ICD, cardiac pacing, triggered pacing, atrial fibrillation, automatic mode switchingA patient with an ischemic cardiomyopathy (left ventricular ejection = 19%) and paroxysmal atrial fibrillation (AF) received a cardiac resynchronization implantable cardioverterdefibrillator (ICD) device () in August 2010. 1 The DDD mode and automatic mode switching (AMS) were programmed. At the time of follow-up in July 2011, the electrocardiogram (ECG) showed AF with a well-controlled ventricular rate (Fig. 1). The relevant programmed parameters confirmed by telemetry printout were as follows: Lower rate = 60 pulses per minute (ppm), upper rate = 130 ppm, paced atrioventricular (AV) delay = 180 ms, sensed AV delay = 140 ms, postventricular atrial refractory period (PVARP) = 250 ms, and atrial upper rate (AUR) = 240 ppm. Lead and shock impedances were determined manually by a programming command and found to be normal. During the procedure testing the impedances, irregular and rapid ventricular pacing occurred with three R-R intervals of about 375 ms (corresponding to a rate of 160 ppm) shorter than the programmed upper rate interval of about 460 ms (corresponding to a rate of 130 beats/min) (Fig. 2). The tracings raise a number of questions: (1) How do you explain the discrepancy of rates with ventricular pacing occurring with R-R intervals corresponding to a rate of 160 ppm, which is faster than the programmed upper of 130 ppm? (2) Why did telemetry reveal an AUR of 240 ppm? (3) Figure 1 shows that in the DDI mode more than one atrial-sensed (AS) event labeled as "S" occurred during the lower rate interval or the interval between two sensed ventricular events. What is the mechanism considering that such a response is contrary to the function of the traditional DDI mode?A Biotronik programmer command executes the manual measurement of pacing and shock impedances. For the pacing impedance measurement (PIM), the device automatically switches to the temporary DDT mode with parameters different from those permanently programmed. 1 During the DDT mode, the device functions with a fixed upper rate of 160 ppm, AV delay = 50 ms, and PVARP = 250 ms and AMS is disabled. During PIM, the pacing amplitude in the affected chamber is set to 2.4 V or the programmed value if >2.4 V. The temporary program for PIM usually takes 5-6 seconds. The manual PIM procedure normally delivers two to three test pulses per channel (shock channel, right atrial, right ventricular, and left ventricular for devices with cardiac resynchronization). Thus, the device limits the number of ventricular paced (VP) beats at 160 ppm to 4-6 cycles. Only nonrefractorysensed events in the atrium can be used to deliver a triggered atrial stimulus in the temporary DDT mode during manual PIM. The test pulses for the pain-free shock impedance always have a lower pacing voltage and shorter pulse width.PIM is performed sequentially in each channel so that triggering occurs in one channel at a time. Atrial triggering with the 50-ms AV delay promotes ventricular pacing for PIM as i...
