Device Therapy: Pacemakers and ICDs
Core concepts
Pacemakers treat bradyarrhythmias; ICDs treat life-threatening ventricular arrhythmias. Know the NASPE/BBG code: first letter chamber paced (A, V, D), second chamber sensed, third response (I inhibited, T triggered, D dual), fourth rate modulation. Common modes: VVI (ventricular pacing/sensing, inhibited), DDD (dual chamber, tracks the atrium), and AAI. Single- vs dual-chamber selection depends on sinus node function and AV conduction.
Understand pacing spikes and capture: a spike followed by a P or QRS means capture; failure to capture, failure to sense (undersensing causing inappropriate pacing), and oversensing (inappropriate inhibition) are the core malfunctions. Pacemaker syndrome (loss of AV synchrony with VVI pacing) causes fatigue, pulsation, and hypotension. ICDs deliver shocks for VF/fast VT; know that shocks can be inappropriate (AF, lead fracture, noise) and that antitachycardia pacing can terminate VT without a shock. Magnet application usually converts a pacemaker to asynchronous mode and suspends ICD tachytherapy.
Question traps
- Confusing failure to capture (no depolarization after spike) with failure to sense (spikes at wrong times).
- Forgetting that ICDs do not treat bradycardia unless they have pacing capability.
- Overlooking lead dislodgement or fracture as a cause of inappropriate shocks.
Rule of thumb
Read the spike: spike + QRS = capture. Spikes where they don't belong = sensing problem. No QRS after spike = capture problem.