Electrocardiogram (ECG)

After taking a history and performing a physical examination, the most commonly and rapidly performed investigation for a patient with chest pain in the ED is an ECG (see Figure 1). An ECG should be performed as soon as possible in all patients presenting with chest pain, particularly if cardiac ischaemia is suspected from the history.

Table 10 presents the likelihood ratios for the association of various ECG changes and AMI(12-14). The presence of ST segment elevation, new Q wave formation, or a new conduction deficit (eg. left bundle branch block) in the context of acute ischaemic chest pain is associated with such significantly positive likelihood ratios for AMI that the diagnosis can be made with confidence and appropriate therapy commenced.

The presence of ST segment depression and/or T wave changes in the context of acute ischaemic chest pain normally indicates myocardial ischaemia (i.e. unstable angina) but is also associated with a positive likelihood ratio for AMI (i.e. non-ST elevation AMI – see Table 9). Approximately 50% of patients with ST depression and 33% of patients with T wave inversion will subsequently be shown to have myocardial infarction as defined by an elevated cardiac marker(20,21).  This group of patients are presenting with an ACS (i.e. unstable angina or non-ST elevation myocardial infarction).

Learning Bite

ST segment elevation is associated with the highest likelihood of AMI followed, in order, by new Q waves, new conduction deficit, ST depression and T wave changes

A normal ECG significantly reduces the probability of AMI(12-14). It does not, however, reduce this probability enough to allow confident safe discharge based upon the history and ECG alone(2,3,4,5,16). Therefore, patients who present with chest pain in whom cardiac ischaemia is suspected and who have a normal ECG should undergo further diagnostic testing (ie. delayed cardiac markers, before they can be confidently ascribed to a low risk group.

Learning Bite

A normal ECG in a patient with chest pain does not allow safe discharge without further investigation