A second degree atrioventricular block is a cardiac abnormality causing normal atrioventricular (AV) conduction impairment. The impulse may be delayed or blocked inside the atrioventricular node or bundle. Depending on the severity of the condition, the patient may have no complaints or may less frequently, experience serious symptoms. This ailment is divided, based on electrocardiography findings, into Mobitz I and II types.
The people with Mobitz I second degree atrioventricular block can be divided into two categories. The first one is represented by trained athletes, with increased vagal tone, that have no symptoms and the heart block is an incidental finding. In their case, the prognosis is excellent. The second one is composed of individuals with underlying heart disease, that may experience dizziness, light-headedness, presyncope or syncope, due to cerebral hypotension. If the condition is due to myocardial ischemia or myocarditis, patients may experience angina. Additional findings include palpitations.
Furthermore, Mobitz II second degree atrioventricular block is more likely to progress to severe bradycardia and complete heart block , especially if associated with large anterior or inferior wall infarction. AV heart block is more often seen with inferior infarctions because in most cases the node is irrigated by the right coronary artery, as is the inferior heart wall. However, five-year survival has been shown to be reduced in unpaced Mobitz I patients compared to their healthy peers . Asystole and sudden cardiac death risk are as high as 35% per year in Mobitz II individuals. Hemodynamic instability requires urgent intervention .
Beta-blockers and digoxin are known medications that reduce heart rate and may cause an atrioventricular block. Digoxin level determinations are widely available and should be used in individuals taking chronic treatment. Blood workup should also include an electrolyte, especially potassium, calcium and magnesium level determination. High potassium levels are a frequent cause of conduction impairment. Those who describe angina, especially if the pain is aggravated or suggestive for acute myocardial infarction, cardiac biomarkers should be urgently tested, since a block that appears in the context of an acute infarction is a poor prognosis factor. Myocarditis can be evaluated using echocardiography and serological markers, like enterovirus or adenovirus polymerase chain reaction, human immunodeficiency (HIV), and Lyme disease serologies, if available. Sarcoidosis needs to be excluded in the differential diagnosis . In patients where congenital heart disease is a concern, thorough transthoracic or transesophageal echocardiography or computer tomography assessment should be performed. The latter is, however, reserved for situations where the first two methods offer incomplete information .
The main diagnostic tool for second degree atrioventricular block is the electrocardiogram. In Mobitz I, also known as Wenckebach block , the PR interval becomes progressively longer until a sinus impulse is blocked. PR interval is longest just before the dropped beat. In Mobitz II, the PR interval does not elongate, but blocked P waves occur. The R-R interval surrounding the blocked P wave measures exactly twice the preceding R-R interval if a single impulse is not conducted, or triple if two beats are dropped. The dropped beats will most often occur at regular intervals . The block is named as a ratio of P waves to QRS complexes, for instance 3:2, 4:3, and so on . In many cases, long electrocardiogram recordings are useful in order to correctly describe the pattern. The QRS complex may be narrow, but is often wide, due to intraventricular conduction abnormalities. A Mobitz II block is often infranodal, but may also be intranodal. A nodal location is suggested if the degree of the block improves with exercise. Infranodal blocks are improved by vagal maneuvers. The exact location of the pathological process is best described by electrophysiological methods, which is always indicated in patients with unexplained syncope.