Applied Evidence

4 EKG abnormalities: What are the lifesaving diagnoses?

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These conditions may not have associated findings on physical exam. See if you can make the diagnosis based on the EKG tracings.


 

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When evaluating a patient with a history of chest pain, palpitations, syncope, and/or new-onset seizures, an electrocardiogram (EKG) may be the key to identifying a potentially life-threatening condition. Here we present 4 cases in which EKG findings were the clue to underlying medical conditions that, if left untreated, could be fatal. Because each of these conditions may not have associated findings on a physical exam, early recognition of these EKG findings can be lifesaving.

CASE 1 › A 15-year-old boy suddenly collapses while walking, and bystanders report seizure-like activity. The patient doesn’t remember the event. Vital signs and physical exam are normal, and his blood glucose level is 86 mg/dL (normal: 70-100 mg/dL). He doesn’t take any medications and denies illicit drug use or recent illness.

What EKG abnormality (FIGURE 1) likely explains the cause of the patient’s collapse?

The EKG abnormality and diagnosis. The patient’s EKG revealed a prolonged QT interval (FIGURE 1, BRACKETS). His QTc (QT interval corrected for heart rate) was .470 seconds, which is at the high end of the normal range for his age and gender.1 The patient had no family history of syncope, sudden cardiac death (SCD), or seizure disorder. Evaluation uncovered a calcium level of 4.4 mEq/L (normal: 4.5-5.5 mEq/L) and a phosphate level of 7.8 mg/dL (normal: 2.4-4.1 mg/dL).

This patient had a low parathyroid hormone from primary hypoparathyroidism. His conduction abnormality was treated with both oral calcium and vitamin D supplements.

Etiology and epidemiology. A prolonged QT interval may be the result of a primary long QT syndrome (LQTS) or an acquired condition from electrolyte imbalance, medication effect, or toxin exposure.

In the United States, the incidence of a genetic mutation that causes LQTS is 1 in 2500 people.2 Patients with LQTS usually remain asymptomatic unless the QT interval is further prolonged by a condition or medication. There are several hundred congenital LQTS subtypes based on specific ion channel defects; the most common is LQTS1, with an inherited defect in the KCNQ1 gene, which regulates the slow potassium ion channel.

Acquired LQTS is much more common than congenital LQTS.3 Many drugs have been linked to an increased risk of LQTS, including certain antiarrhythmics, antibiotics, and antipsychotics (TABLE 1).4 In addition, electrolyte disturbances such as hypokalemia, hypocalcemia, and hypomagnesemia can be etiologic factors.

Be aware that an acquired LQTS may mask an underlying congenital LQTS. Therefore, patients in whom the offending agent or condition is corrected should still have a follow-up EKG. Screening family members for LQTS is worthwhile, even in those without symptoms.

Clinical features. Patients with symptomatic LQTS may have dizziness, palpitations, and syncope. SCD also is possible. These signs and symptoms may be triggered by strong emotions (in LQTS2) or physical activity (in LQTS1). They likely are caused by torsades de pointes and ventricular fibrillation. A brief aura may precede these arrhythmias, and patients may experience urinary or fecal incontinence.5

A common error in measuring the QT interval occurs when physicians inadvertently include a U-wave in the measurement.The key to making a diagnosis of LQTS is correctly measuring the QT interval. The QT interval is measured from the beginning of the Q-wave to the end of the T-wave as measured from the intersection of a line tangent to the downslope of the T-wave and the isoelectric line. This can be difficult to determine in EKGs showing bundle branch block or an irregular rhythm, such as atrial fibrillation (AF).6,7 A common error in measuring the QT interval occurs when clinicians inadvertently include a U-wave in the measurement.1 Some EKG machines may provide QT interval and QTc measurements. Normal QT intervals are ≤.450 seconds for men and ≤.470 seconds for women.8

It is essential to confirm the QT interval by using the Bazett formula (QTc equals the QT in seconds divided by the square root of the RR interval in seconds) for all patients with a history that suggests a possible arrhythmia.

First rule out or treat any causes of acquired LQTS by taking a medication history and evaluating the patient's electrolytes.Our patient had hypocalcemia, which on an EKG can cause T-wave widening with a normal ST segment, rather than a normal T-wave with a long ST segment, as is typically seen in LQTS. This distinction may be difficult to discern and should not preclude the search for either an acquired prolonged QTc or an underlying LQTS.9

Treatment. First rule out or treat any causes of acquired LQTS by taking a careful medication history and evaluating the patient’s electrolytes. Once these have been addressed, a beta-blocker is first-line therapy for symptomatic patients.5

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