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 pacemaking 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).