Q: My dad fainted and they told him needed a pacemaker. I would like to understand more about pacemakers.
A: The first step is to understand the normal electrical activity of the heart. Each of the half-billion cells in the heart plays a role in its mechanical contraction (of the atria, which ‘passively fill’ with blood and then squeeze blood out to load the ventricles, and of the ventricles, with the right ventricle pushing the blood to the lungs and the more powerful left ventricle pushing the blood to the rest of the body) as well as its electrical system (which relays the signal to coordinate the timing of all this mechanical activity). Normally, the initiation of this electrical signal starts in the sinoatrial (SA) node (a specialized area in the wall of the right atrium), passes through the atria (which contract in response to this signal), is delayed a touch at the atrioventricular (AV) node located between the atria and the ventricles (to allow the blood to leave the atria to load the ventricles), and then on to the ventricles (passing through the bundle of His, branching to the left and right bundles and then on to the Purkinje fibers, all designed to time things just right to coordinate the squeeze of the ventricles).
A problem with the functioning of this electrical system can cause issues with the overall heart rate or with the timing needed to coordinate of the contractions of the heart chambers to provide optimal function. In patients whose electrical system is not working correctly, electrical impulses supplied by a pacemaker may be of benefit.
The need for a pacemaker is usually identified by a combination of identifying an issue with the electrical conduction system, typically by seeing an abnormality on the patient’s electrocardiogram (ECG) or on a longer evaluation of their heart rhythm using a monitor they wear for days/weeks/even a month, and by evaluating the patient’s symptoms (including the sometimes difficult task of associating the symptoms with an electrical conduction issue). A pacemaker is usually only needed when there is an electrical conduction issue associated with symptoms, but there are exceptions where the risk of a particular conduction abnormality is felt to be very worrisome.
Pacemakers must be able to sense what electrical activity exists in the heart (for example, if the problem is at the AV node the pacemaker needs to ‘see’ the signal coming from the SA node), and then provide an electrical impulse to stimulate the desired electrical activity. The details of the function of an individual’s pacemaker are specified in its five-digit code.
The first digit of the code specifies what part of the heart is to receive the pacemaker’s electrical output signal (“A” for atrium “V” for ventricle or “D” for dual, i.e. both). The second digit specifies the location of sensing (identifying a signal that exists in the heart, again with “A”, “V” or “D”). The third digit gets into the ‘logic’ utilized by the decision algorithm of the pacemaker, with “I” indicating its electrical output is inhibited by sensing a signal, “T” indicating that a sensed signal triggers its output, “D” indicated it may sometimes be inhibited and sometimes triggered, and “O” meaning no sensed signal matters (usually used when no sensing is utilized). The fourth digit uses “R” if the rate of the pacemaker’s output is modulated depending on the patient’s activity level, “O” is used when this is not the case. Finally, the fifth digit (rarely used) indicates when there are multiple sites of pacing, with “O” meaning there are none, “A” meaning there are multiple sites in the atria, “V” for multiple sites in the ventricle(s), and “D” for multiple sites in both.
Pacemakers consist of three main components; the battery (which needs to be replaced about every five to ten years), the generator (implanted under the skin in the abdominal wall or chest, and which functions to perform the necessary logic analysis of the sensed signals and then to create the required electrical signals), and the leads (which are commonly inserted through the deep blood vessels in the chest into the heart muscle, although some newer systems attempt to do all this by sensing and sending signals without leads).
The specific pacemaker, lead placement and logic used depends on the underlying reason the patient needs a pacemaker. The most common reason is an inappropriately slow heart causing symptoms (such as fainting), which most commonly occurs from ‘sick sinus syndrome’ (a problem with the SA node causing it to initiate activity at too low a rate). In this condition the pacemaker is utilized to sense the atrial signal, and if/when it is too slow then to stimulate the atria and ventricles to contract at a faster rate (stimulating both so as not to lose the atrial contraction which better loads the ventricles so their output is optimized). Many other electrical conduction problems may benefit from implanting a pacemaker.
Although pacemakers are overall quite reliable, problems can occur. There may be complications from the pacemaker insertion procedure (for example a collapsed lung if it is unintentionally ‘nicked’ when the leads are placed). In about 1 percent of patients one (or more) of the cardiac leads may break/fail. Other possible lead complications include infection, blood clot development, or other issues. Possible generator problems include issues with the utilized logic being misapplied (for example, the device may interpret a sensed signal incorrectly, causing inappropriate or absent signal output), infections, and other complications.
Most modern pacemakers generate a ‘report’ (obtained non-invasively from a device that can wirelessly communicate with the pacemaker), which is periodically evaluated to evaluate the correct functioning of the pacemaker, as well as to identify what the patient’s need for pacing has been. It also reports on the remaining battery life of the pacer, as well as other parameters.
Pacemakers do not “shock” the heart if a lethal heart rhythm occurs, that is the function of an automated implantable cardioverter defibrillator (AICD). Some patients have a device that can act as a pacemaker and an AICD.
Jeff Hersh, Ph.D., M.D., can be reached at DrHersh@juno.com