A heart block is a situation when the electrical signals from the top heart chambers (atria) that make the heart contract reach the bottom heart chambers (ventricles) with delay or not at all. A heart block is therefore also called atrioventricular or AV block. There are three degrees of heart block and symptoms usually appear only in the third (complete heart block). Patients with complete heart block may present with:
Sinus node, a cluster of specialized heart muscle cells in the right atrium, is the heart’s natural pacemaker. Sinus node fires in regular intervals electrical impulses that pass through atria to the atrioventricular node (AV node). AV node is the only electrical connection between atria and ventricles. Electrical impulses then continue through the AV node and via specialized fibers (His bundle and Purkinje fibers) to the ventricles and trigger their contraction.
Heart block occurs when there is an delay or interruption in the transmission of electrical impulses from AV node to the ventricles. Even complete heart block normally does not lead to an immediate cardiac arrest but the back-up (escape) rhythm is usually slow (bradycardia) and unstable. Heart block is diagnosed with ECG, in case of intermittent episodes 24-hour ECG monitor or longer term monitoring with ILR may be needed.
A first degree AV block means only a minor delay in conduction between atria and ventricles leading to prolongation of PR interval on ECG above 200 ms. A first degree AV block can be a part of sick sinus syndrome (together with episodes of bradycardia = slow heart beat and/or atrial fibrillation) but it does not necessarily mean any underlying cardiac problem. First degree AV block is often seen in athletes, does not cause any symptoms and does not require any treatment.
In this case some but not all impulses from atria do not reach ventricles which leads to skipped beats. There are further two subtypes of the second degree AV block:
Third degree AV block is also called complete heart block because the electrical impulses from atria do not reach ventricles. Third degree AV block can be congenital (present from birth, very uncommon) or acquired (develops later in life due to wear and tear or injury, e.g. in heart attack). Third degree AV block usually generates symptoms of tiredness, breathlessness, dizziness and blackouts and in some cases it can be a life-threatening emergency requiring prompt treatment with pacemaker.
First degree AV block and second degree Mobitz I AV block normally don’t require treatment and can be entirely normal in fit and active people. Second degree Mobitz II AV block may not generate many symptoms but it can progress unpredictably to the third degree AV block which may manifest with dramatic symptoms and lead to sudden death.
Treatment of Mobitz II AV block and complete heart block is a permanent pacemaker implantation. Pacemaker reconnects electrically atria and ventricles and restores normal coordination between heart chambers. Pacemaker insertion in these indications leads to normal quality of life and excellent prognosis.