ECG/EGM Challenge Answer:

Atrial threshold test

Holly Daw – Chief Cardiac Physiologist
Barts Heart Centre, West Smithfield, London EC1A 7BE

Atrial threshold test

Although recorded as an episode of mode switch the electrogram displayed shows an atrial capture control test occurring.  This is the reason for the shortened AVD during atrial pacing (AP) seen after AP markers.  At the 3rd AP marker there is loss of atrial capture which results in the next atrial event, an atrial sense (AS) marker (sinus activity) being conducted to the ventricle via the intrinsic conduction system.  This is aided by IRS plus / AV hysteresis algorithms remaining active after an AS event resulting in a long PR interval.  At times the intrinsic sinus activity falls in the post ventricular atrial refractory period (PVARP), these events do not trigger an AV delay timer but are counted as atrial events contributing to the mode switch criteria. All other options can be ruled out as the trace does not show atrial bigeminy or ventricular ectopy. There is no atrial oversensing present and dynamic AV delays would not display the short AV delay seen after AP markers at the pacing rate displayed. Overall this is normal device behaviour and subsequent assessment of the atrial lead confirmed a normal functioning atrial lead with atrial capture at a high but stable output.