The artificial cardiac pacemaker

Our heartbeat is set by a pacemaker which is a group of cells called the sinoatrial (SA) node located in the right atrium of the heart. The SA node generates electrical impulses that initiate each heartbeat. It does so automatically and rhythmically, usually setting the pace for the heart at a regular rate of 60 to 100 beats per minute in a healthy adult at rest. First, the pacemaker triggers the depolarization and contraction of the two atria (which pump blood into the ventricles). This signal is then passed to the atrioventricular (AV) node and then to the two ventricles (which pump blood to the lungs and body).

If the SA node is malfunctioning, the heart may beat too slowly (bradycardia), too fast (tachycardia), or irregularly (arrhythmias).  In that case, doctors may implant an artificial cardiac pacemaker that helps correct these issues by delivering electrical impulses to the heart, ensuring it maintains a normal rhythm and rate. The simplest type of artificial pacemaker tracks the heart's natural rhythm. If the pacemaker does not sense electrical activity in either the atrium or ventricle within a typical time frame (about a second), it will deliver a brief, low-voltage pulse to either the atrium or ventricle to stimulate the heart.

The basic components of a pacemaker are the pulse generator, leads, and electrodes. The pulse generator is the main part of the pacemaker and contains a battery and the electronic circuitry to generate the electrical pulses. The leads are electrical wires that carry the electrical pulses from the pulse generator to the heart (Figure 1). Finally, at the tip of each lead are the small electrodes that deliver the electrical pulse to the heart muscle and also sense the heart's natural electrical activity.

An artificial cardiac pacemaker is relatively small, about the size of a matchbox or a large coin. The typical dimensions of a modern pacemaker are approximately 5 cm by 5 cm. An artificial cardiac pacemaker is typically implanted under the skin in the chest area, just below the collarbone (clavicle) in a subcutaneous pocket created just beneath the skin and subcutaneous tissue, usually on the left side of the chest which is closer to the heart. The leads are then threaded through a vein into the heart, typically into the right atrium and/or right ventricle (Figure 1).

There are three basic types of pacemakers:

• Single-Chamber Pacemaker has one lead placed in either the right atrium or right ventricle for when only one chamber of the heart needs pacing.
• Dual-Chamber Pacemaker has two leads, one in the right atrium and one in the right ventricle. It helps coordinate the timing of contractions between the atria and ventricles.
• Biventricular Pacemaker (CRT-P) is used for patients with heart failure and has three leads, placed in the right atrium, right ventricle, and left ventricle. It helps synchronize the contractions of both ventricles to improve the efficiency of the heart’s pumping action.

An artificial cardiac pacemaker is used to treat a variety of medical conditions that affect the heart's ability to maintain a normal, consistent rhythm and rate which include the following:

• Bradycardia is a condition where the heart beats too slowly, usually fewer than 60 beats per minute. It can result from aging, heart disease, or issues with the heart's natural pacemaker (SA node). The artificial pacemaker helps to increase the heart rate to a normal range, ensuring adequate blood flow to the body.
• Heart Block (Atrioventricular Block) occurs when the electrical signals between the atria and ventricles of the heart are delayed or blocked. In complete heart block no signals from the atria reach the ventricles resulting in a very slow heart rate. A pacemaker is often required in more severe cases to ensure the ventricles receive regular electrical signals to beat effectively.
• Atrial Fibrillation with Slow Ventricular Response is a special type of atrial fibrillation (AFib) when the ventricles beat too slowly. AFib is characterized by irregular, often rapid heart rate that occurs when the upper chambers of the heart (atria) beat out of coordination with the lower chambers (ventricles). If the ventricles do not beat fast enough, a pacemaker might be needed to maintain an adequate heart rate.

To function properly, the artificial pacemaker must both sense and stimulate the heart. The pacemaker continuously monitors the heart's electrical activity through the electrodes on the leads. When the pacemaker senses that the heart is beating normally, it remains inactive. If the pacemaker detects that the heart is beating too slowly or skipping beats, it sends a small, painless electrical impulse through the lead to the heart muscle which stimulates the heart to contract. This sensing and stimulating activity continues on a beat by beat basis and is called "demand pacing", which represents the most basic form of heart rate control.

More modern pacemakers extend this basic control to take into account circumstances that may change the heart rate. In particular they are often equipped with built-in ECG sensors that can record the electrical activity of the heart. This feature allows the pacemaker to monitor the heart's rhythm over an extended time frame (i.e. multiple beats) and adjust its pacing accordingly.

Finally, in a recent article in Science Magazine, researchers devised a system that allowed the pacemaker to adjust not only to heart behavior but also other physiologic variables such as physical activity, breathing, body temperature, and blood oxygen saturation. This was accomplished using a network of sensors placed at various positions in the body that sent this information to the pacemaker via wireless signaling. The rate adaptation allows the heart rate to meet the body's demands in a more robust fashion.

In summary, an artificial cardiac pacemaker is a sophisticated device that mimics the natural pacing function of the heart, ensuring a consistent and appropriate heart rate for individuals with heart conditions that may lead to cardiac rhythm disorders. At its core, it continuously monitors the heart's electrical activity and provides necessary impulses to maintain a steady and efficient heartbeat. More recent control innovations have allowed the artificial pacemaker to adapt to a range of heart and body activity profiles making the system more robust.

Figure 1. Schematic diagram of artificial cardiac pacemaker showing the basic components. The device is composed of the pulse generator, leads, and electrodes at the end of the leads. There can be one, two, or three leads that are threaded into different chambers of the heart. The pulse generator sends electrical pulses through the leads/electrodes to the heart muscles stimulating heart contraction (Wikipedia).