The question of what heart rate requires placement of a pacemaker is somewhat complicated.

A “normal” heart rate is defined as 60-100 beats per minute. It is very clear, however, that many people have heart rates lower than this normal range. The most common example usually cited is that of endurance athletes who tend to have large cardiac chambers and deliver more blood with each heartbeat. They therefore need less beats per minute to deliver the blood flow that the body needs.

A “normal” heart rate for a professional endurance athlete may be in the range of 40-50 beats per minute. Tour de France cyclists have been documented to have resting heart rates in the 30s. Heart rates can also be slow from medications eg, beta blockers for hypertension, systemic infections, metabolic conditions, etc.

Symptoms of a slow heart rate

The symptoms of a slow heart rate are varied and depend on the effect of the damage to the electrical system of the heart.

In many cases, the electrical system can be intermittently affected, leading to symptoms that can be separated by periods of months to years. In this situation, diagnosis of the problem can be difficult as the heart rhythm can be normal otherwise.

In other cases, the effect of the damage is such that the heart is consistently slow, and symptoms are present continuously. Prominent symptoms of slow heart rates can include the brain, which utilises about 15 to 20 per cent of the blood flow of the heart at rest.

If the heart is beating too slowly, this can result in inadequate blood flow to the brain, leading to passing out or to symptoms of dizziness. The other large group of symptoms include the inability to exercise or, in severe cases, to do simple activities of daily living. Whenever we undertake any activity, our muscles need an increase in the amount of blood flow to provide the necessary nutrients.

The heart is the organ responsible for delivering this increase and the primary way it does this is by increasing the heart rate. If the heart rate is unable to increase due to electrical system disease, then patients may get symptoms which can include shortness of breath on exertion, inability to exercise, or dizziness on exertion. In severe cases of very slow heart rates at rest, the patient may have severe fatigue, confusion, organ dysfunction and ultimately death. It is important to recognise patients who have slow heart rates and symptoms as they can have increased mortality and morbidity compared to the general population.

The electrical activity of the heart

An understanding of the electrical activity of the heart is necessary to understand the need for and usefulness of cardiac pacemakers.

Much like your home, the heart has an electrical system. This consists of an area to generate electrical activity and specialised wires (muscle fibres) that distribute the electrical current. This current has the important role of causing contraction of heart muscle.

Simply put, the heart beats because an electrical current runs through it. This electrical current starts in the upper chambers of the heart at a point known as the sinus node. The electrical current first runs through the upper chambers triggering them to contract, the current then arrives at the junction of the upper and lower chambers at a region called the atrioventricular node. The current is then slowed down before passing to the lower chambers. This allows the heart to beat in a coordinated way with the upper chambers beating first, followed by the lower chambers.

The rate of current generation of the sinus node determines how fast the heart is beating. The rate of sinus node activity can be affected by hormones, medications, blood chemistry and nervous activity allowing the heart to beat at a rate that is appropriate for the body’s activity eg, the heart rate gets faster with exercise and slows with sleeping.

Disease processes affecting the electrical system of the heart

Problems with the electrical activity of the heart can broadly be divided into problems with the initiation of the electrical activity or its conduction. It may be helpful to think of the Jamaica Public Service grid where your home can lose electricity when the power generation plant shuts down (sinus node problems) or when there is damage to the power lines (muscle fibres) taking current to your home. Either of these scenarios can result in the heart rate being too slow. A heart rate that is too slow can cause a number of symptoms that we will discuss below.

What kind of processes can result in the heart rate being too slow? The common factor is some kind of damage to the sinus node, atrioventricular node or the muscle fibres that conduct (distribute) the electrical signal. This can simply be a matter of ageing with scarring of the electrical system.

Heart attacks which cause death of heart tissue can cause the tissues of the electrical system to die impairing conduction. Enlargement of the heart can “stretch” the electrical system and affect function. This can be seen in heart disease like hypertension or enlargement of the heart chambers in congestive heart failure. Less commonly there can be disease processes which result in infiltration of the heart muscle again impairing electrical function.

What is a Pacemaker?

A pacemaker is a device that monitors the heart rate and will intervene to stimulate heart contraction if the heart rate falls too low. Pacemakers are more and more commonly being inserted in cardiac patients.

Worldwide, it is estimated that one million pacemakers are inserted each year. The exact number of pacemakers implanted each year in Jamaica is unknown as there is no reliable database that tracks local utilisation. However, many patients in Jamaica who need pacemakers are still denied this option for many reasons, including access, health literacy, and finance. Pacemakers have a role in improving the quality of life and reducing morbidity and mortality in patients whose hearts are unable to maintain a normal cardiac rhythm.

When is a pacemaker necessary?

Pacemaker placement is usually considered when symptoms are present along with the slow heart rate or when the heart rate is dangerously slow (usually less than 30 beats per minute when awake) and a reversible cause cannot be identified. Pacemakers can be placed less commonly for other reasons, for example, essential medications that will result in the heart rate being dangerously slow, situations with a high risk of sudden conduction failure or treatment for certain types of structural heart disease.

It is not uncommon for cardiologists to see patients with slow heart rates — particularly elderly patients with presumed early degeneration of their conduction system — who have no symptoms and who are appropriately monitored for many years without pacemaker placement.

In a future article we will review the role of pacemakers in the treatment of slow heart rates.

Dr Ernest Madu, MD, FACC, and Dr Paul Edwards, MD, FACC are consultant cardiologists at Heart Institute of the Caribbean (HIC) and HIC Heart Hospital. Correspondence to info@caribbeanheart.com or call 876-906-2107