What is the interpretation of an EKG showing a prolonged QTc interval?

EKG Interpretation: Borderline QTc Prolongation

Your EKG shows a borderline prolonged QTc of 425 ms that requires manual verification and clinical evaluation to determine significance, as automated measurements are frequently inaccurate and this value falls in the indeterminate zone where clinical context determines management. 1

Immediate Action Required

Visually validate the computer-generated QT measurement manually, as the AHA/ACCF/HRS guidelines emphasize that automated QT measurements are unreliable and must be confirmed by manual measurement, particularly when prolongation is reported. 1

• Measure the QT interval manually in leads V2 or V3, which typically show the longest QT interval 1
• If measurements differ by >40 ms between leads, remeasure in adjacent leads to verify accuracy 1
• Use the tangent method: draw a line along the steepest downslope of the T wave to the baseline to identify the true end of the T wave 1, 2

Rate Correction Considerations

The Bazett formula (likely used by your machine) systematically overcorrects at higher heart rates and should not be relied upon at your ventricular rate of 87 bpm. 1

• At 87 bpm, Bazett's formula produces falsely elevated QTc values with a positive residual correlation of r=0.32 1
• Linear regression formulas are superior and recommended over Bazett's formula for accurate rate correction 1
• The Fridericia formula can be approximated by subtracting 5% from the uncorrected QT for each 10 bpm increase above 60 bpm 3
• For your heart rate of 87 bpm (27 bpm above 60), subtract approximately 13-14% from the uncorrected QT of 354 ms, yielding a QTc-Fridericia of approximately 306-308 ms, which would be normal 3

Clinical Significance Assessment

A QTc of 425 ms falls in the borderline zone (440-480 ms for definitive diagnosis) and requires evaluation for secondary causes before considering congenital Long QT Syndrome. 4

Exclude Secondary Causes First:

• Electrolyte abnormalities: Check potassium, calcium, and magnesium levels immediately, as hypokalemia and hypocalcemia prolong the QT interval 1
• QT-prolonging medications: Review all medications including macrolide antibiotics (erythromycin, clarithromycin), prokinetics (cisapride), antiarrhythmics, antipsychotics, and antidepressants 1, 4
• Structural heart disease: Left ventricular hypertrophy commonly causes slight QT prolongation 1
• Other metabolic causes: Hypothyroidism, hypocalcemia, central nervous system abnormalities 1

Risk Stratification

Your current QTc does not meet diagnostic criteria for Long QT Syndrome, which requires QTc ≥480 ms on repeated ECGs or QTc ≥500 ms as unequivocal diagnosis. 4

However, evaluate for these high-risk features:

• Personal history: Unexplained syncope, seizures, cardiac arrest, or hearing loss 4
• Family history: Early sudden death, fainting spells, congenital deafness in relatives 4
• T-wave morphology: Notched T waves in precordial leads, T-wave alternans, or 2:1 AV block 4

Management Algorithm

If Secondary Causes Identified:

• Correct electrolyte abnormalities immediately 1
• Discontinue QT-prolonging medications when possible 1, 4
• Repeat ECG after correction to reassess baseline QTc 4

If No Secondary Causes and QTc Remains 425-460 ms:

• Obtain detailed three-generation family history focusing on sudden death, syncope, and seizures 4
• Perform family screening ECGs on parents and siblings 4
• Consider 24-hour Holter monitoring to assess QT variability 4
• If family history positive or unexplained syncope present, refer for genetic counseling and testing 4

If QTc >460 ms with Syncope or >480 ms Without Syncope:

• Initiate beta-blocker therapy (first-line treatment for LQTS) 1, 4
• Refer to electrophysiology for comprehensive evaluation 4
• Avoid strenuous/competitive exercise 4
• Provide continuous cardiac monitoring if hospitalized 4

Critical Pitfalls to Avoid

• Do not rely solely on automated QTc measurements - they are frequently inaccurate and must be manually verified 1
• Do not use Bazett's formula at heart rates significantly different from 60 bpm - it produces systematic errors 1, 3
• Do not dismiss borderline QTc in the presence of syncope or positive family history - up to 25-30% of genetically confirmed LQTS patients have QTc <440 ms due to low penetrance 4
• Do not assume normal parental ECGs rule out familial LQTS - 30% of cases are de novo mutations 1, 4