This ECG shows a narrow QRS tachycardia at a rate of around 180/min. No P waves are seen to precede the QRS complexes. Then it has to be taken as supraventricular tachycardia.
Close scrutiny of the tracing shows probable negative P waves after the QRS complex in lead II (red arrow). They are called ‘pseudo s‘ waves produced by retrograde P waves. This pattern is seen in atrioventricular nodal reentrant tachycardia (AVNRT). Here the RP is shorter than PR (from retrograde P to next R wave), qualifying for the short RP tachycardia of the slow-fast variety of AVNRT. In the slow-fast variety, an atrial ectopic beat is conducted down the slow pathway as it finds the fast pathway refractory after the previous sinus beat. It may be noted that the slow pathway recovers fast while fast pathway recovers next. This feature is seen in those with dual AV nodal physiology. The sudden increase in PR interval after an atrial ectopic beat (an atrial extrastimulus during an electrophysiology study) is known as ‘PR jump‘. PR jump has to be more than 50 ms to be significant. This tachycardia can be called as a short RP tachycardia as RP is shorter than PR.
The R waves in V1 (green arrow) are a bit too tall to be called as the corresponding ‘pseudo r’, but the r waves in aVR are of the size usually seen with pseudo r (black arrow). The taller R wave in V1 could be because there is an element of incomplete right bundle branch due to the fast rate (phasic aberrant conduction).