Arrhythmia Recognition: The Art of Interpretation

INTERMEDIATE

Clinically, there are some commonly used pacemaker types. Each has its own strengths and weaknesses, making them ideally suited for certain conditions. In this text, we shall review the most popular types and show you some examples of each for your review. As is customary, we will present various clinical strips at the end of the chapter.

Atrial Demand Pacemaker

When a patient has problems with bradycardia-related symptoms but has a normally functioning AV node, the atrial demand pacemaker (AAI) may be the ideal solution. The AAI pacemaker senses the atria and paces the same atria in an inhibitory fashion. Basically, the pulse generator senses the atria for spontaneous activity. If no spontaneous atrial activity is detected during a specified period (the atrial escape interval), then the pacemaker fires an atrial depolarization wave. The wave then spreads normally through the rest of the atria, the AV nodes, and the ventricles. If spontaneous atrial activity is detected, the pacemaker is inhibited from firing, allowing the normal complex to proceed unimpeded (Figure 39-6).

The ECG complex shows two sets of atrial escape intervals. — Figure 39-6 An AAI pacemaker senses an atrial complex and begins to time out an atrial escape interval (AEI 1). The interval is broken by a normally occurring P wave, and the interval is reset (AEI 2). AEI 3 and 4 are not reset and so the pacemaker fires an atrial impulse.

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Description

Ventricular Demand Pacemaker

The ventricular demand pacemaker (VVI) is the ventricular counterpart of the AAI pacemaker discussed previously. This type of pacemaker senses and paces only the ventricles in an inhibitory manner. The pacemaker senses the ventricles for any type of spontaneous activity. When it senses activity, the pacemaker begins to time out a specified period (the VEI, discussed before). If no spontaneous ventricular activity is detected during the VEI, the pacemaker fires a ventricular depolarization wave. If spontaneous ventricular activity is detected, the pacemaker is inhibited from firing, allowing the normal complex to proceed unimpeded (Figure 39-7).

An ECG complex shows two sets of ventricular escape intervals. — Figure 39-7 A VVI pacer senses the ventricles for spontaneous activity. If ventricular activity is sensed, no complex is fired by the pacer (see VEI 1). If ventricular activity is not sensed during the VEI (VEI 2 and 3), the pacer fires and causes a ventricular contraction.

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Description

Atrial activity will in no way interfere with the functioning of a VVI pacemaker. If a P wave occurs but does not conduct to the ventricles, the pacemaker will not be affected in any way and does not get reset. Only ventricular activity is sensed and only the ventricles are paced. These are the main functional characteristics of the VVI pacer.

The main functional characteristics mentioned earlier are also the cause of the major complications associated with this type of pacer, the so-called pacemaker syndrome. The lack of atrial contribution to ventricular filling and the inability of the heart rate to compensate for any form of exertion or exercise will give rise to various clinical symptoms related to decreased cardiac output and hypotension. These include fatigue, light-headedness, syncope, dyspnea, exercise intolerance, congestive heart failure, and anginal symptoms due to myocardial ischemia.

AV Sequential

The AV sequential or DVI pacemaker was the first attempt to maintain AV synchrony. The DVI paces both the atria and the ventricle in a sequential manner when it does fire (Figure 39-8). The atria are depolarized first, and then a few milliseconds later, the ventricles are stimulated. This allows for ventricular filling and helps to maintain a more efficient and effective cardiac output. It does counter some of the events that lead to the pacemaker syndrome but is not as effective as the DDD pacemaker, which we will examine next.

The ECG complex shows two sets of ventricular escape intervals. — Figure 39-8 A DVI pacemaker is reset by a ventricular complex in VEI 1. During VEI 2, there is no spontaneous ventricular activity and the pacemaker fires a sequential atrial and then a ventricular complex. The same occurs in VEI 3.

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Description

The DVI pacemaker senses the ventricle for any spontaneous ventricular activity and then paces in an inhibitory manner. If spontaneous ventricular activity is sensed by the pacemaker during the VEI that is set for that particular patient, then the pacemaker holds off and does not fire. If the pacemaker does not sense any ventricular activity during the VEI, then it fires a sequence of an atrial and then a ventricular depolarization.

The DVI pacemaker does not sense atrial activity in any way. As such, a supraventricular complex will be completely disregarded by the pacer unless it triggers a ventricular complex. This is the main drawback of the DVI pacer. By disregarding supraventricular complexes, the pacer will compete with the normal heart in some cases. PVCs and premature junctional contractions (PJCs) are sensed and will inhibit the pacer.

Automatic Pacemaker

The automatic, or DDD, pacemaker is the “top-of-the-line” model of artificial pacemakers. The DDD pacer senses both the atria and the ventricles (Figure 39-9). It also has the ability to pace either the atria, the ventricles, or both in a sequential manner. The DDD pacemaker is basically a combination of the AAI, the VVI, and the DVI pacemakers due to the presence of the dual sensing/dual pacing leads.

The ECG complex shows multiple atrial and ventricular escape intervals. — Figure 39-9 A DDD pacemaker senses spontaneous atrial and ventricular activity during atrial escape interval (AEI) 1 and does not fire. During AEI 2, there is no spontaneous atrial activity and the pacemaker fires a paced atrial complex. The pacer then waits an appropriate amount of time, and, when it does not sense any conduction to the ventricles, the pacer fires a sequential ventricular complex. During AEI 3, the pacemaker senses some spontaneous atrial activity and does not pace the atria. However, since conduction to the ventricles did not occur, a sequential ventricular complex was fired. Conducted PACs, PJCs, and PVCs will reset the pacer. This is what occurs during AEI 4—a PVC resets the timer. The sequence then continues as expected.

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Description

The beauty of this pacemaker is that it will sense the heart for any atrial or supraventricular activity. If it doesn’t sense any, it will trigger a supraventricular complex to stimulate the atria. The pacer will then wait an appropriate amount of time for the supraventricular complex to be normally conducted to the ventricles; if it doesn’t make it through, the ventricular lead will then pace the ventricles sequentially.

Any premature atrial contraction (PAC) that occurs will be sensed and will inhibit the atrial response. It will not, however, inhibit the separate ventricular sensing and pacing function. If the PAC is not conducted to the ventricles, the pacer will fire a sequentially timed ventricular complex. Likewise, PJCs and PVCs will be sensed by the pacer, but these will inhibit both atrial or ventricular responses.

The exceptional capability of the DDD pacemaker to sense and respond to atrial and/or ventricular activity gives it a tremendous clinical edge over all of its competition. The DDD pacer will sense a sinus tachycardia that can develop due to a physiologic increase in the heart rate, as can occur when the patient is anxious, febrile, or exercising, and make sure that the ventricles match the rate in a near-normal sequential manner. This ability to respond to physiologic demand, along with the sequential nature of the atrial and ventricular depolarizations, essentially negates most complications seen in the pacemaker syndrome and allows for near-normal functioning. These advantages give the DDD pacer a preferential status as the pacemaker of choice for most patients.

We should mention that for protective measures, an upper limit to the rate of ventricular conduction of a supraventricular tachycardia is always defined in order to protect the patient from a potential disaster. For example, if the patient were in atrial flutter, a 1:1 conduction rate to the ventricles could result in a ventricular response of 300 beats per minute (BPM). As you can imagine, this rate could be life-threatening. Because of this, the upper limits of conduction can be programmed into the pulse generator by the clinician, allowing for maximal physiologic benefit for the patient under normal circumstances, while still maintaining an upper limit protection against unimpeded ventricular conduction.