Page 135 - Rapid Review of ECG Interpretation in Small Animal Practice, 2nd Edition

Answer 56 New Advanced-Level ECG Cases

Answer 56
VetBooks.ir 1 ECG 56a shows that the pacemaker is undersensing the dog’s intrinsically generated (native) QRS

complexes. This occurs when the pacemaker does not sense or detect the intrinsic QRS complex
(black arrow) and continues to pace and deliver an electrical output (i.e., pacing spike) (blue arrow)
when it should be resting (ECG 56b). This pacing spike also demonstrates loss of capture; it does
not cause a subsequent QRS complex because the pacemaker delivered the stimulus during the ST
segment (i.e., ventricular refractory period) of the preceding intrinsic beat and could not stimulate a
ventricular depolarization. This is a potentially dangerous scenario as delivery of pacing spike during
this vulnerable period of repolarization can induce ventricular fibrillation (VF). The greens arrows
represent a pacing spike followed by subsequent capture and a paced QRS complex.
2 Sensitivity refers to the minimum electrical voltage that the pacemaker will detect as a native
depolarization. For example, a sensitivity of 3 mV means that the pacemaker will only sense a
signal equal to or greater than 3 mV. The pacemaker ignores all other voltages. Thus, one cause of
undersensing is that the sensitivity setting is higher than the voltage of the native QRS depolarizations
at the pacemaker lead tip. To correct undersensing, the ventricular sensitivity of the pacemaker is
increased by decreasing the threshold value. In this case, the sensing value was decreased to 2.5 mV.
ECG 56c shows that the change in sensitivity resulted in the pacemaker appropriately sensing the
intrinsic beats and only pacing when the intrinsic rate falls below 110 bpm.

56b II 50 mm/sec
10 mm/mV
F 60~ 0.5–100 Hz W

aVR

V1

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