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    STs
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    STs

    ST segment: J point, ischaemic ST elevation and depression thresholds and other causes of ST changes.

    The ST segment runs from the end of the QRS complex (the J point) to the start of the T wave. During this period all ventricular cells are depolarised and the electrical potential is nearly the same everywhere, so the segment is normally flat and on the baseline (level with the PR and TP segments). Its typical duration is about 80 ms. Displacement of the ST segment is one of the most important signs on the ECG, because it can indicate acute myocardial ischaemia or infarction.

    ECGIschaemiaMyocardial infarction
    Normalon the baseline
    Measured atJ point, relative to PR/TP
    Ischaemic depression≥ 0.5 mm, horizontal or down-sloping
    Ischaemic elevation≥ 1 mm (≥ 1.5–2.5 mm in V2–V3)
    One ECG beat with the ST segment highlighted from the J point to the start of the T wave J point P PR seg. QRS ST seg. T PR interval ST interval QT interval
    Figure 1. The ST segment (highlighted) begins at the J point and ends at the start of the T wave; normally it lies on the baseline.

    ST elevation

    New ST elevation at the J point in two contiguous leads of ≥ 1 mm in all leads except V2–V3 suggests acute myocardial infarction. In V2–V3 the thresholds are ≥ 2 mm in men aged 40 or older, ≥ 2.5 mm in men under 40 and ≥ 1.5 mm in women. Other causes of ST elevation are acute pericarditis (widespread, concave, with PR depression), early repolarisation (benign in most people), left ventricular hypertrophy, left bundle branch block, left ventricular aneurysm, Brugada syndrome and hyperkalaemia.

    ST depression

    New horizontal or down-sloping ST depression of ≥ 0.5 mm in two contiguous leads suggests myocardial ischaemia. ST depression also occurs as a reciprocal change to ST elevation elsewhere, with digoxin, left ventricular hypertrophy ("strain"), bundle branch block, hypokalaemia and during tachycardia.

    Normal ST segment compared with ST elevation and with horizontal ST depression Normal ST on baseline ST elevation ≥ 1 mm in 2 leads ST depression ≥ 0.5 mm, horizontal
    Figure 2. Left: normal ST segment on the baseline. Middle: ST elevation, as in acute infarction. Right: horizontal ST depression, as in ischaemia.
    Seek emergency care

    Call emergency services at once for chest pain or pressure lasting more than a few minutes, pain spreading to the arm, jaw or back, breathlessness, cold sweat or fainting. In a heart attack, every minute of delay causes more loss of heart muscle.

    References

    1. Thygesen K, Alpert JS, Jaffe AS, et al. Fourth universal definition of myocardial infarction (2018). Eur Heart J. 2019;40(3):237–269. doi:10.1093/eurheartj/ehy462
    2. Rautaharju PM, Surawicz B, Gettes LS. AHA/ACCF/HRS recommendations for the standardization and interpretation of the electrocardiogram: part IV: the ST segment, T and U waves, and the QT interval. Circulation. 2009;119(10):e241–e250. doi:10.1161/CIRCULATIONAHA.108.191096
    3. Adler Y, Charron P, Imazio M, et al. 2015 ESC Guidelines for the diagnosis and management of pericardial diseases. Eur Heart J. 2015;36(42):2921–2964. doi:10.1093/eurheartj/ehv318

    This article is for general information and does not replace a consultation with a doctor. ECG findings must be interpreted by a physician together with symptoms and other tests.

    Published on 7 July 2019