Atrial fibrillation (AFib) is the most common sustained heart rhythm disorder, characterized by an irregular and often rapid heartbeat. This chaotic electrical activity in the upper chambers, the atria, prevents them from contracting effectively. While AFib causes palpitations and shortness of breath, profound fatigue and weakness are among the most frequent symptoms patients report. This exhaustion stems from a complex interplay of mechanical inefficiency, metabolic overload, and various non-cardiac factors that drain the body’s energy reserves.
How Inefficient Pumping Causes Low Energy
AFib-related fatigue stems from a significant reduction in the heart’s pumping efficiency, which compromises the delivery of oxygen and nutrients throughout the body. In a healthy heart, the atria contract forcefully to “top off” the ventricles, the main pumping chambers, before they eject blood. This coordinated action is called the atrial kick, which contributes up to 25% of the total volume of blood pumped with each beat.
During AFib, the atria merely quiver or fibrillate chaotically instead of contracting. This loss of the atrial kick means the ventricles begin each contraction with a reduced blood volume, resulting in a reduced stroke volume.
Since cardiac output is the total volume of blood pumped per minute, the reduced stroke volume often leads to a decrease in overall cardiac output, sometimes by as much as 25%. This compromised circulation means that peripheral tissues, muscles, and the brain receive an inadequate supply of oxygenated blood. The resulting oxygen deficit and build-up of metabolic waste products manifest as profound weakness, exercise intolerance, and persistent fatigue.
The Metabolic Burden of a Rapid Heartbeat
AFib frequently involves a rapid heart rate, or tachycardia, which places a severe metabolic strain on the heart muscle itself. Chaotic electrical signals travel quickly to the ventricles, causing them to beat rapidly and irregularly, often between 120 and 175 beats per minute or more.
A heart beating this quickly consumes an excessive amount of energy, primarily adenosine triphosphate (ATP), to sustain the rapid contractions. This high energy demand on the myocardium, or heart muscle, leads to a systemic energy drain. The heart shifts from using fatty acids toward a less efficient glucose metabolism to meet these intense energy requirements.
The rapid beating also drastically shortens diastole, the time the heart spends relaxing between beats. The heart muscle receives its own blood supply and oxygen through the coronary arteries during this relaxation phase. A shortened diastolic filling time starves the heart muscle of needed oxygen and energy replenishment, contributing to myocardial fatigue and exacerbating exhaustion throughout the body.
Non-Cardiac Causes of AFib Fatigue
Fatigue in AFib is often compounded by factors extending beyond the heart’s mechanical and metabolic issues, including medication side effects, sleep disruption, and the psychological burden of living with the condition. Medications used to control the heart rate and rhythm are a frequent contributor to patient fatigue.
Beta-blockers, such as metoprolol or carvedilol, are commonly prescribed to slow the ventricular rate. These drugs work by blocking the effects of adrenaline, which reduces the heart’s force of contraction and lowers the heart rate. While effective for rate control, this mechanism can also cause a generalized reduction in energy and lethargy, and sometimes a slow heart rate, or bradycardia, which further contributes to feeling tired.
Sleep is frequently disturbed in patients with AFib due to several related symptoms. Palpitations and the anxiety associated with an irregular heart rhythm can prevent a person from falling or staying asleep. Furthermore, the increased pressure in the atria during AFib can trigger the release of a hormone that increases urine production, causing patients to wake up frequently during the night, a condition known as nocturia.
The psychological impact of living with an unpredictable and potentially serious heart condition also drives chronic fatigue. The persistent worry about when the next episode might occur, combined with the physical symptoms, creates a state of chronic anxiety and mental exhaustion. This constant state of stress elevates the body’s fight-or-flight response, which can trigger the release of adrenaline, further worsening the physical and mental fatigue experienced by patients.