Cardiac Pacemakers

Basics

Description

  • A device that uses external electrical impulse to contract the myocardium and provide a perfusing rhythm
  • Methods of cardiac pacing:
    • Transcutaneous pacing
      • Two pads are placed on the chest in the anterior-lateral or anterior-posterior position
      • The pacing current is gradually increased until electrical capture occurs and produces a palpable pulse
      • Emergent intervention used only until transvenous pacing or another definitive therapy can be applied
    • Temporary transvenous pacing:
      • A pacemaker wire is floated through single-lumen central venous catheter into the right atrium (RA) or right ventricle (RV) and connected to an external generator outside of the body
      • Used as a bridging therapy until a permanent pacemaker (PPM) can be placed or there is no longer a need for a pacemaker
  • Implanted PPM has 3 components:
    • Battery-powered energy source: Lithium batteries last 7–10 yr
    • Generator: A sophisticated computer with pro-grammable parameters
    • Leads connected to the RV/RA: Typically sense intrinsic electrical cardiac activity and pace the myocardium as needed
  • Pacemaker magnet:
    • If placed over PPM, reverts pacer to asynchronous mode at a constant predetermined rate regardless of the heart’s native activity
    • Useful if pacer spikes not present on ECG
    • A magnet’s pacing rate may indicate low battery (rate decreased by <10% of predetermined rate)
    • A completely depleted battery will not respond to a magnet
  • MRI-conditional pacemakers:
    • Devices shown to pose no known hazard under certain specified conditions according to the FDA:
      • Nonthoracic MRI at 1.5 T was recently shown to have a wide safety margin with implanted non-MRI-conditional cardiac devices

Pacemaker Terminology

  • Fixed mode:
    • Pacemaker is set to fire at a specific rate regardless of underlying rhythm
    • Uncommon
  • Demand mode:
    • Senses the underlying rhythm and fires only when necessary
    • Begins pacing if the intrinsic rhythm is absent or less than the set rate
  • Sensing:
    • Identification of intrinsic rhythm
    • Pacemakers have a 5-letter code that describes their function. The first 3 letters are relevant to ED management
    • First letter in code indicates which chamber is 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 the pacemaker’s response to the sensed intrinsic activity:
      • T: Trigger (a sensed beat results in a pacing response)
      • I: Inhibit (a sensed beat precludes pacemaker function)
      • D: Dual (pacemaker is capable of both functions)
      • O: No response
      • Most pacemakers are VVI (single lead) or DDD (2 leads)

Complications

  • Pacemaker-associated infection:
    • Infection of pacemaker components often associated with endocarditis
    • Infection rate is ∼2%
    • Staphylococcus epidermidis and Staphylococcus aureus account for >90% of infections
    • Transesophageal echocardiography is the preferred diagnostic modality
  • Venous thrombosis:
    • Overall incidence rate of 30–50%
    • Acute obstruction rate <3%
    • Pulmonary embolism is rare
  • Failure to pace:
    • Component failure and battery depletion are rare with routine checks
    • Lead fracture or displacement
    • Oversensing activity or external electrical interference
  • Failure to capture:
    • Lead dislodgement is common
    • Twiddler syndrome: Twisting of the pacemaker pulse generator causing wires to coil around the generator resulting in lead displacement
  • Elevated myocardial threshold
    • Ischemia
    • Hyperkalemia
    • Hypoxia
  • Antiarrhythmics or chemotherapy
  • Fibrosis
  • Pacemaker-mediated tachycardia:
    • Reentry dysrhythmia that occurs with dual-chamber pacemakers
    • Usually caused by PAC or PVC that disrupts normal cycle resulting in retrograde conduction
    • Built-in safeguards typically set max rate to 140 bpm
  • Runaway pacemaker:
    • Ventricular tachycardia triggered by battery depletion or component failure
    • May see heart rate (>200 bpm) with hemodynamic compromise

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