What is the rhythm?
a) Sinus bradycardia
b) Idioventricular rhythm
c) Junctional rhythm
d) None of the above
Answer: c) Junctional rhythm
Regular narrow QRS rhythm at 60 per minute is seen with normal QRS and T waves. P waves are not seen. The first possibility is a junctional rhythm. In a mid junctional rhythm the P waves will be within the QRS and not visible. In a high junctional rhythm the P waves will be inverted in leads II, III and aVf, occuring with a shor PR interval. In low junctional rhythm the P waves are inverted in these same leads, but will occur after the QRS with a short RP interval. In mid junctional and low junctional rhythm there will be regular cannon waves in the jugular venous pulse as the atrial contraction is during ventricular systole when the AV valves are closed, resulting in back flow of blood from right atrium to the superior vena cava.
A slow and regular narrow QRS rhythm with absent P waves can also occur in atrial paralysis due to severe hyperakalemia or due to any other reason. But in this case hyperkalemia is unlikely as the T waves are normal in amplitude. Moreover, the QRS can also become wide in severe hyperkalemia. Atrial standstill can rarely occur as an idiopathic disorder, in some muscular dystrophies and following radiofrequency ablation procedures in the right atrium.
A third possibility for a slow, regular narrow QRS rhythm with absent P waves is fine atrial fibrillation with complete heart block. In that situation, the QRS is regular because it is initiated by a junctional focus and not by the irregularly conducted fibrillary waves. It is unlikely that all the 12 leads will not pick up the fibrillary waves in fine atrial fibrillation.
In this particular case this rhythm was transiently seen after conversion of supraventricular tachycardia with intravenous verapamil. Normal sinus rhythm was recorded after some time.