The correct response is option 3: this is an algorithm of Medtronic cardiac resynchronization therapy devices with automatic threshold-capture determination.1 Daily, at 01:00hours, the device first assesses the right ventricle (RV) with 3 support pacing pules (Vp) followed by 1 test pulse and 1 safety pulse at 90ms (Figure 1, asterisk). After completing this task, the frequency stability and V-V delay are checked (not apparent in the telemetry) and the device then searches for the left ventricular (LV) threshold. To do this, 3 support pacing pulses are applied followed only by another test pulse, without a safety pulse. Loss of capture from the left ventricle therefore leads to a pause if the patient does not have any intrinsic conduction or escape rhythm. In these cases, it is recommended to deactivate automatic threshold capture control, as it can cause ventricular arrhythmias triggered by sequences of short-long-short cycles.2
Response 2 is incorrect because the RV safety pulse prevents loss of capture. Response 4 is incorrect because there is no QRS complex and the warning is appropriate. Response 1 is also incorrect because the supposed dysfunction follows a pattern (3:1), indicating an issue with the algorithm and not true device dysfunction, which would who erratic behavior.