Cardiac Pacemakers

Basics

Description

  • 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

Etiology

  • 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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