You're called to a newborn cared for at the surgical intensive care unit following abdominal surgery. The nurse reports sudden onset tachycardia.
On arrival you are presented the following ECG:
You can appreciate a narrow complex tachycardia (NCT) of 300 bpm (tachycardia cycle lengths - TCL - 200ms). Also you see sudden onset, initiated by premature atrial complex (PAC). However, there is no AH-jump (sudden prolongation of AVN conduction due to dual AV nodal physiology) present. The rhythm right before tachycardia onset seems to be sinus rhythm, as regular P waves are visible.
The newborn is hemodynamically stable. Therefore you try vagal maneuvers first and apply an icebag to the face.
Tachycardia suddenly stops. It ends with a QRS complex, no retrograde P wave on the last beat (ends on a "V" - ventricular event that is not conducted back to the atria). Looking at the tachycardia you find retrograde P waves on top of the T waves. The shape of the T waves changes with tachycardia termination. T waves become more rounded. Those during tachycardia appear more blunted, probably due to upright retrograde P waves atop.
In summary, all the findings above are in favor of trio ventricular reentry tachycardia (AVRT). Accessory pathway most probably concealed (retrograde conduction only, no preexcitation on the ECG). We started the infant on beta blockers for prophylactic reasons.