Sinus bradycardia is a general term describing a slowed heart rate due to irregularities in transmission of the electrical impulse from the sinus node. In a healthy heart, the electrical impulse originates from the sinus node, located in the top right atrium. The pulse passes to the ventricles (lower chambers) via the AV node, and is distributed by the bundle of His or bundle branches. AV block is a specific type of sinus bradycardia, usually involving problems in either the AV node, bundle of His, or bundle branches.There are three recognized states of AV block:
First Degree: With an incidence of 1:1000, this type of AV block is very common, and is so slight that it often goes unnoticed. Electrical impulses are slower to pass through the AV node, but all ultimately reach the ventricles. Heart rate and rhythm remain normal. First degree heart block is often found in well-conditioned athletes, presumably because their hearts are more efficient and require fewer heartbeats. Additionally, some medications, such as digitalis, can contribute to this condition.
Second Degree: In this state, the impulses are slowed to the extent that a heartbeat is skipped. Depending on the frequency that beats are skipped, the patient may be diagnosed with either Type 1 (minor) or Type 2 (more serious) second degree AV block. According to the severity, patients may not notice the irregularity, or they may experience symptoms such as dizziness, fainting, confusion, fatigue, or physical imbalance.
Third Degree: This condition represents a complete AV block, in which no impulses generated in the sinus node reach the ventricles. Third degree block carries a very high, almost absolute risk for cardiac arrest. Barring immediate medical intervention, the only method of maintaining the heartbeat in this situation is via “ventricular escape beats,” in which the ventricles generate their own impulses in an effort to maintain function. These beats are very slow however, and cannot adequately replace the normal electrical impulses. Symptoms include all those of second degree AV block, plus those usually associated with heart attack.
A physician can diagnose AV block by examining a patient’s medical history, giving a physical exam, and evaluating blood samples for electrolyte imbalances, the presence of enzymes associated with cardiac arrest, and abnormal levels of certain medications. Most significantly, an EKG will determine the presence and extent of AV block. In an EKG, the "P" wave occurs first, and represents the electrical impulse traveling through the upper, atrial chamber, while the "QRS" wave follows, and signifies the impulse reaching the lower, ventricle chambers. The skipped beat typical of second degree AV block, for example, would visualize as a "P" wave without the following "QRS" wave. In third degree, there would be no causal relationship between the generation of "P" and "QRS" waves.Treatment varies depending on the severity of the existing problem. Often, the less severe forms of the condition can be addressed by correcting the underlying problems, such as an electrolyte imbalance or medication overdose. For more serious cases, portable heart monitors may be used to record and monitor abnormal activity, while third degree block always requires immediate implantation of a pacemaker.
