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218 P R I N C I P L E S A N D P R A C T I C E O F C R I T I C A L C A R E

sound due to decreased contractility. A fourth heart sound electrodes, V7–V9, may be placed over the posterior of
is common, whereas a third heart sound is uncommon. the left chest to view the posterior wall. Other indicative
Many patients develop a pericardial rub after about 48–72 signs of posterior wall damage are a small r wave in V1
hours due to an inflammatory response to the damaged and/or ST depression in V3 and V4 as these may be recip-
myocardium. Additional findings occur with complica- rocal changes. The endocardial surface of the posterior
tions, and these are discussed in that specific section wall faces the praecordial leads of the ECG so the signs
below. of ischaemia and infarction are reversed or reciprocal
such as ST depraession or a small r wave. If these signs
Electrocardiographic examination are present a left-sided ECG, V7–V9, should be done to
Patients with chest discomfort should be assessed by an confirm or rule out a posterior infarction.
appropriately qualified person and have an ECG recorded Continuous ECG monitoring is essential to detect arrhyth-
within 5 minutes of arrival at a healthcare facility to mias, which often accompany AMI and are a common
determine the presence and extent of myocardial isch- cause of death. The arrhythmia may be due to poor perfu-
aemia, the risk of adverse events and to provide a baseline sion of the conduction tissue. More often, arrhythmias
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for subsequent changes. Most importantly, the ECG is occur because ischaemic tissue has a lower fibrillatory
essential to determine whether emergency reperfusion is threshold and ischaemia is not being managed. Arrhyth-
required, and is recommended as the sole test for select- mias also result from left ventricular failure.
ing patients for PCI or thrombolysis. Where ST segment
monitoring is available, this should be continuous. Alter- Typical ECG evolution pattern
natively, if chest discomfort persists, ECGs should be
repeated every 15 minutes. Even when chest pain resolves The initial ECG features of myocardial infarction are ST
it is important to record a series of 12-lead ECGs during segment elevation with tall T-waves recorded in leads
admission to determine changes over time. (The normal overlying the area of damaged myocardium. These changes
ECG is covered in Chapter 9, whereas this section gradually change, or evolve, over time, with ST segments
addresses ischaemic changes in the ECG.) returning to baseline (within hours), while Q waves
develop (hours to days) and T waves become inverted
Myocardial ischaemia, injury or infarction cause cellular (days to weeks). The time course for the evolutionary
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alterations and affect depolarisation and repolarisation. changes is accelerated by reperfusion, e.g. PCI, throm-
Myocardial ischaemia may be a transient finding on the bolysis or surgery. Thus an almost fully-evolved pattern
ECG. Ischaemia results in T wave inversion or ST segment may be seen within hours if successful reperfusion has
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depression in the leads facing the ischaemic area. Isch- been undertaken (see Figures 10.2–10.4 for an example).
aemic T waves are usually symmetrical, narrower and Given the expected time course for evolution, it is possi-
more pointed. ST segment depression of 1 mm for 0.08 ble to approximate how recently infarction has occurred,
seconds is indicative of ischaemia, especially when which is essential in determining management:
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forming a sharp angle with an upright T wave. These
changes are reversible with reduction in demand (e.g. by ● acute (or hyperacute): there is ST elevation but Q
rest, nitrates). waves or T inversion have not yet developed (see
Figure 10.5).
On acute presentation, myocardial injury (infarction) is
most commonly associated with ST segment elevation on ● recent: Q waves have developed. ST segment elevation
the ECG, although this is not universal. In addition, a may still be present. Evolution is underway. The infarc-
typical pattern of ECG changes over time (evolution of tion is more than 24 hours old.
the ST segments, Q wave development and T wave inver- ● old (fully evolved): Q waves and T inversion are
sion) are often seen (described below), but these changes present. ST segments are no longer elevated. Infarction
too are not universal. The distinction between the various occurred anything from a few days to years ago.
acute coronary syndromes, including ST elevation acute
coronary syndrome (STEACS), ST elevation myocardial Biochemical markers
infarction (STEMI) and non-ST elevation myocardial Intracellular cardiac enzymes enter the blood as ischaemic
infarction (non-STEMI), is important for ensuring appro- cells die, and elevated levels are used to confirm myocar-
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priate assessment and protocol-based treatment for the dial infarction and estimate the extent of cell death. The
various presentations. cardiac troponins T and I (cTnT and cTnI) have been
The location and extent of ischaemia or infarction may found to be both sensitive and specific measures of cardiac
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be evident on the ECG leads overlying the affected area, muscle damage. Troponin I is rapidly released into the
as follows: bloodstream, so it is especially useful for the diagnosis
and subsequent risk stratification of patients presenting
● anteroseptal wall of left ventricle, V1–V4; with chest pain in the early stages. Troponin I is also a
● anterior wall of the left ventricle, V1-V6, I and aVL; more appropriate marker to use in postoperative and
● lateral wall of left ventricle, I, aVL,V5 and V6; trauma patients than creatine kinase–MB (CK-MB), as
● inferior wall of left ventricle, II, III and aVF.
CK-MB levels will be affected by muscle damage. However,
Additional leads are needed to view the right ventricle CK-MB is less costly and more readily available, and so
and posterior wall. Chest electrodes can be placed on the is still often used, particularly in the presence of a non-
right chest wall using the same landmarks as the left chest diagnostic ECG. C-reactive protein assays may prove to
to view the right ventricle (see Chapter 9). Further be useful, as baseline and discharge levels are predictive