What is a normal QRS (Quadrature Amplitude Modulation to ventricular Response time) interval?

Normal QRS Interval

In adults, a normal QRS duration is ≤110 ms, with a wide QRS defined as ≥120 ms and values between 110-119 ms considered borderline or incomplete conduction delay. 1

Age-Specific Normal QRS Duration Values

The normal QRS duration varies by age:

• Adults: ≤110 ms is normal, 110-119 ms is borderline, ≥120 ms indicates complete conduction delay 2, 1
• Children 4-16 years: <100 ms is normal 2, 1
• Children <4 years: <90 ms is normal 2, 1

Clinical Significance of QRS Duration

QRS duration represents ventricular depolarization time and has important clinical implications:

• Normal QRS (≤110 ms): Indicates normal ventricular conduction
• Prolonged QRS (≥120 ms): Associated with:
  • Intraventricular conduction delays (RBBB, LBBB)
  • Higher risk of cardiovascular mortality in cardiomyopathies 3
  • Decreased left ventricular systolic function 4

QRS Morphology Patterns

When QRS duration is prolonged, specific patterns may be observed:

Complete Right Bundle Branch Block (RBBB)

• QRS ≥120 ms in adults
• rsr', rsR', or rSR' pattern in V1/V2
• S wave duration > R wave or >40 ms in leads I and V6
• Normal R peak time in V5/V6 but >50 ms in V1 2

Complete Left Bundle Branch Block (LBBB)

• QRS ≥120 ms in adults
• Broad notched/slurred R wave in leads I, aVL, V5, V6
• Absent Q waves in leads I, V5, V6
• R peak time >60 ms in leads V5/V6 2

Incomplete Bundle Branch Blocks

• RBBB: QRS 110-119 ms with RBBB morphology
• LBBB: QRS 110-119 ms with LBBB morphology 2

Measurement Technique

For accurate QRS measurement:

• Measure from earliest onset to latest offset in any lead (global measurement)
• QRS is typically wider in precordial leads than limb leads
• Use the widest QRS complex for assessment 1

Clinical Implications

• QRS duration ≥120 ms may indicate need for cardiac resynchronization therapy in heart failure patients 2, 5
• QRS prolongation is associated with increased mortality in various cardiac conditions 3, 6
• In patients with wide QRS complex (≥120 ms), JT interval (QTc minus QRSd) may be a better predictor of coronary events than QTc 7

Common Pitfalls

• Failing to use age-appropriate norms for children
• Not measuring the widest QRS complex from earliest to latest deflection
• Overlooking borderline values (110-119 ms) which may have clinical significance
• Ignoring QRS morphology which provides additional diagnostic information 1

The QRS interval is a fundamental ECG measurement that provides important information about ventricular conduction and has significant prognostic implications across various cardiac conditions.