What Is Heart Block?
The normal electrical conduction pathway through the heart originates at the sinoatrial (SA) node in the right atrium, propagates through the atria (right to left) but also downward, inferiorly toward the ventricles. At the junction between the atria and the ventricles is the atrioventricular (AV) node, and from there, the conduction wave travels throughout the ventricles via the Bundle of His which splits into two main tracts which further subdivide into the Purkinje fibers. Thus, the time it takes for this wave of conduction to travel, although very brief, is enough to skew the contractions of the atria and ventricles, effecting a sequential pumping action that engenders a vector force outward from atria to ventricles and from ventricles to the rest of the body.
The very nature of electrical depolarization includes an excitatory phase and an refractory phase, so that a dominant impulse is not hampered by competing impulses. The automaticity of the SA node makes its impulse the dominant impulse.
Atrioventricular Heart Block (AV Block)
A block can occur at any point in the conductive pathway due to congenital abnormalities or heart disease (infarction). The blocks typically interfere with the atrial and ventricular contraction coordination. A complete block at the AV node would mean the ventricles would use their own automaticity to contract, as a “rescue” mechanism. Incomplete blocks result in varying degrees of heart pump inefficiency, leading to symptoms from dizziness and lightheadedness to syncope and/or death.
Causes of Heart Block:
- Structural abnormality in the SA or AV node or the conductive pathways associated with them
- Drugs (digoxin, beta blockers, and some calcium channel blockers)
- Vagal inhibitory influences (autonomic nervous system)
- Myocardial infarction
- Cardiomyopathy and myocarditis
Delayed vs. Intermittent vs. Complete AV Block
The severity of an AV block, clinically, is based on the extent of the block:
- First degree AV block: not a true block, but delayed conduction from atrium to the ventricles without interruption in atrioventricular conduction
- Second degree AV block: intermittent block of atrial conduction to the ventricles, often in patterns such as 2:1, 3:1, etc
- Third degree AV block: complete block of signal from the atria to the ventricles
Signs and symptoms are along a continuum based on the degree of heart block, from rare if any symptoms to severe disturbances in cardiac output associated with other arrhythmias and sudden cardiac death.
Diagnosis of Heart Block?
Diagnosis of heart block begins with suspicious symptoms:
- Chest pain
- Pre-syncope (dizziness, light-headedness)
- Cardiac arrest
Electrocardiography (ECG) is the standard for diagnosing the presence of an AV heart block after a patient presents with the above symptoms that prompt suspicion.
- First degree AV heart block: which is often benign and asymptomatic, will show the delay in conduction from the atria to the ventricles. The characteristic waves that show on an ECG will have an extended interval for the run between the atrial and ventricular contractions (the P-R interval).
- Second degree AV block: which is an intermittent block, can present in patterns, such as 2:1, 3:1, etc., as applied to atrial/ventricular contraction sequences.
Second degree block is categorized as either:
- Mobitz I intermittent block: the interval between atrial and ventricular depolarizations (contractions) progressively lengthens until the refractory period summates in a skipped impulse to the ventricles. There is a skipped ventricular contraction. One Mobitz I type of block is Wenckebach arrhythmia.
- Mobitz II intermittent block: intermittent, non-conducted P waves (without the progressive prolongation of the PR interval as in Mobitz I).
Whereas Mobitz I second degree AV block is usually from AV node abnormalities, Mobitz type II second degree AV block usually means there is disease in the lower conductive pathway. Thus, Mobitz I block occurs at the AV node, but Mobitz II block occurs below the AV node in the ventricles.
- Third degree AV heart block: on ECG, will show no relationship between atrial and ventricular contractions.
There is complete failure of the AV node to conduct any of the atrial conduction pulses to the ventricles. The atria and ventricles contract independently of each other and are not synchronized enough to create an effective pump for the heart. Because the ventricles are electrically isolated from the normal pacemaker automaticity of the atrium, their own automaticity becomes the heart’s pacemaker, albeit with a slower “escape” rhythm of 40-60 beats per minute (normal heart rate = 60-100 bpm).
First Degree Heart Block Management
- Asymptomatic patients: Require no therapy. “Pacemaker syndrome” patients, whose quality of life is affected by the disconcerting awareness of their own heartbeat, can have a permanent pacemaker implanted, but this is actually rare.
Second Degree Heart Block Management
- Asymptomatic Mobitz I patients: Require no treatment.
- Symptomatic Mobitz I patients: Can be treated with medicinal pacing of the ventricles (atropine, dopamine) or temporary cardiac pacemaking in cases of myocardial infarction to mitigate the mortality risk associated with the occurrence of Mobitz 1 at the time of myocardial infarction.
In Mobitz I patients who do not have this arrhythmia from a reversible cause and are symptomatic, a permanent pacemaker can be implanted.
Third Degree Heart Block Management
In many ways, management of third degree AV (complete) block is the same as for second degree Mobitz II AV block.
- Stable patients: Since ventricular escape rhythms are unreliable and unstable, patients should be monitored continuously in the event of deterioration. Any symptoms due to bradycardia should indicate the need for a permanent implantable pacemaker.
- Unstable Patients: Hemodynamic instability mandates urgent, emergency treatment with atropine and a temporary cardiac pacemaker (transcutaneous or transvenous). Dopamine can be given for those with hypotension. Once stability is achieved clinically, a permanent pacemaker can be implanted.
Prevention of Heart Block
Prevention as pertains to heart block means prevention of either uncomfortable symptoms or dangers that run from dizziness to sudden cardiac death.
The range of heart block disease begins with first degree heart block, in which there are usually no symptoms, to second degree, in which patients may complain of palpitations and shortness of breath, to late second degree and third degree, in which there there may be life-threatening arrhythmias that interfere with cardiac output.
First Degree Heart Block
- Asymptomatic first degree heart block: Requires no treatment beyond mere follow-up and continued surveillance for the emergence of inconvenient or concerning symptoms.
- Symptomatic first degree heart block: Can present as “pacemaker syndrome” which is the frightening awareness of one’s own heart beat. Based on patient preference after informed consent, risks vs benefits of general anesthesia and implantable devices, and patient-perceived quality of life so affected, a permanent implantable pacemaker can be placed to counter the arrhythmia.
Second Degree Heart Block: Mobitz I
Like asymptomatic first degree heart block, asymptomatic Mobitz I patients require no treatment.
- Symptomatic Mobitz I patients: Can be treated with medicinal pacing of the ventricles, using atropine (anticholinergic) or dopamine (adrenergic agonist); or temporary cardiac pacemaking in cases of myocardial infarction to mitigate the mortality risk associated with a Mobitz 1 arrhythmia coincident with a myocardial infarction.
Symptomatic patients that have a reversible cause (medication, electrolyte imbalance, or athleticism) are treated by reversing this cause. Also symptomatic patients with a non-reversible cause can have a permanent pacemaker implanted.
Second Degree Heart Block: Mobitz II
- If a patient has Mobitz II arrhythmia due to reversible hyperkalemia, the potassium should be addressed, which often coincides with insulin management of hyperglycemia in diabetics
- Stable patients: Stable Mobitz II patients do not require immediate pharmacologic or pacemaker therapy, but they should be observed closely because Mobitz II itself is unstable and can progress to complete (third degree) heart block
- Symptoms from even a stable bradycardia will require an implantable pacemaker.
- Unstable patients: in cases in which are present hypotension, altered mental status, shock, chest pain, or pulmonary edema, immediate pharmacologic therapy (atropine) and a temporary pacemaker are required to increase heart rate and cardiac output. Once stable, any potentially reversible causes should be treated, followed by a permanent pacemaker, especially in patients without a reversible cause of the arrhythmia
- Patients with a high grade AV block (advanced Mobitz II, with more than one dropped beat) require permanent implantable pacemakers
Third Degree Heart Block: Complete Heart Block
In many ways, management of third degree AV (complete) block is the same as for second degree Mobitz II AV block:
- Stable Patients: Since ventricular escape rhythms are unreliable and unstable, patients should be monitored continuously in the event of deterioration. Any symptoms due to bradycardia should indicate a need for a permanent implantable pacemaker for the prevention of adverse cardiac events
- Unstable Patients: Hemodynamic instability mandates urgent, emergency treatment with atropine and a temporary cardiac pacemaker (transcutaneous or transvenous). Dopamine can be given for those with hypotension. Once stability is achieved, a permanent pacemaker can be placed