User's Manual

Cardiac Airbag Technical Manual 27
2.2.4 Onset and Stability
In addition to the standard tachycardia detection parameters
previously described, the VT Monitoring Zone incorporates two
additional detection enhancements: Onset and Stability. Both
Onset and Stability are preset to standard values and are not
programmable for the VT Monitoring Zone.
2.2.4.1 Onset
inus tachycardia (often characterized by a gradual rate
a 20% adaptive value
(expressed as a percentage of the latest cardiac cycle length).
t and any additional detection
criteria are satisfied.
terval with the three preceding cardiac
within the range.
The Onset function provides an additional discrimination test that
must be satisfied before a VT tachyarrhythmia can be declared.
The purpose of this detection parameter is to discriminate
between s
increase) and a ventricular tachycardia, which typically begins
with an abrupt rate change.
Onset criterion evaluates the most recently sensed cardiac
intervals and compares it to the previous four-interval sliding
average. Onset will be satisfied if a change in cycle length
exceeds the preset Onset value (as compared to the average)
and is followed by a cycle that lies within the corresponding VT
zone. Onset criterion is defined as
VT is not declared until Onse
2.2.4.2 Stability
The purpose of Stability is to assist in the discrimination of stable
ventricular tachyarrhythmias from SVTs that conduct irregularly
down to the ventricles (i.e., atrial fibrillation). Stability evaluates
sudden changes in the cardiac cycle length. The Stability
criterion compares each in
cycles to determine if they remain within the Stability range (as
defined by the parameter setting of ± 24 ms). A rhythm is
declared stable after the number of intervals (equal to the
Interval Count) is found to be stable