Cardiac Rhythm Strip Reference: Sinus & Atrial Arrhythmias

Normal Sinus Rhythm

  • Heart rate: 60–100 bpm60\text{–}100\ \text{bpm}
  • PR interval: .12–.20 s.12\text{–}.20\ \text{s}
    • Visually ≈ 3–53\text{–}5 small ECG boxes.
  • QRS duration: .06–.10 s.06\text{–}.10\ \text{s}
    • Visually ≈ 1.5–2.51.5\text{–}2.5 small boxes.
  • Rhythm: Regular.
  • RR interval (peak-to-peak distance): 3–53\text{–}5 boxes.

Sinus Bradycardia

  • Heart rate: <60 bpm<60\ \text{bpm}
  • PR interval: normal (same range as sinus rhythm).
  • QRS duration: normal.
  • Rhythm: Regular.
  • RR interval: >5>5 boxes (longer distance between R waves reflects slower rate).

Sinus Tachycardia

  • Heart rate: >100 bpm>100\ \text{bpm}
  • PR interval: normal.
  • QRS duration: normal.
  • Rhythm: Regular.
  • RR interval: <3<3 boxes (shortened distance between R waves reflects faster rate).

Premature Atrial Contractions (PACs)

  • Defining feature: Early ectopic P wave (“PAC P”) that differs in shape/location from normal sinus P wave.
  • Heart rate: varies (depends on underlying rhythm).
  • PR interval: normal for intrinsic beats; PAC may alter PR slightly.
  • QRS duration: normal (impulse still travels normal ventricular pathways).
  • Rhythm: "Irregularly irregular"—isolated premature beats disrupt otherwise regular pattern.

Premature Ventricular Contractions (PVCs)

  • Defining feature: Wide, bizarre QRS with no preceding P wave and no subsequent T wave matching morphology of normal beats.
  • Heart rate: varies (depends on frequency of PVCs and underlying rhythm).
  • PR interval: not applicable for the PVC itself.
  • QRS duration: often >.12 s.12\ \text{s} (wide).
  • Rhythm: "Irregularly irregular" due to premature ventricular impulses.

Atrial Flutter

  • Atrial activity: Classic "saw-tooth" F-waves (flutter waves) instead of discrete P waves.
  • Atrial rate often 240–350 bpm240\text{–}350\ \text{bpm} (not explicitly listed in transcript but typically taught; useful contextual note).
  • Ventricular response: Varies depending on AV-block ratio (e.g., 2:12:1 →  150 bpm~150\ \text{bpm}).
  • PR interval: not identifiable.
  • QRS duration: normal.
  • Rhythm: Regular if flutter waves conduct at a fixed ratio; may become irregular with variable block.

Atrial Fibrillation

  • Atrial activity: Chaotic baseline with no discernible P waves; fibrillatory (f) waves.
  • Heart rate: varies (controlled vs rapid ventricular response).
  • PR interval: absent.
  • QRS duration: normal (unless pre-existing bundle-branch block).
  • Rhythm: Irregularly irregular.
Quick Visual Mnemonics
  • "3–5 boxes = 60–100 bpm" (normal sinus).
  • ">5 boxes" → bradycardia; "<3 boxes" → tachycardia.
  • Saw-tooth baseline → think Flutter; chaotic baseline → think Fibrillation.
Practical/Clinical Implications (High-Yield)
  • Sinus bradycardia may be physiological in athletes; treat only if symptomatic (e.g., atropine).
  • Sinus tachycardia usually compensatory (fever, pain, hypovolemia)—address underlying cause.
  • Frequent PACs can precede atrial fibrillation; consider electrolyte/thyroid evaluation.
  • PVC burden >10%10\% of total beats may induce cardiomyopathy; evaluate with Holter.
  • Atrial flutter/fibrillation raise stroke risk—necessitate anticoagulation per CHA<em>2DS</em>2VASc\text{CHA}<em>2\text{DS}</em>2\text{VASc} scoring.