prognosis and treatment (Marieb and Hoehn [110], Marini and Dries [112]).
Types of ECG monitoring
12‐lead ECG
A 12‐lead ECG provides 12 different views of the heart from 10 electrodes placed on the patient's chest and limbs (Aehlert [3], Wesley [209]) (Figure 14.11). Electrical changes take place as the cardiac muscle depolarizes and repolarizes, and the 12 views show the three‐dimensional electrical activity occurring in the heart (Aehlert [3]). A 12‐lead ECG provides a snapshot of myocardial activity and is used for diagnostic purposes (Wesley [209]).
Figure 14.11 A 12‐lead electrocardiogram (ECG). Source: Reproduced from https://meds.queensu.ca/central/assets/modules/ECG/the_12_lead_ecg.html.
Three‐ and five‐lead ECG
Three‐lead ECG monitoring is most commonly used for cardiac rhythm assessment and continuous cardiac monitoring (Coviello [48]). Three leads are placed on the torso: one on each shoulder and one on the lower left chest (Figure 14.12). A five‐lead ECG is used for similar purposes but has the addition of an extra two leads/views, which gives a more detailed picture of the activity in the heart; it is commonly used in critical care settings (Bunce and Ray [33], Wesley [209]).
Figure 14.12 Three‐lead electrocardiogram (ECG) electrode placement.
