Catching a 3rd Degree AV Block on an ECG A 3rd degree AV block (complete heart block) is electrical divorce between the atria and ventricles. What is a 3rd Degree AV Block? In a normal heart, the atria and ventricles work in sync — the SA node fires, the AV node passes it along, and the ventricles contract. In 3rd degree block, that connection is completely lost. ✅ Atria fire on their own (P waves from the SA node) ✅ Ventricles fire on their own (escape rhythm from AV junction or below) 🚫 But t