What Is a Pacemaker? A Computer Deciding When Your Heart Beats
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The heart generates its own electrical signal and needs no instruction from the brain to beat. When that internal wiring fails, a small implanted device takes over the job, watching for beats that should have happened and supplying an electrical pulse when they do not, which is a far more conditional role than the name suggests.
The heart's own wiring
A cluster of cells in the right atrium called the sinus node depolarises spontaneously at a rate set by the autonomic nervous system, producing the heartbeat's timing signal. That signal spreads across the atria, making them contract, and reaches the atrioventricular node, which is the only electrical connection between the upper and lower chambers and which deliberately delays the signal briefly so that the atria finish emptying before the ventricles contract. From there specialised conducting fibres distribute the signal rapidly through the ventricles so they contract as a coordinated unit rather than in a slow wave. Failures occur at each of these points. The sinus node can fire too slowly or unreliably. The atrioventricular node can block the signal intermittently or completely, in which case backup pacemaker cells lower down take over at an inadequate rate. The conducting bundles can fail, producing uncoordinated ventricular contraction. Each pattern produces different symptoms, from fatigue and breathlessness to fainting, and each calls for a different pacing configuration.
What the device actually does
A modern implanted pacemaker is a sensing and decision-making device that paces only when necessary:
- •A pulse generator containing a battery, circuitry and a processor, implanted under the skin below the collarbone, typically lasting eight to twelve years before replacement
- •One or more leads threaded through a vein into the heart chambers, which both sense the heart's own electrical activity and deliver pulses
- •Demand pacing, meaning the device waits a set interval for a natural beat and paces only if none arrives, which is why many patients are paced a small percentage of the time
- •Rate response, using an accelerometer or a breathing sensor to raise the paced rate during activity, since a device cannot otherwise know that the person is climbing stairs
- •Dual chamber coordination, pacing atrium and ventricle in the correct sequence to preserve the contribution of atrial contraction to filling
- •Leadless devices, self-contained capsules placed directly inside the ventricle with no wires, and biventricular devices that resynchronise the ventricles in certain kinds of heart failure
What it is not
A pacemaker treats a heart beating too slowly or in a badly coordinated sequence, and it does not treat a heart beating dangerously fast and does not restart a heart that has stopped in the sense usually depicted. That job belongs to an implantable cardioverter defibrillator, a device that looks similar and is implanted the same way but which monitors for dangerous fast rhythms and delivers a shock to terminate them, and which is implanted in people at risk of sudden cardiac death rather than people whose hearts beat too slowly. Many modern devices combine both functions. The distinction matters for expectations, since a pacemaker prevents fainting and fatigue from slow rhythms and does not prevent heart attacks, which are caused by blocked coronary arteries and are an entirely separate problem. Device therapy has also raised end-of-life questions that medicine took time to address, since a defibrillator will continue delivering shocks to a dying patient unless it is deactivated, and guidance now treats discussing deactivation as part of end-of-life care.
Living with one
The practical restrictions are narrower than commonly believed. Microwave ovens, airport security and everyday electronics are not a problem, and mobile phones are safe if not carried directly over the device. Strong magnetic fields are the genuine concern, so magnetic resonance imaging was long contraindicated and most devices implanted now are certified as conditionally safe for scanning under specified protocols, which was an important change since scans are frequently needed. Certain industrial equipment, arc welding and some medical procedures using electrosurgery require precautions. Devices are interrogated at follow-up visits and increasingly transmit data remotely, which detects lead problems, battery depletion and recorded arrhythmias without requiring a visit. Complications are uncommon and include infection, which is serious because it can require removing the entire system, lead displacement in the early period, and pocket haematoma. The cybersecurity of implanted devices became a genuine research topic after demonstrations of wireless vulnerabilities, and manufacturers have issued firmware updates to address them, which is an unusual sentence to write about a piece of the human circulatory system.
The takeaway
The heart generates its own timing signal, and the device steps in only when a beat that should have happened does not, which is why many people are paced a fraction of the time. Leads both sense and deliver pulses, and accelerometers raise the rate during activity. It does not treat dangerously fast rhythms or restart a stopped heart, which is a defibrillator's job. Scanning in a magnetic resonance machine is now conditionally possible.