CHAPTER 64 Pacemakers and Internal Cardioverter Defibrillators
1 Explain the letters in the NBG coding system for pacemakers
The first three letters refer to chamber paced, chamber sensed, and response to sensed events. The fourth letter generally refers to programmability and rate responsiveness. The fifth letter is rarely used (Table 64-1).
2 What does AOO, VOO, or DOO mode mean?
AOO means pacing in the atrium alone, VOO means pacing in the ventricle only, and DOO means pacing in the atrium and ventricle without sensing or response. Such a device would simply pace at a set lower rate regardless of underlying rhythm.
These modes (ending in OO) are known as asynchronous modes and are useful in the operating room because they allow pacing to be maintained and prevent interference of electrocautery caused by the absence of sensing.
3 What is the result of VVI pacing?
This mode results in ventricular pacing, with sensing in the ventricle alone, and the response is inhibition. This means that the device will release a pacing stimulus unless a sensed event occurs before the expiration of a timer based on the lower rate. If a sensed event occurs, the only response by the device is to inhibit pacing, allowing for the intrinsic rhythm.
4 What is the result of DDI pacing?
DDI means pacing in the atrium and ventricle, sensing in the atrium and ventricle, and the response is constrained to inhibition. The device will thus pace at a set rate in the atrium unless a sensed event occurs at a faster rate in the atrium, which will result in inhibition. The device will then pace in the ventricle at a set rate unless sensing in that chamber occurs at a faster rate, leading to inhibition. This allows for some atrial and ventricular synchrony to be maintained. However, because the response is only inhibition, the pacemaker will not pace in the ventricle as a result of a sensed event in the atrium, which is known as a tracking or triggered response. Thus competitive atrial pacing and variable atrioventricular (AV) timing can occur. This mode is helpful when atrial arrhythmias would lead to rapid heart rates if tracked and is a frequent default mode.
5 What does DDD pacing mean?
DDD pacing means pacing in atria and ventricle, sensing in atria and ventricle, and a dual response (both triggered and inhibited). This is the most common pacing mode. A sensed atrial event occurring before the lower rate timer has expired will inhibit atrial pacing. If no event is sensed in the atrium above the lower rate, an atrial stimulus is released. Both events will initiate a timer expiring before ventricular pacing. If a ventricular-sensed event occurs before this timer expiring, ventricular pacing is inhibited, and intrinsic conduction is maintained. If the AV timer expires, a ventricular pacing stimulus is delivered and is triggered from the previous atrial paced or sensed event. This is known as tracking. If no underlying sensed events occur, atrial and ventricular sequential pacing occur at the lower programmed rate.
6 Describe the difference between a unipolar and bipolar pacemaker
Both scenarios detect electrical signals between two electrodes. In the case of unipolar pacemakers, one electrode is in the heart at the lead-tissue interface, and the other is comprised of the pacemaker generator. Sensing occurs from both electrodes. Thus sensing from the generator may lead to sensing extracardiac signals, muscle myopotentials, external noise, and interference. In the case of bipolar leads, the electrodes are closely spaced at the lead tip within the heart. Sensing between the electrodes is limited mainly to cardiac signals with much less oversensing or external interference.
7 What is an implantable cardioverter defibrillator?
Implantable cardioverter defibrillators (ICDs) are devices much like pacemakers that can pace and support the heart rate in bradycardia. However, these devices have leads with shocking coils, usually one or two, that allow the device to administer internal shocks for tachyarrhythmias (which may be lifesaving). These devices also have complex programming. They allow rapid pacing to terminate regular, fast, more organized arrhythmias in a painless fashion. Algorithms in these devices may even allow differentiation of supraventricular arrhythmias from more serious arrhythmias from the ventricle. These devices are implanted for the treatment of life-threatening arrhythmias and prevention of sudden death.
8 What are common indications for permanent pacing?
In general patients with symptoms from slow heart rate of any type that are not reversible require pacing.
Patients with AV block located below the AV node require pacing. These patients have wide escape complexes, and their escape mechanisms are unreliable and can lead to asystole or death.
Patients with arrhythmias caused by bradycardia such as torsades de pointes responsive to pacing require pacing.
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