A heart block is 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:
The 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 fibres (His bundle and Purkinje fibres) to the ventricles and trigger their contraction.
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 heartbeat and/or atrial fibrillation). Still, 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, leading 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 a 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 any symptoms. Still, 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 permanent pacemaker implantation. Pacemaker reconnects atria and ventricles electrically and restores normal coordination between heart chambers. Pacemaker insertion in these indications leads to a normal quality of life and an excellent prognosis.