ventricular AutoCaptureTM and automatic ventricular threshold measurement

N° 17
Manufacturer Abbott Device PM

69-year-old man implanted with an AccentTM DR pacemaker for complete atrioventricular block; programming of ventricular AutoCaptureTM; interrogation of the pacemaker and analysis of the monitoring curves of the automatic ventricular threshold measurement.

Graph and trace

The AutoCaptureTM curve shows low (<1 V) and regular ventricular threshold values; the tracing of the last automatic measurement shows an intermittent loss of capture at 0.5 V and an effective capture at 0.625 which therefore constitutes the ventricular threshold value.


The device uses 4 complementary algorithms for the functioning of ventricular AutoCaptureTM:

1) Capture confirmation which is based as explained previously on an analysis of the Evoked Response; if a ventricular pacing configuration is programmed to bipolar mode, the area under the curve is used (Paced Depolarization Integral); if a ventricular pacing configuration is programmed to unipolar mode, the signal slope (DMAX) is used; to verify the presence of the capture, there is an initial blanking period of 14 ms followed by an evoked response detection window of 46 ms; if the device detects an evoked response in this window, the capture is confirmed; if no evoked response signal is sensed, the device emits a 5 V backup pulse within 80 to 100 ms after the initial pulse to ensure capture;

2) Loss of capture Recovery; if capture verification confirms two consecutive losses of capture, the device launches the loss of capture recovery algorithm; on the ensuing cycle, the pacemaker delivers a backup pulse and then increases the automatic pulse amplitude by 0.25 V and searches for a capture; if no capture is confirmed, the device increases the pulse amplitude by 0.125 V on the next cycle and searches for the capture; when two successive captures are confirmed at the same voltage, the apparatus begins a threshold search; if no capture is confirmed before the device automatically increases the pulse amplitude to 3.875V, the device switches to High output mode: the pulse amplitude is set to 5V and the duration pulse to 0.5 ms (or longer if the programming value is higher); after 128 cycles, the device reinitiates a threshold search;

3) Fusion avoidance: fusions with intrinsic activation can skew threshold measurements with untimely activation of the loss of capture recovery algorithm; in dual-chamber mode, a single absence of evoked response sensing, requiring the delivery of a 5 V backup pulse, automatically causes the next cycle to prolong the paced and sensed AV delay by 100 ms in order to search for intrinsic conduction; in first line treatment, the algorithm « makes the assumption » that the loss of capture is due to a fusion and not to a lack of pulse strength; this function acts in a manner similar to VIPTM; if the loss of capture is confirmed after extending  the paced or sensed AV delay (2 consecutive beats requiring the delivery of a 5-V safety pulse), the pacemaker triggers its capture recovery algorithm;

4) Periodic measurement of the pacing threshold: to automatically determine the pacing threshold, the device decreases the pulse amplitude by 0.25 V every two cycles; if this leads to a loss of capture, the device emits a 5 V backup pulse (safety margin) 80 to 100 ms after the first test pulse; in the event of loss of capture over two consecutive cycles at the same amplitude, the algorithm then increases the pulse amplitude in steps of 0.125 V every two cycles; two consecutive captures at the same amplitude must be confirmed to determine the new pacing threshold value; if the decremental search fails to determine a loss of capture at the lowest pulse amplitude setting, i.e. 0 V, the device switches to « High output mode » for a duration of 128 cardiac cycles, then reinitiates the threshold search; if the pacing threshold determined by the algorithm exceeds 3.875 V for a given pulse duration, the ventricular AutoCaptureTM is automatically deactivated and the amplitude is reprogrammed to 5 V (High output mode); the threshold search is repeated a) after each loss of capture recovery operation, b) automatically every 8 hours, c) upon removal of the telemetry head, d) upon removal of the magnet, e) when the operator performs the pacemaker AutocaptureTM threshold test.

Message to remember

The programming of ventricular AutoCaptureTM enables 1) regular monitoring of pacing thresholds, 2) a reduction in battery consumption (margin of only 0.25V compared to the measured value), 3) a gain in terms of safety in pacemaker-dependent patients presenting significant threshold variations within the capacity limits of the algorithm (4.5 V for the programmed pulse duration).