For most adults at rest, the heart rhythm falls into a specific range of contractions per minute. When the heart rate deviates significantly from this expected range, either beating too quickly or too slowly, it signals a problem with the heart’s electrical system. Both conditions—an abnormally fast heart rate, known as tachycardia, and an abnormally slow one, called bradycardia—are classified as types of cardiac arrhythmia, which is a broad term for any irregularity in the heart’s rhythm.
Defining the Extremes
A healthy adult’s resting heart rate typically ranges from 60 to 100 beats per minute (bpm). Tachycardia is defined as a resting heart rate consistently exceeding 100 bpm. This condition represents the heart’s electrical system generating impulses at an accelerated pace.
In contrast, bradycardia is characterized by a resting heart rate falling below 60 bpm. While a slower rate is often a sign of high cardiovascular fitness, a pathologically slow heart rate indicates that the heart’s natural pacemaker, the sinoatrial (SA) node, or the electrical conduction pathway is malfunctioning.
Underlying Causes and Risk Factors
Tachycardia can result from the body’s attempt to compensate for a systemic problem, such as anemia, where a low red blood cell count forces the heart to beat faster to deliver sufficient oxygen. An overactive thyroid gland (hyperthyroidism) directly stimulates the heart’s intrinsic rate by increasing the sensitivity of cardiac cells to adrenaline-like hormones.
Other causes include fever, which speeds up metabolism, and intense emotional stress or anxiety, which triggers the body’s sympathetic nervous system response. Damage to the heart tissue from coronary artery disease or a previous heart attack can also create abnormal electrical circuits, leading to types of tachycardia like ventricular or supraventricular tachycardia.
Bradycardia often arises from degenerative changes or external depressants acting on the heart’s electrical control center. Aging is a primary risk factor, as it can lead to sick sinus syndrome, where the SA node degenerates and fails to fire impulses reliably.
Certain medications, particularly beta-blockers, are designed to treat high blood pressure or heart failure by intentionally blocking the effects of stimulating hormones on the heart, thus slowing the rate. An underactive thyroid (hypothyroidism) causes a systemic slowdown of the body’s functions, including a decreased heart rate and contractility. For highly conditioned athletes, a slow heart rate is a sign of an exceptionally strong and efficient heart that can pump an adequate volume of blood with fewer beats.
Manifestations and Immediate Impact
The physical sensations experienced during an arrhythmia are directly related to the compromise in the heart’s ability to pump blood effectively. In tachycardia, the heart beats so quickly that the chambers do not have enough time to fully relax and fill with blood between contractions. This poor filling time drastically reduces the amount of blood pumped out with each beat, leading to a sudden drop in cardiac output.
The immediate impact of this reduced output is often felt as palpitations, a fluttering or pounding sensation in the chest, and shortness of breath. Because the brain and other vital organs receive less oxygenated blood, individuals may experience lightheadedness, dizziness, or even fainting (syncope). This rapid, inefficient pumping can also increase the heart muscle’s oxygen demand, potentially leading to chest pain.
Conversely, the symptoms of bradycardia stem from the heart rate being too slow to meet the body’s demand for oxygen, especially during activity. The primary issue is a hypoperfusion, or insufficient blood flow, to the body’s tissues. This is most noticeable in the brain, manifesting as chronic fatigue, confusion, or memory problems.
When the heart beats too slowly, the body’s response is often a feeling of profound weakness or an inability to tolerate exercise because the muscles are not receiving enough oxygen. Severe bradycardia can lead to syncope, as the heart fails to maintain the blood pressure necessary to sustain consciousness.
Management Approaches
For many forms of tachycardia, management often begins with lifestyle changes to eliminate triggers like excessive caffeine or stress. Medications, specifically antiarrhythmic drugs, are frequently used to restore a normal heart rhythm or to control the rate.
In cases where medication is ineffective or the risk is high, procedural interventions may be needed. Catheter ablation is a minimally invasive procedure that uses heat or cold energy to destroy the small area of heart tissue causing the abnormal electrical signals. For life-threatening tachycardias, a procedure called cardioversion, which delivers a controlled electrical shock, may be used to reset the heart’s rhythm.
Management for bradycardia, especially when symptomatic, is less reliant on medication and more focused on mechanical assistance. If the slow rate is caused by a medication, adjusting the dosage or switching to a different drug is the first step.
When the electrical system is damaged, as with sick sinus syndrome, a permanent pacemaker is often the definitive treatment. This small implanted device monitors the heart rate and delivers an electrical impulse to ensure the heart beats above a predetermined safe threshold.