DESCRIPTION 
- QT intervals reflect time from onset of cardiac depolarization to completion of repolarization.
- Onset of Q wave (or R wave) to end of T wave on 12-lead EKG
- Do not use U wave unless merged with T wave and/or of high amplitude.
- Corrected for heart rate, many formulas available: Clinically most widely used is Bazett formula: QT (msec) divided by square root of RR (in seconds)
- Prolonged QT interval may cause polymorphic ventricular tachycardia (torsade de pointes). Ventricular tachycardia onset is often preceded by a prolonged RR interval (ie, post premature ventricular contraction [PVC] pause)
Geriatric Considerations
More susceptible to drug–drug interactions and more prone to electrolyte abnormalities
Pediatric Considerations
In children 1–15 yr old, the QT corrected is <460 msec.
EPIDEMIOLOGY
Normal QT corrected interval is <450 msec in men and <470 msec in women.
RISK FACTORS
- QT interval may be prolonged by electrolyte abnormalities:
- Hypokalemia
- Hypomagnesemia
- Hypocalcemia
- QT interval may be prolonged by various medication groups (see www.qtdrugs.org):
- Antiarrhythmic drugs
- Psychotropic drugs
- GI motility drugs
- Antibiotic/antifungal drugs
- QT interval may be prolonged in cardiomyopathy
Genetics
- Patients with known long QT syndrome are at increased risk for prolonged QT interval.
- Patients who develop a long QT interval may be silent carriers of a long QT gene (research ongoing).
GENERAL PREVENTION
- Avoid QT-prolonging drugs.
- Serial EKG monitoring
PATHOPHYSIOLOGY
- Possible genetic predisposition (genetic mutations in cardiac ion channels)
- Drugs combined with electrolyte shifts
- Drug–drug interaction (inhibition of drug metabolism)
- Bradycardia and female gender
Outline
Signs and symptoms:
- Asymptomatic
- Palpitations
- Syncope
- Sudden death
History
- Careful review of recent drug exposure (prescribed and OTC drugs)
- Review of family history for syncope or premature sudden death
DIAGNOSTIC TESTS & INTERPRETATION
- EKG
- Holter monitor
- ECG to rule out structural heart disease
Lab
- Electrolytes
- Drug level screen if clinically suggested
Imaging
ECG
Pathological Findings
Prolonged QTc interval on the 12-lead EKG
DIFFERENTIAL DIAGNOSIS
- QT interval in patients with bundle branch block may be prolonged due to wide QRS complex.
- QT interval is difficult to evaluate in patients with a paced QRS complex.
Outline
Replace electrolyte abnormalities.
First Line
IV magnesium suppresses VT.
Second Line
Increasing heart rate (typically by temporary transvenous pacing) shortens QT interval—it may be indicated in patients with long QT interval and polymorphic VT, especially if bradycardic.
Outline
ADDITIONAL TREATMENT
General Measures
- Discontinue suspected QT-prolonging drug.
- If symptomatic and/or QT interval markedly prolonged, admit for cardiac monitoring.
Referral
Cardiac evaluation is often needed to rule out congenital long QT syndrome and advise in safety of alternate drug therapy.
IN-PATIENT CONSIDERATIONS
Admission Criteria
If symptomatic and/or QT interval markedly prolonged, admit for cardiac monitoring.
Outline
FOLLOW-UP RECOMMENDATIONS
Patient Monitoring
Serial EKG and telemetry monitoring until QT interval is normal and precipitating factors (drugs, electrolytes) have resolved
PATIENT EDUCATION
Avoid QT-prolonging drugs (www.qtdrugs.org).
PROGNOSIS
- Good if QT interval prolongation is diagnosed on time
- Discontinuation/avoidance of responsible medication should resolve QT prolongation.
COMPLICATIONS
Syncope, sudden death
Outline