ST-segment elevation: Differential diagnosis, caveats

EARLY REPOLARIZATION

Early repolarization is a normal variant of ST elevation that equals or exceeds 1 mm (measured at the J point). It is highly prevalent in people under age 40 and remains prevalent in middle-aged people.

Two distinct and sometimes coexistent forms of early repolarization have been described: (1) ST elevation in the anterior leads V₁ to V₃,^16–19 and (2) ST elevation in the lateral leads (V₄ to V₆, I, aVL) or inferior leads.^18–22 The prevalence of the first form—ie, ST elevation of 1 mm or more in any of the leads V₁ through V₃—is 60% to 90% in men  age 45 and younger, 20% to 40% in men over age 45, and about 10% in women of any age.¹⁶ Thus, this form of early repolarization is called “normal male pattern.”

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Even early repolarization that involves the lateral or inferior leads is common, with a prevalence of about 15% in people ages 30 to 40 and about 5% to 10% in those 40 to 65.^20–23 It is two to four times more prevalent in men and three times more prevalent in African Americans. It is also highly prevalent in athletes younger than 25 (about 30% to 40%).²²

Either way, early repolarization closely resembles the ST elevation of pericarditis and has the following features (Figure 4):

• The ST segment is concave upward, and the J point is well demarcated and may be notched or slurred (Figure 1).
• ST elevation is usually no more than 3 mm.
• ST elevation may be limited to the anterior leads or, in many instances, may extend to the inferior or lateral leads. Early repolarization is very rarely limited to the limb leads, and involvement of some precordial leads is the rule.^18,19 The ST segment is depressed in lead aVR in 50% of patients.^18,19

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  The T wave is usually ample and may be more than 10 mm tall in the precordial leads in one-third of patients,¹⁷ but as opposed to the ample T wave of STEMI, it is not broad and remains smaller than the QRS complex. The ample T wave distinguishes early repolarization from pericarditis, and explains the low ST-T ratio in lead V₆. In up to 10% of young black men, the T wave has a terminal inversion in leads V₃ to V₅, and occasionally in V₁ and V₂, mimicking infarction (Figure 5).²⁴
• The QRS complex tends to have prominent precordial voltage, in sharp contrast to STEMI, in which QRS shrinking occurs.^3,17,22

The early repolarization pattern may be intermittent, may vary among serial electrocardiograms, may decrease with a rise in sympathetic tone, as observed during exercise, and may increase with a rise in vagal tone.^18,19,25,26  Although it is usually a benign finding, the early repolarization pattern in leads other than V₁ to V₃ has been associated with an increased risk of sudden death, particularly when the ST elevation is horizontal-descending rather than upsloping and, possibly, when early repolarization involves the inferior leads with a J point that is notched or elevated 2 mm or more.^20,22

PERICARDITIS

In pericarditis, ST elevation is concave upward and is widespread to more than one region without reciprocal ST depression, except for the frequent ST depression in leads aVR and V₁ (64%)²⁷; ST elevation is seldom greater than 4 to 5 mm (Figure 6).^27,28 Since the subepicardial injury is diffuse in pericarditis, the axis of the ST segment follows the anatomic axis of the heart and is generally +45° in the frontal plane. Thus, ST depression is seen in leads aVR and V₁; ST elevation is highest in leads II, V₅, and V₆ and is less in leads III and aVL, where the ST segment may occasionally be depressed.²⁹

Transient PR depression greater than 1 mm is often seen, particularly in leads II, aVF, and V₄ to V₆, and represents atrial subepicardial injury. PR depression in those leads is always associated with PR elevation in lead aVR and sometimes V₁. PR changes often coexist with ST changes but may be isolated and may precede ST changes.³⁰ PR depression is characteristic of pericarditis but may be seen in early repolarization, where it is less marked than in pericarditis (< 0.8 mm) and implies early repolarization of the atrial tissue,³¹ and in MI, where it implies atrial infarction with atrial injury pattern.

Classically, it is said that in pericarditis, unlike in STEMI, the T wave does not invert until the ST elevation subsides. In reality, up to 40% of patients develop a notched or biphasic positive-negative T wave before full return of the ST segment to the baseline.^27,32 And if T-wave inversion antedates pericarditis, concomitant ST elevation and T-wave inversion may be seen once pericarditis develops. However, the T wave inverts less deeply and less completely than in STEMI, and the corrected QT interval remains normal even when the T wave inverts.

Three criteria distinguish pericarditis from early repolarization (but not from STEMI):

• PR depression greater than 1 mm
• ST-segment depression in lead V₁
• A ratio of ST-segment height to T-wave height of at least 25% in lead V₆, V₅, V₄, or I. This feature distinguishes pericarditis from early repolarization with a high sensitivity and specificity. In pericarditis, the T waves have normal or reduced amplitude, and the ST-T ratio is therefore high,³³ whereas in early repolarization the T waves are tall, so the ST-T ratio is less than 25%.

Widespread ST elevation may be seen with both pericarditis and early repolarization. ST elevation limited to the anterior leads is more likely to be early repolarization than pericarditis.

LEFT BUNDLE BRANCH BLOCK

In left bundle branch block, a deep and wide S wave is seen in leads V₁ to V₃ and sometimes in the inferior leads, with ST elevation and T waves that are discordant with the QRS complex—ie, directed opposite to the QRS (Figures 7–9). The ST elevation is typically concave upward.^8,34 Occasionally, ST elevation may be straight or convex, mimicking the dome of STEMI. In the lateral leads, the discordant ST segment is depressed, mimicking a reciprocal ST change.

The following findings imply MI:

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  ST elevation or depression that is concordant with the QRS complex. Moreover, since ST deviation is mandatory with left bundle branch block, a “normal-looking” ST segment implies ischemia.
• Inverted T waves concordant with the QRS in more than one lead, or biphasic T waves in more than one lead (eg, V₁ to V₃). Across the precordial leads, T waves may transition from positive to negative one lead earlier or later than the QRS and ST transition. Therefore, even in the absence of ischemia, the T wave may be inverted in lead V₃, in which the QRS is deeply negative and the ST is still elevated (negative T-wave concordance in one lead). Also, the T wave may be upright in leads V₅, V₆, and I where QRS is upright and the ST segment is depressed (positive T-wave concordance does not imply ischemia).

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  In addition to concordance, a discordant ST segment or T wave that is very large may imply ischemia. For example, a discordant ST segment or T wave that is larger than the QRS height implies ischemia. A discordant ST elevation greater than 5 mm has been suggested by Sgarbossa et al³⁵ as a diagnostic feature of STEMI; however, this feature is seen in 10% of control patients with left bundle branch block and no STEMI, and it is thus poorly specific and also poorly sensitive, frequently missing STEMI.^35–37 Smith et al³⁶ have suggested that a discordant ST elevation of at least 25% of the S-wave depth is a far more sensitive and accurate feature but one that may still be found in up to 10% of control patients.³⁶