Pace Maker

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What does a pacemaker do?
As mentioned above, a pacemaker prevents the heart from going too slow. In some cases, it prevents the heart from stopping. The pacemaker lead is capable of pacing when a small amount of electrical current is delivered from the generator: just enough energy is delivered to electrically capture enough heart cells at the tip of the lead so as to result in the generation of an electrical signal that can propagate throughout the electrically connected heart chambers. Of note, this amount of energy is so small that it typically is not felt by the patient (although the patient might feel the heart beating, they will not experience any sensation from the pacemaker lead directly). Rarely, if the pacemaker lead is close to the nerve that supplies the diaphragm, the patient may experience diaphragmatic stimulation, resulting in an uncomfortable hiccup-like movement (another potential complication of placement of the device). The leads also can record the electrical activity of the heart that might occur due to the patient’s own heart beat, and that recorded electrical activity can be communicated back to the generator and thereby be detected or sensed. Finally, the pacemaker can react (and adjust) to the paced or sensed beats.

In brief, through the pacemaker leads the device essentially can do three things: pace, sense intrinsic heart beats, and react to the paced or sensed beats. There are a few other functions that might be employed, but these three activities are the primary building blocks. In fact, the way physicians discuss the programming or mode of the pacemaker involves an abbreviation that can immediately communicate the function of these capabilities. This abbreviation scheme breaks down the description of the programming into three letters: the first letter describes where the pacemaker can pace, the second letter describes where the pacemaker can sense an intrinsic beat, and the third letter describes what the reaction to a sensed or paced beat will be (if any).

For example, if a patient has a single lead in the ventricle that paces, senses, and is inhibited from pacing if an intrinsic beat is sensed would be described as having a pacemaker that is programmed to the VVI mode: the first letter will be a “V” for ventricle (the pacemaker can pace in the ventricle); the second letter will be a “V” (the pacemaker can sense beats in the ventricle); and the third letter will be an “I” for “inhibit” (when an intrinsic beat is sensed, it inhibits pacing). A pacemaker that is programmed similarly with a lead only in the atrium could be programmed to the AAI mode (“A” for atrium).

It gets a bit more complicated when two leads are present. In such cases, the most common programming is DDD (“D” here is for “dual”): the pacemaker can pace in the atrium and the ventricle and the pacemaker can pace and sense in the ventricle. The third “D” means that the pacemaker is inhibited in a given chamber if it senses intrinsic activity in that chamber (e.g., an intrinsic electrical signal in the atrium will inhibit atrial pacing) and it means that the ventricular lead will track the atrial activity.
The tracking process in dual chamber pacemakers also requires a bit more explanation.

After atrial activation (whether it’s a paced atrial beat or sensed atrial activity), if no intrinsic ventricular activity is sensed after a specified amount of time, the pacemaker will pace in the ventricle. So if a patient has a dual chamber pacemaker programmed in the DDD mode and the patient’s own electrical activity happens to be working just fine for the time being, both intrinsic atrial and ventricular activity will be sensed, the pacemaker will be inhibited, and no pacing will occur. In fact, if the intrinsic conduction system is working fine, pacing should indeed be avoided for two reasons: first, it avoids unnecessary depletion of the battery and, second, it is typically more healthy to allow the more organized electrical activity of the heart’s native conduction system to activate the heart than to generate an impulse from the pacemaker (see Section VII).

If that same patient with a dual chamber pacemaker programmed to the DD mode develops a very slow sinus rate such that the intrinsic heart rate sensed in the atria and the ventricles drops below a pre-programmed lower rate limit (often programmed to 60 beats per minute), the pacemaker will begin to pace. Typically, the first paced beat will be in the atrium. If the AV node is working well, that paced beat will travel throughout the atrium (much like a heart beat of sinus node origin would), down the AV node, along the specialized conduction system, and activate the ventricles. An AV delay is programmed into the dual chamber device that is programmed to the DDD mode: this is a period of time that begins when the atrium is paced and will activate a ventricular paced beat only if a sensed ventricular beat does not occur during that period of time. For example, the AV delay may be set to 180 milliseconds (ms). If, after a paced atrial beat, an intrinsic ventricular beat is sensed before the 180 ms have elapsed, the device will inhibit ventricular pacing. However, if that 180 ms transpires without a sensed ventricular beat, the pacemaker will track that atrial paced beat and follow it with a ventricular paced beat.

Another scenario occurs when the sinus node is working fine, but the AV node conduction is slowed or blocked. In this circumstance (again, with a DDD device), the intrinsic atrial beats will be sensed, inhibiting atrial pacing. In addition to programming an AV delay (relevant to when the atrium is paced), a similar delay (sometimes called a PV delay) is in place for intrinsic atrial beats. After sensing of the atrial event, if a sensed ventricular signal occurs prior to the duration of the PV delay, ventricular pacing will be inhibited; if, after an atrial signal is sensed, the PV delay duration elapses without a sensed ventricular beat, the pacemaker will track that atrial beat and deliver a paced ventricular beat.

One other programming option that is useful to understand is called rate response. Some will place an “R” as a fourth letter (e.g., DDDR) in order to designate that this has been turned on. Rate response means that the lower rate limit (the rate below which the pacemaker will disallow by pacing) is adjusted depending on the patient’s activity. For example, if the primary problem is chronotropic incompetence (discussed above as the inability to mount an appropriately fast heart rate with activity), simply disallowing the heart from going less than 60 beats per minute probably will not be a lot of help when a patient is running up a hill (in other words, the patient may remain very fatigued if the heart rate does not increase above 60 beats per minute). Most pacemakers are equipped with some sort of sensor of patient activity (either from the heaviness of their breathing or the amount of movement or, potentially even the content of components inside the blood stream) that can reflect the degree of activity. With rate response, the pacemaker can then increase the lower rate limit as needed in response to that activity.

Of note, there are many other pacemaker capabilities and programming options that are beyond the scope of this review. Primarily, these are complex topics pertinent primarily to the device manufacturers, cardiologists, and cardiac electrophysiologists.

To reiterate the point made at the beginning of this section (perhaps the most important point), note that even in light of the relatively complex discussion above, the pacemaker is only preventing the heart from going too slow or from stopping. In other words, it only speeds up the heart rate, it does not slow heart rates that are too fast. Addressing fast heart rates is an entirely different (albeit sometimes related) topic and typically requires medicines or, in some cases, other invasive procedures such as catheter ablation or an implantable cardioverter-defibrillator.