How do pacemakers set the hearts pace?


2207520000 – that’s roughly how many times your heart will beat if you live to be 70. When things go wrong with your heart's natural pacemaker, the tempo of your heartbeat can be corrected by a nifty device known as a pacemaker.

Nature’s magnificent pump, the human heart, is programmed to beat at a certain rate and rhythm. It also has an override system built in, which allows the heart to adapt to different stresses, such as exercise, shock, fever or emotional stress.

The part responsible for setting and controlling your heart rate is called the sino-atrial (SA) node. A conducting system – like a circuit wire - connects this SA node with the rest of the heart. The result is precise, coordinated heart contractions.

If this organic pacemaker (the SA node) or wiring system becomes faulty, the heart may beat at a dangerously irregular or slow rate (bradycardia). Known causes for abnormally slow arrhythmias include a defective SA node or a defect in the conduction system of the heart, including the atrio-ventricular (AV) node, the bundle of His, or the bundle branches running alongside the walls of the lower heart chambers.

These defects may be congenital or hereditary, caused by age, due to damage from a heart attack, or due to a defective heart valve.

Abnormally slow heart rhythms may lead to fatigue, dizziness, loss of conciousness and even death.

Whatever the cause or the symptoms, a specialist cardiologist must be consulted. A pacemaker may need to be inserted to correct an irregular or slow heart rhythm.

There is no national database for South Africa, but in 1998 in the USA, 175 000 pacemakers were inserted countrywide. This indicates that up to 28 000 South Africans may benefit from pacemaker implants each year.

What is a pacemaker?
A pacemaker is a sophisticated device, temporary or permanent, which stimulates the heart to maintain or restore a normal heartbeat by sending precisely-timed electrical signals to the heart. Pacemakers do not take over the work of the heart. It merely helps to regulate the exact timing of each heartbeat.

A pacemaker consists of three major parts:

  1. The power source or generator is a tiny computer (the size of a R2 coin, but thicker) with its battery. The battery life is 5 – 8 years.
  2. The leads (insulated electrical wires) are connected to the generator at one end and to electrodes in the heart at the other end. The leads (usually two) carry the electrical impulse to the electrodes in the heart chambers.
  3. The electrode/s are placed in one or two of the heart chambers, depending on the specific condition.

The settings of the pacemaker can be programmed to suit the patient's needs. A "demand" pacemaker will only kick in when the heart's own rate drops below a certain level. A "fixed rate" pacemaker will emit an impulse at a fixed rate, regardless of any external circumstances. These are the most commonly used types.

New, technologically advanced "rate-responsive" pacemakers can determine the optimal heart rate during sleep, exercise and other activities and set the pace according to the optimal heart rate. These "rate-responsive" pacemakers can adapt heart rate to body movement and breathing tempo.

How are pacemakers implanted?
Inserting a pacemaker is quick and safe. It is performed under local anaesthesia and may take less than an hour. A patient might need to stay overnight to fine-tune the pacemakers and check that it is functioning properly.

Most commonly, the cardiologist makes an incision (usually about 5 – 7 cm long) under the collarbone. The computer and battery are implanted beneath the collarbone. The leads from the generator are threaded into the heart via a large vein. The electrode tip resting against the inside of the heart chamber (usually the right atrium) stimulates the heart muscle to contract using electrical impulses. If two electrodes are used, they will be placed in the right upper chamber (atrium) and right lower chamber (ventricle) respectively.

Complications are uncommon and can usually be treated very effectively. Within two to four weeks (once the incision has healed), the person can return to a fairly normal life. Most patients experience relief from fatigue and other symptoms of a very slow heartbeat.

Pacemaker check-ups are done at least once a year, and the entire generator will be changed every five to ten years. Depending on the original cardiac problem, blood-thinning medication may be needed to prevent clots forming in the heart, as they could cause strokes.

Some devices can interfere with a pacemaker
With a pacemaker in place, everything is back to normal, right? Well, not quite. Like any other sensitive apparatus, a pacemaker can be affected by electromagnetic impulses, which we come across in daily life.

Make note of the following devices:

  • Microwave ovens, TV’s, CB radios, electric drills, electric blankets, electric shavers, remote control TV channel changers and other home appliances are adequately shielded from interference by these appliances. Consult your cardiologist if you have any concerns
  • Cellphones can cause problems if kept in a pocket directly over the battery. But don’t worry; normal use - when held near the ear - is safe.
  • Security systems and metal detectors as those found in banks, shopping centres and airports are safe provided a patient walks briskly through the system and does not get too close to the source of the signal.
  • MRI-scanners use large magnets which could change pacemaker settings if the proper precautions have not been taken.
  • Radiation therapy for cancers can damage the pacemaker. The pacemaker needs to be shielded from the radio-active beams by a lead apron.
  • Shock wave technology to break up kidney stones can damage a pacemaker if implanted in the abdomen.
  • Arc welding equipment and heavy duty industrial motors generate magnetic fields that can inhibit the function of pacemakers.
  • Electrical stimulation to relieve acute or chronic pain is generally safe, but your generator might need to be reprogrammed.

- (Dr Anna Hall, Health24, January 2009)

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