Cardiac Arrhythmia -Second Degree Atrioventricular Block

Second Degree Atrioventricular Block

Sometimes excitation completely fails to pass through the atrioventricular node or the bundle of His. When this occurs intermittently second degree atrioventricular block is said to exist.
There are three main variations of this.

Mobitz I (Wenckebach Phenomenon)

The QRS duration is normal and the P is upright and uniform. The PP interval is constant, but not all P waves are 1:1 with the QRS complexes. The PR interval gets longer until an impulse is not conducted due to the refractory state of the bundle of His. This cycle is then repeated (and known as a Wenckebach phenomenon).

                                                                              Conduction                

                                                                    Electrocardiogram





Mobitz II (Constant 2:1 Block)
here may be alternate conducted and nonconducted atrial beats, giving twice as many P waves as QRS complexes. This is called 2 to 1 conduction and may exist in different ratios. As with other rhythms, the P wave may only show itself as a distortion of a T wave.

                                                                             Conduction 

                                                                 Electrocardiogram






Mobitz II (Periodic Block)
Here we find that most beats are conducted with a constant PR interval, but occasionally there is an atrial contraction without a subsequent ventricular contraction. 

                                                                             Conduction 

                                                                     Electrocardiogram