Artificial pacemakers can sense, depolarize, and pace either the atria, the ventricles, or both in a sequential fashion. The pacemakers can also either automatically trigger responses at a certain specified amount of time or they can function in an inhibitory manner. The pacemaker functions in an inhibitory manner when any electrical activity sets off an internal alarm clock that will not fire a complex for a certain length of time. The artificial pacemakers can also be programmed to perform certain functions and to respond differently in the presence of a tachyarrhythmia.
In order to organize all of the various functions of the artificial pacemakers into a usable system, the Intersociety Commission for Heart Disease came up with a system that is presently in use. This system uses a five-letter code with each letter representing a specific function (Figure 39-2).
Figure 39-2 Pacemaker codes established by the Intersociety Commission for Heart Disease.
© Jones & Bartlett Learning.
DescriptionThe first letter in the code refers to the chamber that is paced—in general, pacemakers will either pace the atria (A), the ventricles (V), both (D for dual), or none (O). If the pacemaker paces both the atria and the ventricles, the pacing occurs sequentially to try to normalize the mechanical contraction of the various chambers as much as possible.
The second letter in the code refers to the chamber that is sensed—in other words, the chamber that the pulse generator will monitor for spontaneous depolarizations. The chambers that are sensed are either the atria (A), the ventricles (V), both (D for dual), or none (O).
The third letter in the code refers to the response that the pacemaker will take when it senses an event. The triggered (T) response is when the pacemaker activates a depolarization wave to a sensed event. An example of a triggered response is when the pacemaker fires an impulse to the ventricles after it senses an atrial depolarization.
The inhibited (I) response is when the pacemaker responds to a sensed event by not firing a response for a certain amount of time. This concept appears at first glance to be counterintuitive; the pacemaker is programmed to fail in response to an event. But, it is actually a fail-safe method to prevent the pacemaker from competing with an intrinsic cardiac pacemaker. Let’s look a little closer at this type of response.
Let’s suppose that a pacemaker senses an atrial depolarization (Figure 39-3). The pacemaker will then automatically set up a time period, known as the ventricular escape interval, or VEI, during which it patiently waits for the ventricles to respond to the atrial impulse. The hope of the pacemaker is that the atrial impulse will proceed normally through the atrioventricular (AV) node and the electrical conduction system and eventually cause a ventricular depolarization. The VEI is usually a little bit longer than the normally occurring PR interval would be, in order to allow for normal conduction to occur and finish through the conduction pathways. After the VEI is completed, if the ventricles failed to respond because of an AV block or whatever, or if an intrinsic ectopic ventricular pacemaker failed to fire, then and only then would the pulse generator fire an impulse.
Figure 39-3 Inhibited response to an event. When the pacemaker senses an atrial complex (see 1 in figure), it sets off a stopwatch. The stopwatch starts to measure a predesignated amount of time during which the pacemaker will not fire. This period is known as the ventricular escape interval (VEI). If after the VEI is over, there has been no ventricular response, then the pacer fires a pacemaker spike and a ventricular depolarization.
© Jones & Bartlett Learning.
The third possibility that can occur is the dual (D) response. This refers to the possibility of the pulse generator to respond in a triggered manner for some pre-programmed cases and in an inhibitory manner for other pre-programmed events.
Finally, the fourth possible response to an event is none (O). In this case, the pulse generator responds in neither a triggered nor an inhibitory way to an event.
The fourth and fifth letters in the pacemaker code relate to advanced functions of certain pulse generators. The review of those features is beyond the scope of this text, and the reader is encouraged to seek additional information in more advanced books on artificial pacemakers or from the individual manufacturers of specific pacemaker models.