Arrhythmias and Sudden Cardiac Arrest

Has your heart “skipped a beat” or “stopped” in a moment of panic? What about a warm “fluttering” sensation when you are in love? These may be cardiac arrhythmias, and they are more than just colloquial expressions.

What is an Arrhythmia?

Cardiac arrhythmia broadly refers to an abnormal heart rate or rhythm. In the case of an arrhythmia, the heart may be beating too fast, too slow, or in an irregular pattern. These abnormal rhythms are caused by disturbances in the electrical signals that control the heart.  

Arrhythmias can occur in healthy hearts and, most of the time, they are both short-lived and harmless. However, serious arrhythmias that are prolonged and significantly disrupt heart function are very dangerous. In fact, certain types of arrhythmia can lead to Sudden Cardiac Arrest (SCA) and death within a few minutes after onset. In these life-threatening cases, the heart becomes unable to effectively pump blood throughout the body and to the vital organs. When the heart no longer delivers sufficient oxygen to the brain, syncope (‘fainting”) and death can occur in quick succession.

Arrhythmia Mechanism: Heart Function 101

In order to understand how and why arrhythmias occur, it is important to briefly review the structure and function of the heart.  

The heart is divided into four asymmetric chambers – the right atrium, the right ventricle, the left atrium, and the left ventricle. These chambers contract in a carefully coordinated sequence so that the heart pumps blood throughout the body every time it beats. On the right side of the heart (which appears on the left in the diagram), blood enters the right atrium from the body. This blood is then pumped to the lungs by the right ventricle where it becomes oxygenated. Oxygen-rich blood then returns to the heart through the left atrium before it is pumped to the rest of the body by the left ventricle. The upper chambers of the heart, the atria, act as the receiving chambers; they fill with blood and deliver that blood to the corresponding ventricle. The ventricles are the pumping chambers that then export the blood to either the lungs (right ventricle) or the systemic circulation (left ventricle).

Classification of Arrhythmias

Arrhythmias can be classified in two primary ways: how it affects the heart rate and where in the heart it originates. A healthy heart will slow down and speed up depending on much oxygen the body needs. The ideal heart rate of a resting, healthy individual is between 60 to 100 beats per minute (bpm).

• Tachycardia: A heart rate that exceeds 100 bpm
• Bradycardia: A heart rate below 60 bpm
• Supraventricular arrhythmia: These arrhythmias are caused by malfunctions in the atria, which are the upper chambers of the heart. Supraventricular arrhythmias are further categorized into atrial flutter, atrial fibrillation, and paroxysmal supraventricular tachycardia.
• Ventricular arrhythmia: These abnormal heart rhythms originate in the lower chambers of the heart, the ventricles. Preventricular contractions, a less-severe ventricular arrhythmia, are extra, abnormal heartbeats that come from the ventricles rather than the sinus node. These beats can cause a fluttering feeling, like your heart skipped a beat; they are relatively common. Two lethal arrhythmias that cause Sudden Cardiac Arrest include ventricular fibrillation and ventricular tachycardia.

What Arrhythmias can cause Sudden Cardiac Arrest?

Sudden Cardiac Arrest (SCA) is an unexpected loss of heart function due to an underlying arrhythmia. During cardiac arrest, the heart is no longer able to pump oxygen to the brain or body. Consequently, victims of SCA become unresponsive and stop breathing normally.  

When someone suffers SCA, their heart is likely in either pulseless ventricular tachycardia or ventricular fibrillation. While many cardiac arrhythmias are not immediately dangerous, both of these lethal ventricular rhythms will lead to death within minutes if left untreated. 

• Pulseless ventricular tachycardia is a life-threatening fast heart rhythm that originates in the lower part of the heart, the ventricles. During pulseless ventricular tachycardia, the ventricles are contracting very rapidly and are not able to effectively pump blood throughout the body.
• Ventricular fibrillation is another ineffectual heart rhythm where the heart beats with erratic, rapid electrical impulses. These chaotic electrical signals cause the ventricles to essentially quiver rather than pump blood to the rest of the body. 

Fortunately, both of these cardiac arrhythmias can be treated with a lifesaving defibrillation shock from an Automated External Defibrillator (AED).

Sudden Cardiac Arrest Risk Factors and Causes

Arrhythmias are caused by a diverse range of conditions and substances that affect the physiology and electrical function of the heart. The lethal arrhythmias that cause Sudden Cardiac Arrests are no different. All the following conditions can increase risk of SCA.

Symptoms and Diagnosis of Arrhythmia

When arrhythmias occur, symptoms may include dizziness, shortness of breath, discomfort in the chest, heart palpitations, and fatigue. In most cases, arrhythmias are episodic, appearing without warning, and, in some dire cases, are only discovered after Sudden Cardiac Arrest!

In diagnosing an arrhythmia, physicians rely on a variety of tests that record and analyze the heart rhythm. Some of the more common tests include:

• Electrocardiogram: The ECG records the heart’s electrical activity and depicts how impulses propagate throughout the heart. It is a simple and common procedure in which electrodes are attached to the bare chest. These electrodes relay electrical signals from the heart to a machine that then displays graphed patterns on a monitor for easy analysis.
• Holter/Event/Patch Monitor: These are small, portable ECG devices that are commonly used to record the heart rhythm for anywhere from 48 hours to 14 days. Holter monitors are useful for patients with transient symptoms or cardiac arrhythmias that may be difficult to detect during a shorter period of time. Event monitors specifically record the heart rhythm when activated during symptomatic episodes. Patch monitors sit directly on the chest and collect ECG data for up to 14 days.
• Stress Test: This test, also known as a treadmill or exercise test, is performed to assess heart function and diagnose potential arrhythmias during physical activity on a treadmill or stationary bike. To better visualize blood flow and heart function, patients might also receive a nuclear stress test where they receive a small and safe amount of radioactive tracer. If a patient cannot exercise on a treadmill or stationary bike, they may also be given pharmacological agents to simulate cardiac stress.
• Electrophysiologic Testing (EP Study): During an EP study, electrode catheters are temporarily threaded through the peripheral veins or arteries and they enter the heart while patients are under local anesthesia. The electrode catheters are then positioned in the atria, ventricles, or both where they record the heart’s electrical signals. This recording provides a highly-detailed map of the cardiac electrical impulses during each heartbeat.
• Tilt Table Test: For patients who often feel faint or dizzy, a tilt table test can help determine whether those feelings are due to an abnormal heart rate or blood pressure. During this exam, patients lie on a rotating table that raises their head 60 to 80 degrees from a horizontal lying position. This movement and positioning intends to trigger symptoms while patients are being monitored.

Treatment of Arrhythmia

Based on the type and severity of the arrhythmia, treatment can take many different forms. Some treatment options include: 

• Lifestyle changes: It is important to have a healthy diet reduce your risk of heart disease at any age. Physicians may advise patients to adopt lifestyle changes that mitigate the risk of worsening an arrhythmia. Some suggestions include eating healthy diets, cessation of smoking, and stress management. 
• Medications: A host of medicines can be administered carefully to treat abnormal rhythms. Of course, caution and proper diagnosis should be taken when administering these (or any) drugs. Some common drugs that can be used to treat arrhythmia are described in the table below. 

Medication	How it Works
Beta blockers	By binding to adrenoreceptors, these drugs reduce heart rate and the strength of heart muscle contraction.
Ion channel blockers	These drugs change the electrolyte balance of calcium, sodium, magnesium, or potassium in the cardiac cells and reset electrical conduction pathways to normal.
Blood thinners	By reducing the propensity of blood to clot, blood thinners lower the risk of blockages that may cause heart attacks and strokes.

• Surgery: If lifestyle changes and medication can’t solve the problem, more involved procedures may be required. Common surgical procedures and implants include:

1. Cardioversion: Cardioversion involves the use of synchronized electrical shocks to stabilize an irregular heartbeat, often atrial fibrillation or atrial flutter. Unlike defibrillation used to treat patients during cardiac arrest, cardioversion is administered to patients who still have a pulse. 
2. Implantable cardioverter defibrillator (ICD): This device is inserted surgically under the skin and continuously monitors the electrical impulses of the heart. If there is an abnormal heart rhythm, a corrective electrical impulse is sent to the heart to stabilize the heartbeat. Patients who go into sudden cardiac arrest often receive an ICD which you can learn more about here.
3. Pacemakers: A pacemaker is implanted in the abdomen and chest with electrical connection to the heart that enables it to monitor and stabilize the heart rate by producing electrical impulses to counteract problematic electrical signals. Unlike an ICD, pacemakers cannot “shock” the patient in case of a pulseless tachycardia or fibrillation.
4. Catheter ablation: This process deliberately creates scar tissue in the heart to stop arrhythmias. The scars block the heart tissue from triggering or conducting abnormal electrical impulses. During surgery, the catheter enters the heart through a vein or artery in the groin and uses either heat or cold to create the scars.

Other Resources

• What is defibrillation?
• What is an AED?
• Why is rapid defibrillation so important?