Feb. 24, 2020

Understand arrhythmia and autonomic dysfunction

Learn about abnormal heart rhythms and how autonomic nervous system dysfunctions can affect your health

Understanding arrhythmias

The heartbeat in a healthy individual is rhythmic and coordinated. Each heartbeat is initiated in the natural pacemaker region of the heart called the sinoatrial node. 

Electrical signals generated here are conducted from the upper chambers of the heart (the atria) to the lower (the ventricles). This allows these regions of the heart to contract and pump blood to the body. Disruptions in the pattern of conduction of these signals can lead to an irregular heartbeat, called an arrhythmia.

Cardiac arrhythmias can cause the heart to beat too slow (called bradycardia), too fast (called tachycardia), with an irregular rhythm (called flutter or fibrillation), or cause an extra heartbeat. Arrhythmias may cause symptoms like dizziness, shortness of breath, palpitations and chest pain. 

Types of Arrhythmias

The heart rate for healthy adults falls between approximately 60-80 beats per minute. If the heart rate drops below this point, the heart can’t pump enough blood to meet the body’s needs. 

Tachycardia, on the other hand, occurs when the heart beats too fast. It can impact the supraventricular or ventricular regions of the heart. Supraventricular arrhythmias are not usually life threatening but can diminish a patient’s overall quality of life.

Ventricular arrhythmias affect the lower chambers of the heart and can be life threatening. Ventricular fibrillation (VF) leads to rapid and ineffective ventricular contractions that prevent the ventricles from pumping blood to the body. Patients with VF require immediate medical attention to shock the heart back into normal rhythm, as the condition can be fatal within minutes. 

The most common sustained cardiac arrhythmia is atrial fibrillation (AF). When the heart is in AF, the atria quiver at rates exceeding 400 beats per minute due to highly disorganized patterns of electrical conduction travelling within the atria. Some, but not all, of these electrical signals travel from the atria to the ventricles, resulting in uncoordinated contractions between the upper and lower chambers of the heart. This means that both the rate and rhythm of the heart are impacted. 

Causes and treatment

Researchers know that cardiac arrhythmias are caused by alterations in the structure and electrical function of the heart. These changes can be inherited, induced by certain medications, or occur in conjunction with different diseases. Patients with heart attacks, for example, can be more susceptible to developing arrhythmias. 

Given the potential danger and loss of quality of life associated with arrhythmias, researchers at the Libin Institute are investigating the specific causes and possible treatment of a number of arrhythmias with the goal of improving patient outcomes. 

Fortunately, arrhythmias are often treatable. While the treatment depends on the specific type of arrhythmia, common therapies include healthy lifestyle choices, medications, or surgical procedures such as ablation. Implanted devices like pacemakers and defibrillators may also be used. 

Dr. Hailey Jansen is a postdoctoral fellow in the lab of Dr. Robert Rose, PhD. 

Understand autonomic dysfunction

The autonomic nervous system is not something you normally think about in your everyday life. In contrast to the voluntary or somatic nervous system, which controls body movement, the autonomic nervous system works “automatically” to control the involuntary processes in your body including breathing rate, digestion, and your heart rate.  

The autonomic nervous system can be divided into two main components: the sympathetic nervous system, which is responsible for the body’s “fight or flight” response, and the parasympathetic nervous system, which can be thought of as the “rest and digest” system.  

When you are startled, for example, the sympathetic nervous system activates the “fight or flight” response by signaling the body to release more adrenaline, increasing your heart rate, breathing rate and perspiration. When you feel your heart pounding in your chest, it is the result of this system. 

The parasympathetic nervous system is responsible for processes when your body is at rest, including digestion. A delicate balance of sympathetic and parasympathetic activity is required to maintain a state of balance in the body. If there is a disruption in the function of the sympathetic or parasympathetic nervous systems, this can lead to autonomic dysfunction. If you have ever fainted before, or felt lightheaded when standing up, you may have experienced a type of autonomic dysfunction.  

Syncope (fainting) and orthostatic intolerance are two common types of autonomic dysfunction, and ones that are studied at the Libin Cardiovascular Institute. 

Although there are many different causes of fainting, a type of syncope called neurocardiogenic or vasovagal syncope is caused by a sudden drop in heart rate and blood pressure, usually triggered by a stimulus such as prolonged standing, emotional distress or the sight of blood.  

Orthostatic intolerance occurs when the autonomic nervous system does not respond appropriately to standing. Patients with orthostatic intolerance have severe symptoms when they stand up and may not be able to do so for long. 

Two common conditions of orthostatic intolerance are Postural Tachycardia Syndrome (POTS) and Orthostatic Hypotension (OH). Patients with POTS experience a rapid heart rate when standing, whereas patients with OH experience low blood pressure. Autonomic dysfunction is diagnosed using autonomic function testing, a series of specialized tests that evaluate the function of the sympathetic and parasympathetic nervous systems. 

Patients with autonomic dysfunction often experience quality of life impacts, and more research is required to try to understand underlying mechanisms and develop treatments for these disorders. 

Dr. Kate  Bourne is a PhD candidate in the lab of Dr. Satish Raj, MD.