Page 139 - Rapid Review of ECG Interpretation in Small Animal Practice, 2nd Edition

Answer 57 New Advanced-Level ECG Cases

Answer 57
VetBooks.ir 1 ECG 57a shows that the pacemaker is oversensing. The pacemaker occasionally detects the T wave

and mistakes it for an intrinsically generated QRS complex. The pacemaker is programmed to pace at
75 bpm (i.e., time between ventricular paced [VP] beats of 800 ms). In this example, the pacemaker
senses the T wave (VS = ventricular sense, horizontal blue arrow) and waits to pace again 800 ms after
the sensed T wave (ECG 57b). This results in a R-R interval that is longer than 800 ms (green arrow)
and heart rate slower than 75 bpm each time the pacemaker senses the T waves.
2 Oversensing can result in pacing at a lower than expected rate as the inappropriate detection inhibits
the pacemaker from pacing at the correct rate. As described in Question 56, sensitivity refers to the
minimum electrical voltage that the pacemaker will detect as a native depolarization. Oversensing
occurs if the pacemaker senses electric signals, such as skeletal muscle myopotentials, electromagnetic
interference, or P or T waves and interprets them as native QRS complexes. The likelihood of
oversensing can be increased by inadequate programming of sensitivity settings or refractory periods,
lead or device failure, or changes in signal amplitude due to change in tissue characteristics that can
occur over time (i.e., fibrosis. hyperkalemia). In most cases, the inappropriately sensed electrical
events are of lower voltage than the true QRS complexes, and decreasing the pacemaker’s ventricular
sensitivity will correct the problem. In this case, the sensing threshold value was increased from 3.8 to
5.0 mV, and the oversensing was corrected.

57b
I

800 ms VS VS
II
800 ms

VP VP VP VP VP
III

aVR

aVL

aVF

v: Speed: 25 mm/sec Limb: 10 mm/mV Chest: 10.0 mm/mV F 60~ 0.

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