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Lesson 5 of 21 · Waves & Intervals

The QRS Complex

The QRS represents ventricular depolarization. Its width tells you whether conduction used the normal His–Purkinje system (narrow) or spread muscle-to-muscle (wide).

01Naming the waves

  • Q: first negative deflection before an R
  • R: first positive deflection
  • S: negative deflection after an R
  • R′: second positive deflection (e.g., RBBB rSR′ in V1)
  • QS: entirely negative complex
The labeled EKG complex
PRQSTisoelectric linePR intervalQRSST segmentQT interval

02Width

  • Normal: < 0.12 s (< 3 small boxes)
  • 0.10–0.12 s: incomplete bundle branch block / nonspecific delay
  • ≥ 0.12 s: BBB, ventricular rhythm, pre-excitation, hyperkalemia, Na⁺-channel blockade, or paced
II
Narrow QRS: normal conduction through the bundles.
II
Wide QRS at 170 bpm: ventricular tachycardia.

03R-wave progression

In V1 the QRS is mostly negative (small r, deep S). Moving to V6 it becomes mostly positive. The transition (R = S) normally occurs in V3–V4. Poor R-wave progression can mean old anterior MI, LVH, or misplaced leads.

Key takeaways

  • Wide = ≥ 0.12 s = 3 small boxes
  • Wide + fast = VT until proven otherwise
  • Transition normally V3–V4