17.2 Defibrillators and Pacemakers

Defibrillators and pacemakers are crucial cardiac devices that save lives. Defibrillators shock the heart to restore normal rhythm during emergencies, while pacemakers regulate heartbeats in patients with slow or irregular rhythms.

These devices work by delivering electrical pulses to the heart. Defibrillators use high-energy shocks for immediate intervention, while pacemakers provide low-energy pulses for ongoing heart rate control. Both are essential in managing various cardiac conditions.

Defibrillation Devices

Defibrillation and Cardioversion

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• Defibrillation delivers a controlled electric shock to the heart to restore normal heart rhythm in cases of life-threatening arrhythmias such as ventricular fibrillation (VF) or pulseless ventricular tachycardia (VT)
• Cardioversion is a procedure that uses electrical shock to convert an abnormally fast or irregular heart rhythm back to a normal rhythm, typically used for atrial fibrillation (AF) or atrial flutter
• Both defibrillation and cardioversion work by depolarizing the heart muscle cells simultaneously, allowing the heart's natural pacemaker to regain control and establish a normal rhythm
• The main difference between defibrillation and cardioversion is that defibrillation is used in emergency situations for life-threatening arrhythmias, while cardioversion is typically a planned procedure for less urgent heart rhythm disturbances

External and Implantable Defibrillators

• Automated External Defibrillator (AED) is a portable device designed for use by laypersons to treat sudden cardiac arrest in out-of-hospital settings
  • AEDs analyze the heart's rhythm and determine if a shock is needed, providing voice and visual prompts to guide the user through the defibrillation process
  • These devices are commonly found in public places such as airports, shopping malls, and sports facilities
• Implantable Cardioverter Defibrillator (ICD) is a small, battery-powered device implanted under the skin to continuously monitor the heart's rhythm and deliver electrical shocks when necessary to treat life-threatening arrhythmias
  • ICDs are programmed to detect specific arrhythmias and deliver appropriate therapy, such as low-energy pacing pulses for bradycardia or high-energy shocks for VF or VT
  • These devices are typically recommended for patients at high risk of sudden cardiac death due to conditions such as previous heart attacks, heart failure, or inherited heart disorders

Pacemaker Components

Pacemaker and Leads

• A pacemaker is a small, battery-operated device that helps regulate the heart's rhythm by sending electrical impulses to the heart when it beats too slowly (bradycardia) or irregularly
• Pacemaker leads are insulated wires that carry electrical signals between the pacemaker and the heart
  • One end of the lead is connected to the pacemaker, while the other end is placed in the heart's right atrium, right ventricle, or both, depending on the patient's specific condition
  • Leads can be unipolar (one electrode) or bipolar (two electrodes) and are designed to be durable and resistant to fracture or damage

Pulse Generator and Programming

• The pulse generator is the main component of the pacemaker, containing the battery and electronic circuitry that generates the electrical pulses sent to the heart
  • Modern pulse generators are small, typically about the size of a half-dollar coin, and are implanted under the skin near the collarbone
  • The battery life of a pacemaker can range from 5 to 15 years, depending on the device's settings and the patient's condition
• Pacemaker programming involves adjusting the device's settings to optimize its performance for the individual patient
  • Physicians use an external programmer to communicate with the pacemaker wirelessly, allowing them to change parameters such as pacing rate, pulse amplitude, and sensitivity
  • Regular follow-up visits are necessary to monitor the pacemaker's function, adjust settings as needed, and assess battery life

Pacemaker Functions

Sensing and Pacing

• Sensing refers to the pacemaker's ability to detect the heart's intrinsic electrical activity
  • The pacemaker continuously monitors the heart's rhythm through the leads and determines when to initiate pacing based on programmed settings
  • If the heart's intrinsic rate falls below the programmed threshold, the pacemaker will generate an electrical pulse to stimulate the heart
• Pacing is the delivery of electrical impulses to the heart to maintain an appropriate heart rate and rhythm
  • The pacemaker generates these impulses according to the programmed mode and settings, which can be customized for each patient
  • Different pacing modes exist, such as single-chamber pacing (atrial or ventricular) or dual-chamber pacing (atrial and ventricular), depending on the patient's specific needs

Cardiac Conditions Treated by Pacemakers

• Bradycardia is a slow heart rate, typically defined as a resting heart rate below 60 beats per minute
  • Pacemakers are commonly used to treat symptomatic bradycardia caused by conditions such as sick sinus syndrome, heart block, or certain medications
  • By maintaining an appropriate heart rate, pacemakers can alleviate symptoms such as fatigue, dizziness, and fainting associated with bradycardia
• Tachycardia is a fast heart rate, typically defined as a resting heart rate above 100 beats per minute
  • While pacemakers are primarily used to treat bradycardia, some advanced devices called antitachycardia pacemakers can also detect and treat certain types of tachycardia
  • These pacemakers can deliver rapid pacing pulses to overtake and terminate the abnormal rhythm, or provide low-energy cardioversion shocks if necessary