An implantable cardioverter-defibrillator, or ICD, is a small medical device placed under the skin of the upper chest to protect people at risk for life-threatening heart rhythms. It is connected to the heart by insulated wires called leads that sense electrical activity and can deliver therapy when needed. ICDs matter because rhythms such as ventricular tachycardia and ventricular fibrillation can stop the heart from pumping blood effectively within seconds.
By acting automatically, an ICD can provide emergency treatment before a person reaches a hospital.
The ICD constantly measures the timing and pattern of electrical signals in the heart. If it detects a dangerous rhythm, it may first try painless or low-energy pacing pulses, then deliver a stronger shock if the rhythm does not stop. The shock briefly depolarizes many heart cells at once, giving the heart's natural pacemaker a chance to regain control.
This technology combines sensors, batteries, pulse generators, electrodes, and computer algorithms in a compact implanted system.
Key Facts
- An ICD is implanted under the skin, usually below the collarbone, with leads threaded through a vein into the heart.
- The device monitors heart rhythm by sensing voltage changes from cardiac electrical activity.
- Heart rate can be calculated from beat interval: heart rate = 60 / period, where period is in seconds per beat.
- Electrical energy delivered by a shock can be estimated by E = 1/2 C V^2 for a charged capacitor.
- Dangerous ventricular arrhythmias reduce cardiac output because the ventricles do not contract in a coordinated way.
- ICD therapy may include antitachycardia pacing, cardioversion, defibrillation shocks, and stored rhythm data for doctors.
Vocabulary
- Implantable cardioverter-defibrillator
- An implantable cardioverter-defibrillator is a battery-powered device that monitors heart rhythm and delivers electrical therapy for dangerous arrhythmias.
- Arrhythmia
- An arrhythmia is an abnormal heart rhythm that may be too fast, too slow, or irregular.
- Lead
- A lead is an insulated wire that connects the ICD to the heart and carries sensing signals and therapy pulses.
- Defibrillation
- Defibrillation is the use of an electrical shock to stop chaotic heart activity so a normal rhythm can restart.
- Capacitor
- A capacitor is an electrical component that stores energy in an electric field and can release it quickly during an ICD shock.
Common Mistakes to Avoid
- Thinking an ICD prevents all heart disease is wrong because it treats certain rhythm emergencies but does not cure blocked arteries, weak heart muscle, or other underlying conditions.
- Confusing an ICD with a standard pacemaker is wrong because a pacemaker mainly treats slow rhythms, while an ICD can detect and shock dangerous fast ventricular rhythms.
- Assuming a shock restarts a stopped heart is wrong because defibrillation is designed to stop chaotic electrical activity, not power a heart with no electrical activity.
- Ignoring lead placement is wrong because the device must sense and deliver current through electrodes positioned so the heart receives effective therapy.
Practice Questions
- 1 An ICD detects ventricular beats every 0.30 s. What is the heart rate in beats per minute using heart rate = 60 / period?
- 2 An ICD capacitor has capacitance 120 microfarads and is charged to 700 V. Estimate the stored energy using E = 1/2 C V^2, with capacitance in farads.
- 3 Explain why an ICD lead must both sense small electrical signals from the heart and deliver much larger therapy pulses during a dangerous arrhythmia.