Pacemakers – Key Vocabulary

Indications for Permanent Pacemakers

• Central governing concept: (CO = HR \times SV). Any pathology that drops heart rate (HR) enough that stroke volume (SV) can no longer compensate becomes an indication for artificial pacing.
• High-yield exam list (know verbatim):
  • Sick-sinus syndrome (brady–tachy, sinus arrest, sinus pauses)
  • AV-conduction delays/blocks (1°, 2°, 3°)
  • Post-cardiopulmonary bypass surgery bradyarrhythmias

Cardiac-Muscle & Electrical Basics (Why the Device Works)

• Both ventricles contain exactly the same number of muscle fibres; LV fibres are thicker (↑diameter) because they chronically work against higher afterload.
• EACH individual fibre—regardless of ventricle—needs:
  • A fixed amount of current (\text{mV} or \text{mA})
  • For a fixed time interval (ms)
  • Delivered electricity above the threshold ➔ ventricular systole ("capture").

Pacemaker Types

• Synchronous ("demand")
  • Fires only when needed; goal = let intrinsic conduction work whenever possible.
• Asynchronous ("fixed-rate")
  • Fires at a programmed rate regardless of native activity ➔ risk of competition arrhythmias.

Pacemaker Coding System (3 letters you must decode)

1. Chamber paced: V (ventricle) / A (atrium) / D (dual—A+V)
2. Chamber sensed: V / A / D (lead(s) monitor intrinsic activity here)
3. Response to sensing:
   • I = Inhibit (withhold pulse if intrinsic beat sensed)
   • T = Trigger (fire when intrinsic event detected)
   • D = Dual (can do either)

Implantation & Threshold-Testing Workflow (high-yield sequence)

1. Lead advanced via subclavian v., across tricuspid valve, tip lodged in RV endocardium.
2. Rep turns generator ON ➔ increases output until 1st capture; record value.
3. Gradually decrease output until capture LOST; record value.
4. WAIT ~5 min (fibres stretch/relax, local irritation subsides).
5. Re-increase output ➔ 2nd capture (almost always at a LOWER threshold).
6. Program this 2nd (lower) threshold into generator; generator ↔ tip continuously communicate.

How Synchronous Pacing Actually Decides to Fire

• SA node → AV node → His → Purkinje → reaches RV-lead tip.
• Lead measures incoming electricity:
  • E{sensed} \ge E{programmed} ➔ Inhibit (native beat adequate)
  • E{sensed} < E{programmed} ➔ Trigger (generator emits pulse)

Classic Pacemaker Modes for the Exam

VVI Mode

• Letters = V (paced) / V (sensed) / I (inhibit).
• ONE lead in RV → implies underlying AV-block problem.
• Rhythm strip clues:
  • Narrow, tall positive pacer spike immediately before a wide, negative QRS (resembles PVC).
  • Test items: “How many leads?”, “Where is the lead?”, “Identify the pacer spike vs QRS.”

DDD Mode

• Letters = D / D / D ➔ TWO leads (RA + RV).
• Covers both atrial & ventricular dysfunction.
• Test usually shows strip and asks location/number of leads.

DVI Mode (mentioned, rarely tested since 4 yrs)

• Know it exists; low probability.

Pacemaker Terminology & Malfunctions

• Capture – EVERY pacer spike followed by the intended chamber’s depolarisation (P-wave or QRS).
• Sensing – Device evaluates intrinsic beats and decides whether to fire.
• Failure to Fire – No spike when one was needed.
  • Etiology: generator end-of-life (EOL) / depleted battery.
• Failure to Capture – Spike present, but no chamber response.
  • Etiology: lead fracture, lead dislodgement, poor tip-myocardium contact.
• Failure to Sense – Generator ignores intrinsic activity; spikes appear haphazardly.
  • Etiology: sensitivity setting too low/high, lead issues, electromagnetic interference.

Biventricular (Bi-V) / CRT Devices

• Purpose: Re-establish synchrony between RV & LV contractions.
  • Physiologic delay ≈ 3{-}4\text{ ms} = 0.003!–!0.004\ \text{s} (RV then LV).
  • Pathology elongates or abolishes this delay ➔ dyssynchrony, ↓CO, pulmonary backup.
• Lead set-up (3 leads):
  • RA lead ➔ pace RA
  • RV lead ➔ pace RV
  • CS (coronary sinus) lead ➔ pace LV

Rapid-Fire Exam Q&A (Likely One-liners)

• Single lead in atrium only ➔ problem = SA-node disease.
• Single lead in ventricle only ➔ problem = AV-block.
• In an A-V pacer, the 1st spike triggers atrial contraction.
• Leads are introduced via the subclavian vein (NOT the IJ).
• Why do paced QRS complexes look like PVCs (negative deflection)?
  • Activation travels bottom→top (Purkinje → myocardium) instead of the normal top→bottom.
• Asynchronous pacing vs demand pacing – know definitions.
• Can you defibrillate a paced patient? YES, just do NOT place paddle directly over generator.

Practical / Ethical / Real-World Points

• Device = "an incredibly intelligent computer" that adapts to changing thresholds over the years.
• Waiting 5–10 min during implant respects myocardial irritation & immune response—reduces need for high chronic outputs, prolongs battery life (ethical use of resources).
• Bi-V/CRT devices improve HF quality-of-life—ties to evidence-based medicine and healthcare economics.