This 64-year-old man received a Medtronic Consulta CRT-D in the context of ischemic cardiomyopathy with complete AV block. He presented with shortness of breath during exercise.
A stress test (squats) was performed during an ambulatory visit, using the telemetry wand placed over the device. Atrial sensing was recently programmed to 0.8 mV because of ventriculo-atrial crosstalk. Atrial sensing at rest was approximately 1.2 mV.
The first channel is an ECG lead with superimposed markers, the second is a bipolar RV EGM, third is the bipolar atrial EGM, and the fourth is the far-field RV coil / LV tip EGM.
This tracing illustrates the importance of an intact atrial lead for an optimal resynchronization. This patient presented with intermittent episodes of atrial undersensing. The analysis of the atrial EGM showed a wide respiratory variability in the atrial signal amplitude. In a patient with preserved AV conduction, atrial undersensing is associated with the reappearance of spontaneous ventricular events, without sudden decrease in HR and prominent symptoms. Conversely, in patients presenting with complete AV block (as in this case), atrial undersensing causes sudden decreases in HR, which are often poorly tolerated. Atrial undersensing is the number one cause of sudden drop in heart rate during exercise, and is much more common than the other causes, including 2:1 AV conduction, ventricular oversensing, and loss of capture by both ventricular leads. In addition, the absence of reliable monitoring of the P wave during exercise may also have an arrhythmogenic effect, should the atrium be paced in the vulnerable period.
