Cardiac Pacemakers

Basics

A device that uses electrical impulses to contract the heart muscles and provide a perfusing pulse
Methods of cardiac pacing:
- Transcutaneous pacing:
  - 2 pads are placed on the chest in the anterior-lateral or anterior-posterior position
  - The pacing current is gradually increased until electrical capture occurs with a pulse
  - Emergency therapy used only until transvenous pacing or another definitive therapy can be applied
- Temporary transvenous pacing:
  - A pacemaker wire is placed through single lumen central venous access into the right atrium (RA) or right ventricle (RV) and connected to an external generator outside of the body
  - Used as a bridge until a permanent pacemaker can be placed or there is no longer a need for a pacemaker
Permanent, implanted pacemaker has 3 components:
- A battery-powered energy source:
  - Lithium batteries last 7–10 yr
- Generator:
  - A sophisticated computer with many programmable parameters
- Leads connected to the RV/RA:
  - Typically sense intrinsic electrical activity of the heart and pace the myocardium as needed
Pacemaker magnet:
- Placed over pacemaker generator
- Converts pacer to asynchronous mode which will revert to a constant predetermined rate regardless of the heart's native activity
- Useful if pacer spikes not present on ECG
- A depleted battery will result in decrease in magnet rate by ∼10%

Pacemaker Terminology

Fixed mode:
- The pacemaker is set to fire at a set rate regardless of patient's underlying rhythm
- Rarely seen
Demand mode:
- The pacemaker fires only when necessary
- It senses the underlying rhythm
- It will only pace if the intrinsic rhythm is absent or less than a set rate
Sensing:
- Pacemaker's ability to determine whether the heart has an intrinsic rhythm
All pacemakers have a 5-letter code to describe their function
For ED purposes, only the first 3 letters of the code are necessary:
- First letter in code indicates chamber being sensed by pacemaker:
  - A: Atria
  - V: Ventricle
  - D: Dual (both chambers)
- Second letter in code indicates chamber that can be paced:
  - A: Atria
  - V: Ventricle
  - D: Dual (both chambers)
- Third letter in code describes pacemaker's response to sensed intrinsic complex:
  - T: Trigger (a sensed beat results in a pacing response as when a sensed atrial beat provokes a subsequent ventricular beat)
  - I: Inhibit (a sensed beat precludes pacemaker function)
  - D: Dual (a pacemaker is capable of both functions)
  - O: No response
- The most common pacemakers are VVI (single lead) and DDD (two leads)

MRI-conditional pacemakers
- Devices shown to pose no known hazard under certain specified conditions, are labeled “MRI-conditional” by the FDA
- Nonthoracic MRI at 1.5 T was recently shown to have a wide safety margin with an implanted non–MRI-conditional cardiac devices

Pacemaker-associated infection:
- Infection of pacemaker components often associated with endocarditis
- Staphylococcus epidermidis and Staphylococcus aureus account for >90% of infections
- Transesophageal echo is the preferred diagnostic method
Venous thrombosis:
- Very common (overall incidence 30–50%)
- Symptomatic, acute obstruction is rare (<3%)
- Pulmonary embolism is rare
Pacemaker failure to discharge impulse
- Component failure is rare
- Battery depletion is rare with routine checks; it is not abrupt
- Lead fracture or disconnection
- Oversensing of muscular activity or external electrical interference
Pacemaker failure to capture myocardium:
- Lead dislodgment is common
- Twiddler syndrome:
  - Unintentional manipulation of pacemaker generator causing lead to be dislodged from myocardium
- Elevated myocardial threshold:
  - Hyperkalemia
  - Ischemia
- Change in cardiac (QRS) morphology
Pacemaker-mediated tachycardia:
- Occurs with dual-chamber pacemakers
- A re-entry rhythm using generator and intrinsic conduction system
- Max rate typically 140 bpm due to built-in safeguards
Runaway pacemaker:
- Rare; triggered by battery depletion or component failure
- Often rapid rates (>200 bpm) with hemodynamic compromise

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