Arrhythmias – Bradycardia Explained (Part One)

Congenital heart defects are frequently accompanied by issues with the electrical conduction of the heart. These issues can cause a slow heart rate( bradycardia), a fast heart rate (tachycardia), or an irregular heart rhythm, also referred to as an arrhythmia. There may be one of these conduction defects present, or several at the same time.This can be due to the structural defects that were present at birth, or related to scar tissue that develops with each surgical intervention.  Fortunately, there are options to treat these irregularities.

Here we will be discussing the most common types of issues related to the electrical system of the heart. In order to discuss these abnormalities, first you should understand how things are supposed to work.

In normal conduction of the heart, the impulse is initiated in a group of specialized cells, called the sinus or sinoatrial (SA) node. This is located in the right atrium, or upper chamber, of the heart. Once initiated, is causes the both the right and left atria to contract. The signal then continues to the Atria-Ventricular (AV) Node. This is found between the upper and lower chambers. The signal slows slightly through here, allowing the blood to empty from the atria and enter the ventricles, or lower chambers. From there, the signal continues down through the lower chambers via the left and right Bundle of His, causing the ventricles to contract. Each time this cycle occurs, it results in one complete heart beat. When working properly, it should repeat this cycle without interruption 60-100 times per minute.

Sinus Bradycardia:

This describes a regular rhythm with a rate of less then 60 beats per minute. Sinus bradycardia is usually a benign finding. It is frequently seen in athletes or during sleep. There is generally no treatment for this unless the patient has symptoms, such as lightheadedness or fatigue. In such cases, further evaluation is done to determine an underlying cause, such as medication side effects or metabolic disorders.

First degree AV block:

This refers to a prolonged delay in conduction between the atria and ventricles. Generally, there are no symptoms associated with this, as the ventricle still does contract with every beat. No treatment is required for this type of heart block.

Second degree AV block:

Type 1- This is the less critical of the two types of second degree blocks and rarely requires treatment. In type 1, there is a progressive lengthening of the time it takes for the signal to travel from the sinus node to the AV node. With each contraction, it takes longer and longer until the ventricle “skips” a beat and resets the cycle.

Type 2-  In type 2 AV block, there is no specific pattern to when the signal is blocked from the atria to the ventricles. When this signal is blocked, the ventricles do not get the message to contract, and thus the blood is not pumped to the body. This often leads to third degree heart block. A pacemaker is implanted to treat type 2 AV block.

Third degree AV block:

Third degree, or complete heart block, refers to when the signal between the top and bottom chambers of the heart is absent. The ventricles use a “back up” electrical system, which produces a much slower heart rate then normal. The atria continue to contract as well, but there is complete dissociation between the top and bottom chamber, so the amount of blood being sent to the body is lessthen usual. Symptoms associated with complete heart block include lightheadedness and fainting. If left untreated, this block can cause death. This type of heart block  is critical and requires emergent treatment with a pacemaker. Often a temporary, external pacemaker is placed until the patient receives the permanent pacemaker.

Bundle Branch Blocks (BBB):

The Bundle branches are the fibers that carry the electrical signal from the AV node down through each ventricle. They are described as either the right or left bundle branch. When there is an interruption in this pathway, it is referred to as a bundle branch block(BBB). Generally, bundle branch blocks themselves do not cause any symptoms or require treatment. Further cardiac assessment should be done, however, to determine if there is an underlying cardiac condition. Right BBB is often seen in Congenital Heart Defects, especially in atrial or ventricular septal defects. Left BBB can frequently be seen in patients with coronary heart disease, or with weakening of the heart muscle.

The cardiac conduction irregularities listed above can be diagnosed in several ways. The most common is by performing a 12 lead electrocardiogram (ECG) This is a non-invasive test where patches, connected to leads, are placed on the skin. The technician ensures that the patient lays still for 15-30 seconds while a reading of the electrical signals through the heart is done. A printout of these readings can immeadiately show the heart rate, rhythm, and any damage to the muscle of the heart.

A holter monitor, similar to an ECG, involves applying electrodes to the skin in specific locations. Generally, there are 5 leads or less. These leads are connected to a small device that has recording capabilities. The patient can wear this device for 24-48 hours, depending on the physicians preference.  The device continuously records during the time that it is worn. Once returned, the ECG recordings are downloaded to a computer for a physician to review. The advantage to this test is that  with a prolonged recording period, there may be abnormalities detected that an ECG, which gives a “snapshot” can not detect.

An event monitor is similar to a holter monitor, in that it it is a small portable device that allows for prolonged recording of heart rate and rhythm. This device is usually worn for up to 30 days, and usually the patient can push a button on the device when he/she is experiencing any symptoms. Then they can call in to the device monitoring company and transmit the reading, which is reviewed and forwarded to the patients physician.

Of the above conduction issues, treatment is generally only required for second degree type 2 and third degree heart block. A small, electronic  device, known as a pacemaker, is used to restore normal conduction. This device is surgically implanted in the upper chest or abdomen, just under the skin, in a “pocket” the physician creates. Leads connected to the pacemaker are then threaded through a vein into the heart, where it is attached. The pacemaker “watches” the heart rhythm, and when it detects abnormal conduction, it sends an electrical signal to the heart. The pacemaker is programmed to not let the heart drop beneath a certain rate. Newer pacemakers transmit data wirelessly to their doctor or pacemaker clinic via a small device that can be kept near the patients bedside at home, This allows the physician office to monitor the functioning of the device without requiring the patient to call in, as was done in the past . Usually, every 6-12 months, an in-office pacemaker check is required to do full testing on the device, but this is quick and non-invasive. Pacemakers can last 7-10 years on average, depending on what percentage of time the pacemaker is in use.

Please join us next week for Part Two – Tachycardia.

Carol Raimondi is an adult CHD patient and nurse, living with Congenitally Corrected Transposition of the Great Arteries.  She has had 4 open heart surgeries in her 40 years, as well as a pacemaker since the age of 6. After spending a large part of her childhood in and out of hospitals, she developed a passion for nursing. She went to school to become a cardiac nurse. Carol’s many hospital experiences helped her as a nurse to better understand what her patients were going through and  to care for them with that much more compassion and empathy.

Due to worsening medical issues, Carol had to give up the profession she loved. That did not stop her from being a patient advocate, however. She joined her local hospital’s’ Patient Family Advisory Council, which she now co-chairs, and shortly thereafter she joined Mended Little Hearts Chicago(MLHC) as an adult CHD liaison. She then expanded her work in the CHD community by starting an adult and teen CHD group within MLHC and became an  Ambassador for the Adult Congenital Heart Association. Currently she sits as the Conquering CHD of IL(CCHD-IL)  President. Her proudest moments are when she is advocating and raising awareness for the CHD community, both on Capitol Hill in D.C. and locally.