A third-degree atrioventricular (AV) block is a severe cardiac condition where the electrical communication between the heart’s upper and lower chambers is completely interrupted. This condition, also known as complete heart block, represents the most serious classification of AV block, demanding immediate medical attention.
Understanding Complete Heart Block
The heart’s electrical system begins with the sinoatrial (SA) node, the natural pacemaker, which generates an impulse causing the atria to contract. This impulse travels to the atrioventricular (AV) node, which delays the signal before passing it to the ventricles. In a third-degree AV block, the electrical signal originating in the atria is entirely blocked and cannot pass through the AV node.
This total communication failure results in complete atrioventricular dissociation, meaning the atria and ventricles beat independently without coordination. Since the ventricles receive no SA node signals, they rely on an emergency backup system called an escape rhythm. This rhythm originates from a lower pacemaker site, typically in the His bundle or Purkinje system, which is slower and less reliable than the SA node.
The rate of this ventricular escape rhythm depends on its origin within the lower conduction system. If the rhythm arises high in the His-Purkinje system, the heart rate may be 40 to 60 beats per minute, which is slow but more stable. If the block occurs lower down, the ventricular escape rhythm may drop to 20 to 40 beats per minute. These slow rates are unstable and can cause the patient to become hemodynamically compromised, meaning circulation is inadequate.
The lack of coordination and the slow ventricular rate reduce the blood the heart pumps, lowering cardiac output. If this escape mechanism fails, the patient is at high risk for asystole, a complete absence of electrical activity leading to cardiac arrest.
Recognizing the Signs and Symptoms
The severely decreased heart rate (bradycardia) leads to physical manifestations indicating a medical emergency. Patients commonly experience dizziness or lightheadedness due to insufficient blood flow to the brain, which can lead to syncope (temporary loss of consciousness).
Extreme fatigue is common, as tissues are not receiving enough oxygenated blood. The heart’s inefficient pumping can cause fluid backup, resulting in shortness of breath and sometimes acute heart failure. Some individuals may also experience chest pain because the heart muscle is not receiving adequate oxygen supply.
Common Underlying Causes
The complete interruption of the heart’s electrical signal results from factors that damage the conduction system. One of the most common causes is chronic degeneration and fibrosis, often called idiopathic fibrosis, where electrical pathways wear out over time. Other causes include:
- Acute myocardial infarction (heart attack), especially those affecting the inferior wall. This damage is often temporary.
- Medication toxicity, particularly from agents used to control heart rate or rhythm, such as beta-blockers, calcium channel blockers, and digitalis.
- Systemic diseases, including sarcoidosis, Lyme disease, and certain collagen vascular disorders, which can infiltrate the heart’s electrical wiring.
- Complications following cardiac surgery, such as aortic valve replacement or other open-heart procedures.
Diagnostic Procedures and Treatment Options
Diagnosis of a third-degree AV block is confirmed using an Electrocardiogram (ECG), which records the heart’s electrical activity. The ECG tracing demonstrates complete dissociation, showing the atria and ventricles beating independently. Physicians observe that the P waves (atrial activity) bear no consistent relationship to the QRS complexes (ventricular activity). Longer-term monitoring, such as a Holter monitor, may be used if the block is intermittent.
The definitive treatment for persistent third-degree AV block is the implantation of a permanent pacemaker. This small, battery-powered device is surgically placed, typically near the collarbone, with leads extending into the heart chambers. The pacemaker monitors the heart’s intrinsic rhythm and delivers an electrical impulse whenever the heart rate falls below a programmed threshold. This steady signal ensures a healthy heart rate and rhythm, restoring adequate blood flow.
In an acute setting, temporary pacing is initiated immediately if the patient is unstable. This may involve non-invasive transcutaneous pacing (stimuli delivered through skin pads) or transvenous pacing (a temporary wire inserted into a vein). The goal of this acute management is to stabilize the patient until a permanent pacemaker can be implanted. Although reversible causes like drug toxicity must be ruled out, a permanent pacemaker is indicated in nearly all cases of symptomatic or persistent complete heart block.