The medical term for a low heart rate is bradycardia. Sometimes a low heart rate is defined as below 60 beats per minute, but it would probably make more sense to have low heart rate defined as below 50 beats per minute.
Patients often ask if they have a low heart rate and whether there are a certain number of beats per minute below which they should be concerned. It’s important to realize however that the significance of a low heart rate is different in every case. Some people can have a heart rate of 40 beats per minute and have no symptoms and no long-term consequences. However in other people this can lead to symptoms and require treatment.
In some patients a low heart rate is found as part of a routine physical exam or study such as an EKG or a heart monitor. In other cases patients may present with symptoms and then found to have a low heart rate as part of the work up. Its very important to ensure symptoms are related to the low heart rate otherwise treatments such as pacemaker will have no effect whatsoever.
When I see a patient with a low heart rate I ask myself the following.
Is the low heart rate a physiologic finding or a pathologic finding? An example of a physiologic low heart rate would be an athlete with a low resting heart rate from being trained, which is absolutely fine. An example of a pathologic heart rate would be a disorder of the internal pacemaker system of the heart such as heart block that would often need a pacemaker inserted as treatment.
Is the low heart rate the likely cause of symptoms? Symptoms of a low heart rate may include dizziness and fatigue. In order to be attributed to a low heart rate the symptoms should occur at the same time the heart rate is low.
Are there any reversible causes for the low heart rate? Medicines such as beta-blockers or disorders such as hypothyroidism may lead to low heart rate and if the heart rate is dangerously low and causing symptoms as a result of this, stopping the medication or treating the underlying conditions will likely reverse the symptoms.
Why Could a Low Heart Rate be Bad in Some Situations?
The heart needs to pump out a certain amount of blood to provide the body with the blood it needs to function. The amount of blood pumped is known as cardiac output and is usually defined as liters per minute. Heart rate of course affects this output. In some patients a low heart rate can lead to a low output and cause symptoms such as dizziness, shortness of breath and fatigue. These symptoms are associated with low output heart failure. In other patients a low heart rate causes no effect whatsoever as the heart simply pumps out more blood with each beat to compensate.
The Conduction System of the Heart
The heart has its own natural pacemaker made up of a specialized collection of cells in the top chamber of the heart known as the SA node. This generates an impulse that travels through another collection of cells in the middle of the heart known as the AV node. The pathways taken by the impulses are known as the conduction system.
Problems with a low heart rate can be caused by dysfunction of the SA node, the AV node or the conduction system! It gets even more complex. The conduction system of the heart has many nerves attached to it; some of these nerves decrease the rate of conduction whereas others increase the rate of conduction. The nerves that decrease the rate of conduction and therefore lower heart rate are known as parasympathetic nerves. An example is when someone vomits; this can increase impulses in the parasympathetic nerves and slow the heart rate significantly for a while. This can even lead to passing out, which is known as a vagal event.
A balance of impulse from the sympathetic and the parasympathetic nerves determine a person’s baseline heart rate. Interestingly, in experiments where a person’s nerve supply is blocked, the heart rate is often higher; this would suggest that the parasympathetic nerve impulses that serve to slow the heart rate down are the predominant force under normal resting conditions. This is particularly evident at night when most people have a significant drop in heart rate.
When searching for a definition of a low heart rate, it’s important to know what is considered a normal heart rate. The normal resting heart rate is variable across the population and even between sexes. Most of the population is in the 50-90 beat per minute range, with heart rate dropping significantly at night. Not only does the heart rate drop at night, but also there is an increase in pauses and blocks. These are common enough to be considered normal variant at night that needs to be taken in to account when interpreting heart monitors.
Patients with sleep apnea, for example, may have very low heart rates at night that can be treated by sleeping the treat apnea.
Causes of Low Heart Rate
Firstly we will discuss things directly affecting the heart tissue and the conduction system called intrinsic disease. Aging is a common cause of slow heart rate, which results from degeneration of the conduction system of the heart. Heart attacks may damage areas of the conduction system also. Conditions that affect many organs of the body such as sarcoid, lupus and others can also affect the conduction system of the heart. Undergoing heart valve surgery such as the TAVR procedure for aortic stenosis, the mitraclip procedure for mitral regurgitation, mitral valve replacement or mitral valve repair, aortic valve replacement, or other complex heart surgeries may also cause trauma to the conduction system of the heart. Sometimes infection of the heart valves can extend in to the conduction system of the heart also.
Next we will discuss outside influences on the heart and conduction system known as extrinsic causes. Certain situations such as coughing, vomiting and others can lead to slow heart rate through the nerve system. Drugs that directly slow the heart rate include beta-blockers, calcium channel blockers and others. Metabolic disorders such as hypothyroidism can lead to a slow heart rate. Levels of electrolytes such as potassium derangement can lead to a slow heart rate.
Treatment of Low Heart Rate
In patients with confirmed or suspected slow heart rate, the underlying possible causes such as those outlined above need to be evaluated carefully. It’s especially important to review the medication list carefully and stop any potentially offending agents. Blood tests such as thyroid function studies may be performed.
An EKG is performed to see if there is just a slow heart rate or any evidence of heart block. Sometimes a monitor is worn to see the heart rate over time. Some people with a slow heart rate are unable to get their heart rate up with exercise known as chronotropic incompetence; this can be diagnosed with exercise testing. An echocardiogram may be performed to evaluate the heart structure and function.
What we do with a slow heart rate really depends on how bad the symptoms are. Its key to make sure the symptoms are related to the slow heart rate and that possible causes are identified and treated. The main indication for a patient without symptoms to get a pacemaker would be advanced heart block, long pauses in the heartbeat or rhythms that have the potential to lead to instability.
In patients that are symptomatic, and in whom underlying reversible causes have been ruled out, insertion of a pacemaker may be required. The choice of pacemaker for those with a low heart rate is different in different people and depends upon the level of block in the heart.