Patient

• 53-year-old man
• Evia DR-T dual-chamber pacemaker (Biotronik) for complete atrioventricular block
• lipothymia on exertion
• sudden drop in heart rate during exercise test

Tracing

• atrial undersensing (P wave not sensed); low amplitude signal
• in the absence of atrial sensing, no ventricular pacing and pause (ventricular rate divided by 2) explaining the discomfort felt by the patient
• important beat to beat variation in the amplitude of the atrial signals (breathing cycles)

Comments

• this patient was very symptomatic on exertion with marked limitation of his exercise capacity
• the tracing showed intermittent atrial undersensing
• atrial EGM analysis revealed a wide variability in the amplitude of atrial signals as a function of the respiratory cycles
• in a patient with complete atrioventricular block, atrial undersensing results in a sudden drop in rate, which is often highly symptomatic
• atrial undersensing is probably much more frequent than a 2:1 block or ventricular oversensing or loss of capture of the 2 leads, which constitute the other causes of a sudden drop in rate on exertion
• the amplitude of the sensed atrial signal may vary according to the position of the patient and as a function of the respiratory cycle
• appropriate atrial sensing may be systematically tested with deep respiration to unmask significant fluctuations of the signal with breath