Inappropriate Ventricular Pacing With Exercise in an 11‐Year‐Old Girl With a History of Idiopathic Paroxysmal AV Block

An 11-year-old girl presented to establish cardiology care after relocation. She originally presented at 4 years of age with abrupt episodes of syncope associated with abdominal pain, and she was found to have paroxysmal atrioventricular (AV) block of unknown etiology. Her baseline electrocardiogram (ECG) and AV conduction were otherwise normal, and she had a structurally normal heart on echocardiogram. She underwent placement of a single-chamber epicardial pacemaker (Adapta, Medtronic, Minneapolis, MN, USA). Since that time, she has had done well with no further syncope. Upon presentation to our clinic, the device interrogation revealed the following settings: VVI at 50 beats per minute (BPM), hysteresis at 40 BPM, capture threshold 1.25 V @ 0.4 ms, R waves 16 to >22.4 mV, sensitivity 5.6 mV, impedance 684 ohms with ventricular pacing <0.1%. As part of her initial evaluation at our center, the patient underwent an exercise stress treadmill. Her baseline ECG demonstrated sinus rhythm. She was stressed according to the Bruce protocol, and during stage 3, as she reached 181 BPM, she began to have isolated ventricular-paced beats (Figure 1). Pacing increased in frequency as her heart rate continued to climb to her maximal rate of 200 BPM and continued as she recovered until her heart rate fell to less than 180 BPM before resolving. Question for the reader: What is the cause of the ventricular pacing? The tracing demonstrates ventricular pacing during intrinsic sinus rhythm with intact AV conduction, which should not occur in VVI mode with appropriate pacemaker function. The differential diagnosis would include undersensing, but in this case, the R waves were robust, and the sensitivity was programmed appropriately. The progressive nature of the pacing, occurring first around 180 BPM, suggested a pacemaker-determined refractory issue. Her device had a nominal ventricular refractory period of 330 ms or 181 BPM. As her heart rate approached 181 BPM, she had individual beats with cycle length ≤ 330 ms, which initiated ventricular refractory events. Once her heart rate consistently crossed 181 BPM, she had a pattern of repetitive ventricular refractory events and ultimate ventricular pacing when she reached the hysteresis rate of 40 BPM (1500 ms, Figure 2). With reprogramming of the ventricular refractory period to 270 ms, the ventricular pacing was eliminated, and all ventricular events were appropriately sensed. This behavior occurred due to the combination of single-chamber pacing with high intrinsic heart rate, as can be seen with devices for pause protection in young patients. While the theoretical risk of inappropriate pacing occurring due to the use of the ventricular refractory period has been described, an actual case has never been reported to our knowledge [1]. Providers should be aware of this possibility to avoid unnecessary and potentially dangerous inappropriate ventricular pacing. C.K. drafted the original article. G.A. participated in the direct patient care and supervised the editing of the article. B.K., L.B., and C.F. all made significant contributions in editing the article. The authors have nothing to report. The authors have nothing to report. The authors declare no conflicts of interest. The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.