Premature Atrial Contractions (PACs)

Definition

• Premature heartbeats originating in the atria (upper chambers of the heart).
• Often occur in patients with lung disorders (e.g., COPD) and in healthy individuals.

Causes

• Caffeine
• Alcohol use
• Certain medications

Symptoms

• Generally asymptomatic
• Occasional palpitations
• No treatment required in healthy individuals

Mechanism

• Atria activation by ectopic site instead of SA node.
• Results in unusual, non-sinus morphology P wave.
• Early activation can merge P wave with preceding T wave, creating a fusion wave.

ECG Characteristics

• Retrograde Conduction:
  • Ectopic site near AV node can cause inverted P wave.
  • Shorter PR interval due to quicker signal reaching AV node.
• Non-Compensatory Pause:
  • Ectopic activation resets SA node timing.
  • Seen as changes in PP intervals on ECG.
• Differentiation from PVCs:
  • PACs affect SA node firing; PVCs do not.
  • PACs show non-compensatory pause; PVCs do not.
  • PVCs do not alter PP interval.

Conduction Variations

• Normal Conduction:
  • Narrow QRS complex, AV node and ventricular conduction unaffected.
• Aberrant Conduction:
  • Widened QRS complex, often with right bundle branch block morphology.
  • Differentiation relies on presence of preceding P wave and non-compensatory pause.
• Non-Conducted PAC:
  • Arrives too early; AV node in refractory period and not activated.