- QT interval: from beginning of QRS to end of T wave (ie, QRS complex + ST segment + T wave)
- Includes ventricular depolarization & repolarization (ie, all events of ventricular systole)
- QTc interval (corrected QT interval): adjusts for variations in heart rate
True measurement of QT interval may be difficult due to:
1. Lead selection
- Lead II most commonly used in single-channel ECG recordings 
- Multi-channel ECG recordings allows for more accurate measurement by detecting earliest QRS onset to T wave end in any lead (excluding measurement of distinct U waves) 
- Note: use multi-channel ECG recordings whenever possible
- If variation exists in leads: use longest QT interval
2. P wave superimposed on preceding T wave
- Often from faster heart rates
- Extrapolate T wave downslope to baseline
3. RR interval variation
- Due to (1) sinus arrhythmia & (2) slow adjustment of QT interval to abrupt heart rate changes 
- Makes determining RR interval for QT interval correction difficult
- Using shortest RR interval with 0.46 s as diagnostic QT threshold was found to include 98.4% diagnosed with long QT syndrome (3.8% controls also met threshold) 
- May be more appropriate to use RR interval averaged over several beats 
- QT dispersion:
- QTc dispersion affected by sinus arrhythmia; therefore, don’t use QTc dispersion values in the presence of sinus arrhythmia 
- No apparent difference in normal QT dispersion value in adults & adolescents  (note: data lacking for younger children)
Methods for determining QTc interval:
- Bazett formula: most common 
- Divide QT interval (seconds) by square root of RR interval (seconds)
- “Overcorrection” in children: QTc interval varies inversely with age & directly with heart rate [8,9]
- Fridericia & Framingham methods: may be preferred, but not widely used (likely due to complexity of formulas) 
- “Shorthand” rule (preferred): normal QTc interval if <1/2 RR interval 
- Adult/adolescents (14-18 y/o): women > men 
- Similar findings not confirmed in children 10-13 y/o 
- No significant gender difference in infants 
Exact QT interval measurement important in:
1. Diagnosis of long QT syndrome
- Use (1) multi-lead ECG recording, (2) QTc interval with RR interval averaged over multiple beats, & (3) Bazett’s formula (normal <0.44 s; borderline 0.44-0.46 s; abnormal >0.46 s) 
- Computer QT interval measurements: unreliable 
- Clinical parameters & genetic studies: improve diagnostic specificity 
- Large pediatric population screening: difficult due to overlap in normal & abnormal populations 
- QT interval response to exercise: measurement 1 minute into recovery suggested to differentiate long QT syndrome from normal [16,17]
- Epinephrine infusion suggested to unmask long QT syndrome (esp. LQT2) by (1) developing protuberance above apex of T wave & (2) significantly increasing QTc interval in LQT1 
- Beta-adrenergic blockers used for congenital long QT syndrome treatment in children do not appear to affect QTc interval or QT dispersion 
2. Drug-treated patients
- QT interval measurement more reliable at comparable heart rates 
- QT60 (QT interval at 60 BPM) proposed to be standard for QTc interval 
- Suggested that QT60 makes no assumptions about nature of QT interval-heart rate relationship, removes dependence of QT interval on heart rate, & maintains biological differences in QT interval
- Determine best mathematical relationship of QT interval to heart rate & then extrapolate QT60
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