Implanted Electrical Cardiac Devices
The Basics of Pacemakers and Implantable Cardioverter-Defibrillators
Dec 13, 2008
Some individuals may have problems that involve heart rate and rhythm. They can be managed with certain medications, procedures, and/or cardiac devices. Regarding devices, two types are available: pacemakers and implantable cardioverter-defibrillators. They can exist as separate devices but are available as combination devices for patients who need both.
Pacemakers
Pacemakers are cardiac devices that stimulate heart contraction. There are many clinical indications for these devices, such as sick sinus syndrome and third-degree heart block. In general, pacemakers are used by patients who have slow or absent heartbeats, which are due to problems with the sinus node firing electrical impulses or the atrioventricular (AV) node conducting impulses from the atria to the ventricles of the heart.
The device consists of a battery-powered pulse generator, implanted under the skin (subcutaneously) or muscle (submuscularly), and leads that connect to the atria and/or ventricles of the heart. After placement, the pacemaker is programmed in a certain way and designated with a shorthand of at least three letters:
- The first letter specifies which heart chambers are paced (A for atria, V for ventricles, or D for dual to involve both chambers).
- The second letter designates which heart chambers are sensed for cardiac events (A for atria, V for ventricles, or D for dual to involve both chambers).
- The third letter refers to the function of the pacemaker in response to a sensed event (T for trigger of pacing, I for inhibition of pacing, or D for dual to involve different functions for each chamber)
- Optionally, the pacemaker code can have a fourth or even a fifth letter for additional attributes, such as the ability to increase the heart rate during exercise.
For example, a pacemaker with the code AAT paces the atrium, senses events in the atrium, and triggers contraction in response to an event. Certain configurations, such as DDD and VVI, are commonly used because of their greater survival benefit compared with other codes.
Nevertheless, pacemakers can be prone to complications. Various problems can occur when pacemakers incorrectly sense noncardiac electrical activity (oversensing), do not sense cardiac electrical activity (undersensing), do not pace as intended (failure to pace), or pace correctly without the intended response from the heart (failure to capture). Other issues, such as device migration over time, are less common.
Implantable Cardioverter-Defibrillators
Implantable cardioverter-defibrillators (ICDs) are devices that attempt to restore normal heart rhythms. They are used for certain abnormally fast heart rhythms (tachyarrhythmias), particularly ventricular tachycardia (VT) and ventricular fibrillation (VF). Patients with persistent VT or VF despite attempts to address any reversible causes may benefit from ICDs.
An ICD is designed similarly to a pacemaker with a pulse generator and leads. The ICD detects a tachyarrhythmia when the sensed heart rate exceeds the heart rate programmed as the threshold. When this happens, the ICD shocks the heart to halt the fast rhythm and restore a normal rhythm.
Like pacemakers, ICDs can have technical problems. The most common is inappropriate cardioversion in which the ICD shocks the heart in response to a fast rhythm that does not need such a response. Otherwise, ICDs can have issues similar to those of pacemakers.
Final Words
Pacemakers and ICDs can mean the difference between life and death for certain individuals. However, they should not be considered foolproof, a reason for patients and doctors to watch them carefully.
References
- eMedicine - Pacemaker and Automatic Internal Cardiac Defibrillator
- Merck Manual - Arrhythmias and Conduction Disorders
