Atrial Fibrillation (Afib) is a common heart rhythm disorder characterized by irregular and often rapid heartbeats. While Afib typically causes the heart to beat too quickly, it can also lead to an unusually slow heart rate. Understanding when a slow heart rate becomes a concern for individuals with Afib is important.
Defining a Low Heart Rate in Atrial Fibrillation
A slow heart rate, medically termed bradycardia, generally refers to a heart rate below 60 beats per minute (bpm) in adults. For many healthy adults, particularly athletes, a resting heart rate in the 40s or 50s can be normal. However, in the context of Atrial Fibrillation, what constitutes a heart rate that is too low is more complex and often individualized.
When a person has Afib, the electrical signals in the atria are chaotic, leading to an irregular and sometimes rapid response in the ventricles. Medications are often used to control this ventricular rate. A heart rate that is “too low” in Afib falls below the generally accepted range of 60-100 bpm and is accompanied by symptoms, or when it falls below a specific target set by a healthcare provider.
The threshold for a concerningly low heart rate depends on several factors. These include a person’s overall health, other medical conditions, specific medications they are taking, and their usual activity level. A healthcare provider considers these individual circumstances.
Common Causes of Slow Heart Rate with Atrial Fibrillation
Several factors can contribute to a slow heart rate in individuals with Afib. Medications frequently prescribed to manage Afib are a prominent cause. Drugs like beta-blockers (e.g., metoprolol, carvedilol) and calcium channel blockers (e.g., diltiazem, verapamil) are designed to slow heart rate and control ventricular response in Afib.
Digoxin, another medication, also works by slowing electrical signals between the heart’s upper and lower chambers, reducing heart rate. While effective in controlling Afib, these medications can sometimes slow the heart rate excessively. Adjustments to dosage or type may be necessary if the heart rate becomes too low.
Underlying electrical issues within the heart can also lead to a slow heart rate in Afib. Sick Sinus Syndrome (SSS) involves dysfunction of the heart’s natural pacemaker, the sinoatrial node, leading to slow heartbeats, pauses, or alternating periods of fast and slow rhythms. Afib and SSS often co-occur, and SSS can directly result in bradycardia.
Another electrical problem is Atrioventricular (AV) nodal dysfunction or heart block, where the electrical signals from the atria are partially or completely blocked from reaching the ventricles. This causes the ventricles to beat much slower than the atria, resulting in a low pulse. Both SSS and AV nodal dysfunction can impair the heart’s ability to maintain an adequate rate.
Medical procedures like catheter ablation, performed to correct Afib, can also lead to a slower heart rate. While ablation aims to normalize heart rhythm, the process of creating scar tissue to block faulty electrical signals can sometimes affect nearby electrical pathways, including the sinoatrial node, resulting in new onset or worsening bradycardia.
Increased vagal tone, a heightened activity of the vagus nerve, is another factor that can slow the heart rate. The vagus nerve is part of the parasympathetic nervous system, regulating bodily functions during rest. High vagal activity, sometimes seen in athletes or during sleep, can cause the heart rate to drop significantly, potentially triggering or exacerbating Afib with a slow ventricular response.
Recognizing Symptoms of a Slow Heart Rate
A heart rate that is too low may prevent the body from receiving enough oxygen-rich blood, leading to various symptoms. Dizziness or lightheadedness is a common symptom, occurring when the brain is not getting an adequate blood supply. This can range from a mild sensation to feeling as though one might faint.
Fatigue and general weakness are also frequently reported, as the heart’s reduced pumping efficiency can limit the supply of oxygen and nutrients to muscles and organs. Shortness of breath can develop if the heart is not effectively circulating oxygen throughout the body.
More severe symptoms include fainting or near-fainting spells (syncope or presyncope), which indicate a significant reduction in blood flow to the brain. Chest pain (angina) can also occur if the heart muscle is not receiving enough oxygen. Confusion or difficulty concentrating may arise due to insufficient blood flow to the brain.
When to Contact a Healthcare Professional
Individuals with Afib should be aware of their heart rate and any new or worsening symptoms. If a consistently low pulse rate is noticed, especially if it falls below a range previously discussed with a doctor, it warrants medical attention. This is particularly true if the low rate is new or differs from the individual’s typical heart rate.
Seek immediate medical help if a slow heart rate is accompanied by severe symptoms. These include fainting or near-fainting episodes, severe chest pain, or extreme shortness of breath. These symptoms suggest the heart is not adequately supporting the body’s needs and require urgent evaluation.
Any new or worsening symptoms associated with a slow heart rate should prompt a discussion with a healthcare provider. Individuals taking heart medications for Afib should always consult their doctor before making any changes to their regimen or if they have concerns about their heart rate.