1. From: Pacemaker Malfunction
Ann Intern Med. 1993;119(8): doi: /
Figure Legend:
Example of functional undersensing.The electrocardiographic tracing was obtained from a patient with a ventricular pacemaker programmed to 70 beats/min. Of the five pacing artifacts shown, the first, fourth, and fifth result in effective ventricular depolarization. There is failure to capture with the second and third pacing artifacts. The third pacing stimulus occurs approximately 850 ms after the preceding intrinsic QRS complex. This timing represents normal sensing, that is, pacing at a rate of 70 beats/min. However, the fourth pacing stimulus occurs approximately 640 ms after the preceding intrinsic QRS complex, indicating that the preceding QRS complex was not sensed because it occurred during the pacemaker's ventricular refractory period. This is an example of functional undersensing; that is, undersensing was a function of the ventricular refractory period.
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