Cardiac Pacemakers

Cardiac Pacemakers is a topic covered in the 5-Minute Emergency Consult.

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Basics

Description

  • A device that uses electrical impulses to contract the heart muscles and provide an adequate 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 therapy can be applied
    • Temporary transvenous pacing:
      • A pacemaker wire is placed through 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
    • 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 1st 3 letters of the code are necessary:
    • 1st letter in code indicates chamber being sensed by pacemaker:
      • A: Atria
      • V: Ventricle
      • D: Dual (both chambers)
    • 2nd letter in code indicates chamber that can be paced:
      • A: Atria
      • V: Ventricle
      • D: Dual (both chambers)
    • 3rd 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).

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's 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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Citation

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TY - ELEC T1 - Cardiac Pacemakers ID - 307123 Y1 - 2016 PB - 5-Minute Emergency Consult UR - https://emergency.unboundmedicine.com/emergency/view/5-Minute_Emergency_Consult/307123/all/Cardiac_Pacemakers ER -