Onset is an SVT discriminator used to distinguish between rapid and gradual onset of fast ventricular rhythms. If the ventricular rate increases gradually, as typically happens during sinus tachycardia, the device does not classify sensed ventricular events that occur in the VT detection zone as VT events. If the ventricular rate increases rapidly, as typically happens during a ventricular tachycardia episode, the device does classify sensed ventricular events that occur in the VT detection zone as VT events. Onset is triggered when a patient's heart rate during exercise reaches the programmed VT Zone.
This feature can be found in some Medtronic ICD and CRT-D devices. Please go to manuals.medtronic.com or consult with your local Medtronic representative regarding device models available in your geography.
Onset is nominally Off and can also be programmed to On or Monitor. To program this feature, go to Params -> Detection (V.).
An additional programming option is Percent, which determines the percent change in rate that must be met in order for Onset to classify the increase in ventricular rate as abrupt. Nominal percent is 81%.
Onset utilizes the theory that sinus tachycardia can generally be distinguished from ventricular tachycardia by the speed of the ventricular rate increase. Typically, sinus tachycardia is characterized by a gradual ventricular rate increase, while ventricular tachycardia exhibits a sudden ventricular rate increase.
This diagram illustrates the difference in rate acceleration between ST and VT.
Onset compares average cycle lengths over time, looking for significant changes in the rhythm that may indicate VT. The averages are calculated using sets of four beats, while a programmable percentage defines what is abrupt. Specifically, it looks at a rolling average of the most recent four ventricular intervals and compares that to the average of the preceding four intervals. If the percent change is greater than the programmed onset percent, then detection can occur. When the percent change is less than the programmed onset percent, detection will be withheld until the percent range changes. The Onset feature applies to both VT detection and VT monitoring.
In order for Onset to declare VT in this example, the average of the 4 most recent ventricular intervals must be faster than 430 msec.
- Onset is operational as long as the intervals are within the VT zone.
- Onset is operational before detection occurs and therefore works before PR Logic™ will operate.
- Onset will operate only for initial detection and is not operational on redetection.
- Onset may delay detection of true VT in patients who experience exercise-induced ventricular tachycardia episodes.
This example illustrates the gradual acceleration of the ventricular rate which eventually enters the VT detection zone. The average cycle length of each set of 4 intervals is never 81% or less than the previous set of 4 intervals (81% is the programmed Onset Percent value). Therefore, the intervals are labeled VS due to Onset intervention, even though they fall into the VT zone.
Sources: Medtronic Protecta™ XT DR Clinician Manual; Medtronic Protecta™ XT CRT-D Clinician Manual.