Third-degree atrioventricular block

Third-degree atrioventricular block (AV block), is a medical condition in which the nerve impulse generated in the sinoatrial node (SA node) in the atrium of the heart can not propagate to the ventricles. Third-degree atrioventricular block (AV block), is a medical condition in which the nerve impulse generated in the sinoatrial node (SA node) in the atrium of the heart can not propagate to the ventricles. Because the impulse is blocked, an accessory pacemaker in the lower chambers will typically activate the ventricles. This is known as an escape rhythm. Since this accessory pacemaker also activates independently of the impulse generated at the SA node, two independent rhythms can be noted on the electrocardiogram (ECG). People with third-degree AV block typically experience severe bradycardia (an abnormally low measured heart rate), hypotension, and at times, hemodynamic instability. Many conditions can cause third-degree heart block, but the most common cause is coronary ischemia. Progressive degeneration of the electrical conduction system of the heart can lead to third-degree heart block. This may be preceded by first-degree AV block, second-degree AV block, bundle branch block, or bifascicular block. In addition, acute myocardial infarction may present with third-degree AV block. An inferior wall myocardial infarction may cause damage to the AV node, causing third-degree heart block. In this case, the damage is usually transitory. Studies have shown that third-degree heart block in the setting of an inferior wall myocardial infarction typically resolves within 2 weeks. The escape rhythm typically originates in the AV junction, producing a narrow complex escape rhythm. An anterior wall myocardial infarction may damage the distal conduction system of the heart, causing third-degree heart block. This is typically extensive, permanent damage to the conduction system, necessitating a permanent pacemaker to be placed. The escape rhythm typically originates in the ventricles, producing a wide complex escape rhythm. Third-degree heart block may also be congenital and has been linked to the presence of lupus in the mother. It is thought that maternal antibodies may cross the placenta and attack the heart tissue during gestation. The cause of congenital third-degree heart block in many patients is unknown. Studies suggest that the prevalence of congenital third-degree heart block is between 1 in 15,000 and 1 in 22,000 live births. Hyperkalemia in those with previous cardiac disease and Lyme disease can also result in third-degree heart block. Atropine is often used as a first line treatment of a third-degree heart block in the presence of a narrow QRS which indicates a nodal block, but, may have little to no effect in an infra-nodal block. Atropine works by reducing vagal stimulation through the AV node but will not be effective in those who have had a previous heart transplant. Other drugs may be utilized such as epinephrine or dopamine which have positive chronotropic effects and may increase the heart rate. Treatment in emergency situations can involve electrical transcutaneous pacing in those who are acutely hemodynamically unstable and can be used regardless of the persons level of consciousness. Sedative agents such as a benzodiazapine or opiate may be used in conjunction with transcutaneous pacing to reduce the pain caused by the intervention.