LECTURES:
1. QT Interval
2. Long QT Syndrome
3. Short QT Syndrome
4. Full ECG Course
5. The EKG Guy Lectures

QT interval: represents time from QRS complex onset to end of T wave

• QT interval = [QRS complex] + [ST segment] + [T wave]
• Cardiac event: ventricular depolarization + ventricular repolarization (note: all of ventricular systole)

QT interval duration: variable (age, sex, HR, electrolytes)

• In general: normal QT interval <½ preceding RR interval
• Best measured in II, V5, or V6 (use maximum interval)
• If TU fusion: use maximum slope-intercept method to define end of T wave (include U wave if amplitude >1 mm)
• Females > males
• Inversely related to heart rate (↓ QT interval with ↑ heart rate; ↑ QT interval with ↓ heart rate)

QTc interval: attempts to “correct” for QT interval variation with heart rate

• Improves detection of patients at ↑ risk of arrhythmias
• Bazett’s formula: QTc = ??/√??
• Normal (varies): males <440 ms; females <460 ms; >350 ms
• QTc >500 ms: ↑ risk of torsades de pointes