lyme disease

Atrioventricular Heart Block: PR, QRS, Mobitz

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Heart block, also known as atrioventricular (AV) conduction system block, refers to decreased conduction of electrical impulses through the heart. This decreased conduction occurs at the atrioventricular node of the heart’s conduction system. Heart block occurs in three flavors: type 1, type 2, and type 3. Each type has different causes and different treatments.

Pathology and Types

In type 1 AV block the PR interval on an ECG is longer than normal (> 200 msec), but every QRS complex has a preceding P-wave indicating that the electrical impulses from the atria (ie: the top chambers in the heart) are "making" it through the AV node to the ventricles. In type 1 block, the AV nodal conduction is slowed compared to normal "healthy" individuals, but the impulses are still able to get all the way through the conduction system of the heart.

Type 1 AV node block = PR interval > 200 msec, no missed beats.

Type 2 heart block occurs in 2 flavors: Mobitz type 1 and type 2. In Mobitz type 1 the amount of delay between the P-wave and QRS complex (ie: the PR interval) gradually increases until a beat is dropped. It is caused by impaired conduction in the AV node. Mobitz type 2 occurs when the PR interval remains stable, and then out of the blue a missed beat occurs; this is caused most commonly by slowed conduction through the bundle of His or bundle branches of the heart’s conduction system.

Mobitz type 1 = increasing PR interval until dropped beat occurs (image below). Mobitz type 1 is also referred to as "Wenckeback" block.

Mobitz Type 1

Mobitz type 2 = PR interval remains the same until a dropped beat occurs (image below). Mobitz type 2 is also referred to as "Hay" block.

Mobitz Type 2

Finally, type 3 AV nodal block occurs when the atria and ventricles of the heart beat independently of one other. The most common causes of 3rd degree block are severe diseases of the AV nodal system due to age, previous heart attack, drug and medication toxicity (ie: digitalis), and untreated Lyme disease. The atria generally beat at a faster rate than the ventricles. In other words, the ECG will show more P-waves than QRS complexes. The QRS complexes may be narrow or wide depending on where in the conduction system the escape rhythm originates.

Type 3 heart block = atria (p-waves) and ventricles (QRS) beat independently of each other; no impulses from the atria make it to the ventricles (image below).

AV nodal block type 3

Symptoms

The symptoms of heart block depend on how slow the rate becomes. Patients with type 1 block rarely become symptomatic because the atria are still setting the pace of the entire hearts rhythm. However, in Mobitz type 2 and type 3 block the heart rate can become very slow. When this occurs symptoms such as syncope (ie: fainting), extreme fatigue, shortness of breath, and dizziness can occur. These symptoms occur because the heart is not pumping blood fast enough to keep up with the body’s demand; the result is a decrease in cardiac output.

Diagnosis

Diagnosis of heart block can be made by looking at the characteristic findings on ECG.

Treatment

Treatment of AV nodal dysfunction is dependent on the type. Most patients with type 1 heart block do not need treatment. Type 1 heart block can be considered a "healthy" variant of AV nodal conduction, which is slower than most people in the population. The only instance where type 1 block may need further work up is in elderly patients who have other signs or symptoms of coronary artery disease.

Type Treatment
Type 1

Usually none, look for causes such as coronary artery disease in elderly patients

Type 2 – Mobitz type 1

 Usually none, but pacemaker if symptomatic. Also discontinue any drugs that slow AV node.
Type 2 – Mobitz type 2 Pacemaker
Type 3 Pacemaker

Mobitz type 1 (ie: one of the 2 subgroups of type 2 heart block) also generally requires no treatment. If patients are symptomatic pacemaker insertion may be considered. Mobitz type 2 is more worrisome because it often progresses to 3rd degree heart block. In these patients, pacemakers are often inserted to ensure that the heart beats at a pre-defined and safe rate.

Type 3 heart block is almost always controlled with pacemaker insertion. The reason is that the ventricles often beat at a rate that is too slow for normal daily activity. A pacemaker will keep the heart beating at a pre-defined rate to ensure that symptoms do not develop.

Overview

Heart block (aka: atrioventricular block) occurs when impulses travel too slowly, or not at all through the AV node. There are different types depending on how severe the block is. In type 1 AV block impulses are slowed, but cause no conduction block. In type 2 missed beats can occur, and in type 3 the atria beat independently of the ventricles indicating complete conduction block. Symptoms depend on the severity of block but can include fainting, shortness of breath, dizziness, and fatigue. Diagnosis is made by ECG. Treatment is dependent on the type of block, but can include placement of a pacemaker.

References and Resources

Ixodus, Burgdorferi, and Lyme Oh My!

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Borrelia burgdorferi is a gram negative spirochete bacterium. Its hosts are mice, deer, and humans. Spread from host to host is via a tick from the species Ixodes. Ixodes scapularis is the most common tick vector in the Northeastern United States, whereas Ixodes pacificus is the most common in the Western United States.

When an infected tick feeds from a human it can pass borrelia burgdorferi bacteria into the bloodstream. From there the bacterium is usually attacked by both the innate and adaptive immune systems. The innate immune system uses toll-like receptors on macrophages to bind to bacterial proteins. The adaptive immune response uses antibodies to bind to bacterial surface proteins, which results in complement activation and bacterial death.

Unfortunately, the immune response is often not enough to entirely eliminate this bacteria. This is because borrelia burgdorferi has the ability to evade the immune system by changing proteins on its cell surface.

Signs and Symptoms

Borrelia burgdorferi causes Lyme disease. Lyme disease manifests in different ways depending on which stage of the disease the patient enters. The first stage of the disease presents with a rash known as erythema chronicum migrans. This rash appears as a red ring around a clear central area (ie: it resembles a bull’s eye). During this stage of the disease many patients also have flu-like symptoms.

If left untreated the disease can progress to stage two. During this stage neurological, joint, and cardiac symptoms can occur. The bacterium can cause cardiac arrhythmias, especially heart block. Facial drooping can be caused by damage to the facial nerve (cranial nerve seven). Meningitis, which ranges in severity, can occur. Finally, a migrating intermittent arthritis can also be seen during this stage of the disease.

If the disease continues to remain untreated, stage three ensues! This stage happens approximately a year or more after the initial tick bite. It is characterized by chronic arthritis, which can cause severe joint damage. Encephalitis (inflammation of the brain tissue) and myelitis (inflammation of the spinal cord) can also occur.

Diagnosis

The diagnosis of Lyme disease is a two step process. A blood sample is taken from the patient and a test known as an ELISA is performed. ELISA stands for “enzyme linked immunosorbent assay”. This test detects antibodies made by the patient against borrelia burgdorferi antigens (ie: it’s surface proteins). If the ELISA is positive, then a second confirmatory test called a "western blot" is performed. This test further separates the different antibodies to borrelia. If both the ELISA and western blot are "positive" the likelihood of Lyme disease is high.

Treatment

Treatment for early stage disease is with an antibiotic known as doxycycline. Children and pregnant women are often treated with amoxicillin. Other medications used include cefuroxime; erythromycin is sometimes used in patients who are allergic to pencillin. Later stages of the disease may need intravenous antibiotics.

If patients do not respond to initial antibiotic treatment it is important to think of co-existent infections with other tick-borne illnesses. Ticks may carry not only borrelia burgdorferi, but also other pathologic entities so treatment may need to be further tailored.

Overview

Lyme disease is caused by the bacterium borrelia burgdorferi. It is transmitted by the Ixodes tick and is common in the northeastern United States. Symptoms include a bull’s eye rash known as erythema migrans. Neurological, joint, and cardiac symptoms can also occur. Treatment for early stage disease is with oral antibiotics, usually doxycycline, although amoxicillin is sometimes used. Intravenous antibiotics may be necessary for later stages.

References and Resources