ARE THE ST SEGMENTS ELEVATED?
CHApTER 15 The ST segment
Normally, the ST segment is isoelectric, meaning that it lies at the same level as the ECG’s baseline, the horizontal line between the end of the T wave and the start of the P wave (Fig. 15.1).
ST segments can be abnormal in one of three ways, so the questions you need to ask about the ST segments when you review them are:
•
Are the ST segments elevated?•
Are the ST segments depressed?•
Are J waves present?In this chapter, we will help you to answer these questions, and guide you about what to do next if you find an abnormality.
ARE THE ST SEGMENTS ELEVATED?
Look carefully at the ST segment in each lead to see if it is isoelectric. If it is raised above this level, the ST segment is elevated.
ST segment elevation should never be ignored, as it often indicates a serious problem that warrants urgent attention. If you see ST segment elevation in any lead, consider the following possible diagnoses:
•
ST segment elevation myocardial infarction•
left ventricular aneurysm•
Prinzmetal’s (vasospastic) angina•
pericarditis•
left bundle branch block•
Brugada syndrome•
high take-off (diagnose this only if you have excluded all other causes).CHApTER 15 The ST segment
ST segment Figure 15.1 The ST segment.
Key point: • Normally, the ST segment is isoelectric.
15The ST segment
Therefore, ST segment elevation can represent anything from a potentially life-threatening condition to a normal variant, making it particularly important to identify the cause. To help you in this task, we describe each of these conditions (together with example ECGs) below.
ST segment elevation myocardial infarction
Patients presenting with acute coronary syndromes are ultimately divided into three categories:
•
ST segment elevation myocardial infarction (STEMI)•
non-ST segment elevation myocardial infarction (NSTEMI)•
unstable angina.All three groups of patients present with similar symptoms – ischaemic chest pain (tight, central), often with breathlessness, sweating, nausea and vomiting – and the initial categorization is made on the basis of the presenting ECG (Fig. 15.2). If there is ST segment elevation, the patient is managed as STEMI. If the ST segment is depressed, or there is T wave inversion, or the ECG is normal, then the working diagnosis is either NSTEMI or unstable angina (and the distinction between these is made later when the troponin results are available).
This section is chiefly concerned with STEMI. More information about NSTEMI can be found on page 174.
CHEST PAIN SUGGESTIVE OF ACUTE CORONARY SYNDROME The key symptoms of acute coronary syndrome are:
•
tight, central chest pain (usually severe, and longer lasting, than that of angina)•
breathlessness•
nausea and vomiting•
sweating (‘diaphoresis’).Ask about a history of previous angina or myocardial infarction, or any other form of vascular disease (stroke/TIA or peripheral vascular disease) and assess cardiovascular risk factors (Table 15.1). An urgent ECG is mandatory in patients presenting with chest pain suggestive of acute coronary syndrome.
Table 15.1 Risk factors for cardiovascular disease Modifiable:
• Cigarette smoking
• Hypertension
• Diabetes mellitus
• Dyslipidaemia
• Overweight and obesity
• Physical inactivity Non-modifiable:
• Age
• Male sex
• Family history
The ST segment 161
15The ST segment
In STEMI, the ECG changes gradually ‘evolve’ in the sequence shown in Figure 15.3.
The earliest change is ST segment elevation accompanied, or even preceded, by tall ‘hyperacute’ T waves. Over the next few hours or days, Q waves appear, the ST segments return to normal and the T waves become inverted. It is usual for some permanent abnormality of the ECG to persist following STEMI – usually
‘ pathological’ Q waves, although the T waves may remain inverted permanently too.
Symptoms indicative of acute coronary syndrome
ECG shows ST
segment elevation ECG shows no ST
segment elevation
Treat as STEMI
Treat as NSTEMI/unstable
angina
Troponin positive Troponin negative
NSTEMI Unstable angina
Figure 15.2 Acute coronary syndromes.
Key point: • The diagnostic category of acute coronary syndrome is based upon the presenting symptoms, the ECG findings and whether there has been myocyte necrosis (as evidenced by the troponin results).
(1) Tall ‘hyperacute’
T waves (2) ST segment
elevation
(3) Q wave formation and T wave inversion as ST segment settles
(4) Residual Q waves with T wave inversion
Figure 15.3 Evolution of a STEMI.
Key points: • Tall ‘hyperacute’ T waves may be the earliest finding, rapidly followed by ST segment elevation.
• As the ST segment settles, the T wave inverts and Q waves form.
• The Q waves and inverted T waves may persist indefinitely.
15The ST segment
Do not forget that acute myocardial infarction can also present with the new onset of left bundle branch block on the ECG (Chapter 14), and patients presenting in this way are managed in the same way as those with ST segment elevation.
The ECG also allows you to identify the area of myocardium affected by STEMI, as the leads ‘looking at’ that area will be the ones in which abnormalities are seen (Table 15.2). Examples of ST segment elevation in different myocardial territories are shown in Figures 15.4–15.6.
Having diagnosed STEMI, waste no time in admitting the patient to a coronary care unit or other monitored area for treatment as indicated. This is discussed later in this section.
If you diagnose an inferior STEMI, go on to ask the question:
•
Is the right ventricle involved?To make the diagnosis, record another ECG, but this time use right-sided chest leads (Fig. 15.7). Look for ST segment elevation in lead V4R (Fig. 15.8). If present, there is a high likelihood of right ventricular involvement.
Patients with STEMI require:
•
pain relief (an opioid intravenously and an anti-emetic)•
oxygen if hypoxic•
aspirin, 300 mg orally•
clopidogrel, 300 mg orally.The main priority in STEMI is urgent primary percutaneous coronary intervention (PCI) or, if facilities for coronary angiography and primary PCI are unavailable, intravenous administration of a thrombolytic agent may be considered. Thrombolysis is indicated (unless contraindicated) in patients whose history suggests a myocar-dial infarction within the past 12 h and whose ECG shows:
•
ST segment elevation consistent with infarction, or•
new left bundle branch block.AORTIC DISSECTION
Beware of missing a diagnosis of aortic dissection. This too can cause ST segment elevation (if the dissection involves the coronary arteries) and chest pain, but patients may also complain of a ‘tearing’ back pain with different blood pressure in each arm, and a chest radiograph will show mediastinal widening. The diagnosis can be confirmed by a CT aortogram.
Table 15.2 Localization of ST segment elevation acute myocardial infarction
Leads containing ST segment elevation
Location of event
V1–V4 Anterior
I, aVL, V5–V6 Lateral
I, aVL, V1–V6 Anterolateral
V1–V3 Anteroseptal
II, III, aVF Inferior
I, aVL, V5–V6, II, III, aVF Inferolateral
The ST segment 163
15The ST segment
Following STEMI, patients should continue with:
•
aspirin, 75 mg daily•
clopidogrel, 75 mg daily (short term)•
a beta blocker•
an angiotensin-converting enzyme inhibitorI aVR
aVL
aVF
V2 V5
V6 V3
V1 V4
II
III
II
Figure 15.4 Inferior STEMI.
Key point: • There is ST segment elevation in leads II, III and aVF, with ‘ reciprocal’ ST segment depression in leads I and aVL.
II
II aVL V2 V5
I aVR V1 V4
III aVF V3 V6
Figure 15.5 Anterior STEMI.
Key point: • There is ST segment elevation in leads V1–V4.
15The ST segment
V6R
V5R V4R V3R
V2R V1R
Figure 15.7 Positioning of right-sided chest electrodes.
Key point: • On the ECG, the chest leads should be labelled V1R–V6R and the ECG itself should be clearly labelled ‘right-sided leads’.
I
II
III aVF
aVR
aVL
V1 V4
V2 V5
V3 V6
IV
Figure 15.6 Anterolateral STEMI.
Key point: • There is ST segment elevation in leads V4–V6, I and aVL.
The ST segment 165
15The ST segment