Objective To investigate the electrocardiogram（ECG） manifestations

significance and characteristics of phase 3 block of atrioventricular conduction pathway or bypass. Methods The ECG of 3-phase block of 27 cases were retrospectively analyzed. The location

critical heart rate

and ECG characteristics of 3-phase block were analyzed. Results There were 5 cases of left bundle branch block

8 cases of right bundle branch block

1 case of left anterior branch block

2 cases of left posterior branch block

5 cases of atrioventricular block

1 case of I degree atrioventricular block and left anterior branch block

and 5 cases of bypass. The critical heart rate of each patient was different. The critical heart rate of 3-phase block was relatively constant in a certain period of time

but there was a certain change from 24 h dynamic observation. Conclusions Phase 3 block can occur in any part of atrioventricular conduction system

and it is a pathological change. The bypass is also a predisposing site for 3-phase block

which may be a pathological change

but its unique physiological characteristics cannot be excluded. When3-phase block occurs in normal atrioventricular pathway

it may affect cardiac function. When 3-phase block occurs in bypass

reverse atrioventricular reentrant tachycardia should be avoided. Three phase block is found when the heart rate changes at the critical heart rate

so dynamic electrocardiogram is the main method to detect it.