Bradycardia, a condition where the heart beats slower than a normal adult resting rate of 60 to 100 beats per minute, can sometimes be a natural occurrence, particularly in highly physically active individuals or during sleep. However, when the heart rate falls below 60 beats per minute and causes symptoms, it may indicate an underlying issue. While various medical conditions can lead to a slow heart rate, certain medications are a common and significant cause.
How Medications Influence Heart Rate
The heart’s rhythm is precisely controlled by its electrical system, which generates and conducts impulses to make the heart muscle contract. The sinoatrial (SA) node, often called the heart’s natural pacemaker, initiates these electrical signals. These signals then travel through the atria to the atrioventricular (AV) node, which acts as a relay station, ensuring the ventricles contract in a coordinated manner.
Medications can influence heart rate by affecting either the SA node’s ability to generate impulses or the AV node’s ability to conduct them. Some drugs can directly slow the firing rate of the SA node, while others can delay the electrical conduction through the AV node.
Medication Classes Known to Cause Bradycardia
Several classes of medications are known to slow the heart rate, either as an intended therapeutic effect or as a side effect. These drugs act on different parts of the heart’s electrical system to reduce its pace.
Beta-blockers are a common class of medications that reduce heart rate by blocking the effects of adrenaline and other stress hormones on the heart. They are frequently prescribed for conditions such as high blood pressure, angina (chest pain), and certain arrhythmias. Examples of beta-blockers include metoprolol, atenolol, and propranolol. By inhibiting the beta-adrenergic receptors in the heart, these drugs decrease the heart’s automaticity and conduction velocity, leading to a slower heart rate.
Calcium channel blockers of the non-dihydropyridine type also frequently cause bradycardia. These medications, such as verapamil and diltiazem, are used to treat high blood pressure, angina, and certain heart rhythm disorders. They work by blocking the movement of calcium into heart muscle cells, which slows the electrical impulses at both the SA and AV nodes. This action directly reduces heart rate and contractility.
Digoxin is a medication primarily used to treat heart failure and atrial fibrillation. It can slow the heart rate by increasing parasympathetic (vagal) activity on the SA node and by inhibiting the AV node, thereby prolonging its refractory period. Due to its narrow therapeutic window, careful monitoring is necessary to avoid toxicity, which often manifests as bradycardia.
Antiarrhythmic medications are specifically designed to correct abnormal heart rhythms, but some can paradoxically cause the heart rate to slow excessively. Drugs like amiodarone and sotalol, classified as Class III antiarrhythmics, work by blocking potassium channels to prolong the heart’s electrical repolarization. Flecainide, a Class Ic antiarrhythmic, can also contribute to bradycardia.
Opioids and sedatives/hypnotics, which are central nervous system depressants, can indirectly slow heart rate, particularly at higher doses. While their primary action is not on the heart’s electrical system, their overall depressive effect on the body’s systems can lead to a decrease in heart rate.
Other medications can also contribute to a slow heart rate. Certain eye drops, particularly those containing timolol, a beta-blocker, can be absorbed systemically and affect heart rate. Timolol eye drops are often prescribed for glaucoma, and systemic absorption can lead to symptomatic bradycardia, especially when used concurrently with oral medications that also slow the heart. Additionally, cholinesterase inhibitors, used to treat Alzheimer’s disease (e.g., donepezil, rivastigmine, galantamine), can increase parasympathetic tone, which may lead to bradycardia by affecting the SA and AV nodes.
Recognizing Symptoms of Medication-Induced Bradycardia
When a medication causes the heart rate to slow significantly, the body may not receive enough oxygen-rich blood, leading to various symptoms. These symptoms can sometimes be subtle or develop gradually, making them easy to overlook.
Common indicators of medication-induced bradycardia include dizziness and lightheadedness, which result from reduced blood flow to the brain. Individuals may also experience persistent fatigue or weakness, finding themselves more tired than usual, especially during physical activity. Shortness of breath can occur as the heart struggles to pump enough blood to meet the body’s oxygen demands. In more severe cases, symptoms might progress to fainting or near-fainting spells (syncope). Other possible signs include chest pain, confusion, or a general feeling of being unwell.
What to Do if You Suspect Medication-Induced Bradycardia
If you suspect that a medication is causing your heart rate to be too slow, it is important to seek medical attention promptly. Do not stop taking any prescribed medication without consulting a healthcare professional, as abrupt discontinuation can lead to serious health complications or worsen the condition they are treating.
Contact your doctor, pharmacist, or emergency services depending on the severity of your symptoms. For instance, if you experience severe dizziness, chest pain, or fainting, immediate emergency care is warranted.
When you speak with a healthcare provider, be sure to provide a complete list of all medications you are currently taking. This includes prescription drugs, over-the-counter medicines, herbal supplements, and any recreational substances. Your healthcare provider will assess your symptoms and may perform tests to determine the cause of the bradycardia. They might adjust the dosage of your current medication, switch you to an alternative drug, or investigate other potential causes for your slow heart rate. Communication with your healthcare team is essential for appropriate care.