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Chapter 5 Programmable Parameters
Evia HF / HF-T Technical Manual
PAGE 52
• If the first SA after activating VCC is completed successfully, but subsequent SA’s are not
completed successfully, then VCC is suspended, and the pacing amplitude is programmed to the
last measured threshold plus the safety margin. The SA will be attempted up to three times. After
the 3rd failure, VCC is disabled, and the pacing amplitude is programmed to the MaxVCCAmp plus
the Safety Margin of 1.2 V. VCC then requires manual reactivation of the feature with the
programmer.
Automatic Threshold Measurement
• The Automatic Threshold Measurement is the component of the VCC feature that measures the
ventricular pacing threshold by stepping down the output until non-capture occurs.
• The Automatic Threshold Measurement occurs over a series of cardiac cycles and begins at the
threshold test start that decreases until capture is lost. AV delay is shortened to 50 ms after a
paced atrial event and to 15 ms after a sensed atrial 4event to ensure ventricular pacing.
• The pacing amplitude decrements with every paced beat by 0.6 V, until the first non-captured beat.
The algorithm then decrements by smaller increments of 0.1 V until the first failed capture, at which
point it determines that the capture threshold is preceding value. .
• If two out of three non-capture events are detected, the pacing amplitude is set to the sum of the
pacing threshold voltage and the programmed safety margin. If two non-captures are detected
when the voltage decrements are greater than 0.1 V, the pacing amplitude is set to the previous
amplitude and then the amplitude decrements by 0.1 V until the pacing threshold is determined.
When another two non-capture are detected, the previous voltage setting is the pacing threshold.
• The pacing amplitude is then set to the pacing threshold plus a programmable safety margin (0.5 V
through 1.2 V in steps of 0.1 V).
• In addition to performing the threshold search after a loss of capture, the search is also conducted
at a programmable interval or a specific time during the day to provide an accurate safety margin
even with gradual changes in the pacing threshold. Note that a CV initiated by spontaneous loss of
capture will reset the timer that triggers the next periodic measurement if the search is
programmed to interval.
Capture Verification (CV)
The Capture Verification function is the component of the VCC feature that provides beat-to-beat
capture verification. (Not available in the ATM mode).
• If VCC determines that capture has been maintained, then the pulse amplitude remains at that
current setting and no action is required.
• If VCC determines that non-capture (NC) occurred, then a safety back-up pacing pulse is delivered
at an increased energy (same output voltage and pulse width extended to 1.0 ms) within 130 ms
after the non-captured pacing pulse.
• If a series of ventricular pacing pulses at varying AV delays result in loss-of-capture, a Signal Analysis
(SA) and Automatic Threshold Search are initiated to determine the current pacing threshold.