cardiovascular disorders
Atrioventricular Block
Third degree AV block
Heart block is complete in third-degree AV block (see figure Third-degree atrioventricular block ). There is no relationship between P waves and QRS complexes (AV dissociation) in third-degree AV block. The P wave rate is greater than the QRS rate. The underlying rhythm here is a junctional escape rhythm with narrow QRS complexes and a rate of approximately 65 beats/minute. There is no electrical communication between the atria and ventricles and thus no relationship between P waves and QRS complexes (AV dissociation). Cardiac function is maintained by an escape junctional or ventricular pacemaker. Escape rhythms originating above the bifurcation of the His bundle produce narrow QRS complexes, relatively rapid (>40 beats/minute) and reliable heart rates, and mild symptoms (eg, fatigue, postural light-headedness, effort intolerance). Escape rhythms originating below the bifurcation produce wider QRS complexes, slower and unreliable heart rates, and more severe symptoms (eg, presyncope, syncope, heart failure). Signs include those of AV dissociation, such as cannonawaves, blood pressure fluctuations, and variation in loudness of the 1st heart sound (S1). Risk of asystole-related syncope and sudden death is greater if low escape rhythms are present. Most patients require a Cardiac Pacemakers . If the block is caused by Most patients with congenital 3rd-degree AV block have a junctional escape rhythm that maintains a reasonable rate, but they require a permanent pacemaker before they reach middle age. Less commonly, patients with congenital AV block have a slow escape rhythm and require a permanent pacemaker at a young age, perhaps even during infancy. 