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It is important to approach an electrocardiogram (ECG) using the same system every time:

1. Rate

  • Definitions: Bradycardic (<60 bpm), normal (60–100 bpm), tachycardic (≥100 bpm)

  • Assessment: 1) 300-150-100-75-60-50 Rule; 2) # QRS complexes × 6 (for a 10-second strip)

2. Rhythm

  • Regular vs. irregular:

    • - If not 1:1, determine if P > QRS (AV block) or if P < QRS (accelerated junctional or ventricular rhythm)

  • Sinus vs. nonsinus:

    • - Sinus rhythms: P before every QRS and QRS after every P. Atrial activity initiating at sinus node: P upright in II, down in avR, sinusoidal in V1

    • - Nonsinus rhythms:

      • Abnormal atrial rhythms: Premature atrial contractions (PACs), ectopic atrial rhythm (<100 bpm), atrial tachycardia (>100 bpm), atrial fibrillation, atrial flutter, AVNRT, AVRT, paced atrial rhythms

      • Junctional rhythms: Junctional escape rhythm (<60 bpm), accelerated junctional rhythm (60–100 bpm), junctional tachycardia (>100 bpm)

      • Ventricular rhythms: Premature ventricular beats (PVCs), idioventricular (escape) rhythm (<50 bpm), accelerated idioventricular rhythm (50–100 bpm), ventricular tachycardia (>100 bpm), ventricular fibrillation, paced ventricular rhythms

3. Axis (Figure 1.7)

  • Normal axis: +I, +II

  • L axis deviation: +I, −II. Ddx: LVH, LBBB, LAFB, inferior MI, chronic lung disease, hyperkalemia

  • R axis deviation: −I, +aVF. Ddx: RVH, LPFB, lateral wall MI, PE, chronic lung disease, arm lead reversal, dextrocardia, ostium secundum ASD

  • Extreme “NW” axis: −I, −aVF, +aVR. Ddx: Ventricular rhythm, pacing, hyperkalemia, lead misplacement


Standard ECG lead placement and elements of a normal waveform.


ECG lead detection displayed in 360 degrees.

4. Interval

  • Measurement: Big box = 0.2 seconds, small box = 0.04 seconds

  • Intervals:

    • - PR: normal is 0.12–0.2 seconds (3-5 small boxes)

      • Ddx long PR: First-degree AV block, high vagal tone, conduction delay, AV nodal agents, hypokalemia

      • Ddx short PR: WPW, high catecholamine state

    • - QRS: Normal is <0.12 seconds (<3 small boxes)

      • Ddx long QRS: 1) Ventricular rhythm; 2) Bundle branch block; 3) Intraventricular conduction delay (IVCD; slowing of both bundles, normal morphology but stretched QRS); 4) Medications (e.g., TCAs, antiarrhythmics); 5) WPW; 6) Paced rhythms

      • Bundle branch blocks: QRS may be narrow or wide; incomplete block = narrow QRS, complete block = wide QRS

        • - Fascicular blocks: Typically narrow QRS (<0.12ms), then look at axis, I/avL, II/III/avF

          • LAFB: LAD (−45° to −90°) with no other cause (diagnosis of exclusion), qR in I/avL, rS in II/III/avF

          • LPFB: RAD with no other cause (diagnosis of exclusion), rS in I/avL, qR in II/III/avF

        • - Unifascicular block: QRS is wide; due to one slow conduction tract; can be normal (0.2% of normal population)

          • RBBB: V1 rSR’ (QRS terminal forces POSITIVE), wide slurred S in lateral leads ...

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