Cardiac Rhythm Strip Reference: Sinus & Atrial Arrhythmias

Normal Sinus Rhythm

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

Sinus Bradycardia

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

Sinus Tachycardia

• Heart rate: >100\ \text{bpm}
• PR interval: normal.
• QRS duration: normal.
• Rhythm: Regular.
• RR interval: <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\ \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\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., 2:1 → ~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\% of total beats may induce cardiomyopathy; evaluate with Holter.
• Atrial flutter/fibrillation raise stroke risk—necessitate anticoagulation per \text{CHA}2\text{DS}2\text{VASc} scoring.