A 33-year-old man is admitted with acute respiratory exacerbation of his cystic fibrosis. While being vigorously treated in the medical ICU, he suddenly develops palpitation, increased dyspnea and weakness:

• Supraventricular tachycardia, 197/min

There is a narrow-QRS tachycardia. No P waves are seen and a specific diagnosis of the nature of the arrhythmia cannot be made. Probably AVNRT (in a large number of cases of AVNRT atria and ventricles are depolarized simultaneously and the P waves are not seen). The patient had no previous episodes of tachyarrhythmia.

The tachycardia did not respond to vagal stimulation. It stopped spontaneously within 25 minutes and did not recur. 

•  Sinus tachycardia, 118/min
•  Otherwise normal ECG