heart block
Heart block is a traffic jam on the road between the upper and lower chambers. The atria fire normally, but the electrical signal struggles to get through the junction — the AV node and the conducting bundle below it — to reach the ventricles. Depending on how bad the blockage is, beats may simply be delayed, occasionally dropped, or completely cut off.
Doctors grade it in three degrees of increasing severity. First-degree block only slows the signal, with every beat still conducted. Second-degree block lets some beats through and drops others. Third-degree, or complete, block stops all conduction, forcing the ventricles to fall back on their own slow, unreliable escape rhythm.
Mild block can be a normal variant or a harmless drug effect, but more advanced block from degeneration, a heart attack or certain medicines can cause dangerously slow rates, fainting or even cardiac arrest. The decisive treatment for symptomatic high-grade block is a pacemaker, which artificially bridges the broken connection.
On a routine ECG, a man's PR interval is long and every few beats one fails to conduct to the ventricles; he is referred to a cardiologist to decide whether a pacemaker is needed.
The grade of AV block, not its mere presence, determines whether a pacemaker is needed.
“Heart block” refers to blocked atrioventricular conduction and is unrelated to a “blocked artery.” The two share the word block but describe completely different problems.