ECG challenge: a paradoxical response

Clinical vignetteA 65-year-old man was referred for incidental finding of bradycardia.Further history revealed subtle symptoms of reduced exercise tolerance, although he was minimally symptomatic.Electrocardiography (ECG) demonstrated second-degree atrioventricular (AV) block with predominant 2:1 conduction and occasional 3:2 conduction with Wenckebach periodicity, together with incomplete right bundle branch block (RBBB) and left anterior fascicular block (LAFB) (Figure 1A).Carotid sinus massage resulted in improvement in AV conduction, from 2:1 to 1:1 conduction (Figure 1B).In contrast, atropine administration worsened AV conduction, with progression from 2:1 conduction to high-grade AV block (Figure 1C). Question 1Which of the following statements is correct? A. The observed response to carotid sinus massage suggests AV nodal disease.B. The observed response to atropine suggests AV nodal disease.C. Mobitz type I second-degree AV block may arise from infranodal disease.D. Mobitz type I second-degree AV block is usually symptomatic.E.An ECG showing only 2:1 AV block can be classified as Mobitz type I or Mobitz type II.The correct answer is C.