Table 1.

Comparison of the 3 forms of paroxysmal AVB.

Features	Intrinsic AV block (I-AVB)	Extrinsic vagal AV block (EV-AVB)	Extrinsic idiopathic AV block (EI-AVB)
	Cardiac syncope	Reflex syncope	adenosine syncope
ECG
Sinus rhythm	BBB frequent	BBB infrequent	Narrow QRS
Before AVB	–Sometimes AVB initiated by APB or VPB –PR unchanged	–Progressive sinus rate slowing (P-P cycle increase) –Generally progressive PR prolongation	–Sinus rate unchanged (P-P cycle unchanged) –PR unchanged
During asystolic AVB	Sinus rate increase	Sinus rate slowing	Sinus rate unchanged
	(P-P cycle decrease)	(P-P cycle increase)	(P-P cycle unchanged)
End of AVB	–Sometimes AVB interrupted by APB or VPB	Sinus rate acceleration (P-P cycle decrease)	Sinus rate unchanged (P-P cycle unchanged)
Follow-up	Progression to persistent AVB	No progression to persistent AVB	No progression to persistent AVB
Syncope
History of syncope	Short (mostly<1 year)	Long (since youth)	Short (average 2 years)
Prodromes	No or very short (≤5 s) prodromes	Always present >10 s	No or very short (≤5 s) prodromes
Structural heart disease	Mostly present	Mostly absent	Absent
Age on presentation	Elderly	Any age	Any age, mostly over 40 years
Efficacy of pacemaker therapy	Effective	Partially effective	Effective
Efficacy of theophylline therapy	Ineffective	Partially effective	Effective
Investigations
Plasma adenosine value	Normal	High	Low or very low
Adenosine (ATP) test	Usually negative	May be positive	Frequently positive (asystolic 3rd degree block)
Tilt table test	Usually negative	Mostly positive	Mostly negative
Electrophysiological study	Frequently positive	Negative	Negative
Carotid sinus massage	Usually negative	Frequently positive	Negative

Abbreviations: AVB=atrioventricular block; ECG=electrocardiography; BBB=bundle branch block; APB=atrial premature beat; VPB=ventricular premature beat