The device achieves rate adaptation through programming of either standard motion-based pacing via a capacitive accelerometer or by the means of the principle of closed loop stimulation (CLS) which involves the translation of myocardial contractility into patient-specific pacing rates.
- for standard motion-based rate adaptation, the pacemaker is equipped with an accelerometer located on the hybrid circuit of the pacemaker; this sensor produces an electric signal during physical activity of the patient
- when in CLS mode, the pacemaker monitors and processes the intracardiac impedance signal associated with myocardial contraction dynamics; changes in the waveform of this impedance signal are associated with changes in the contraction dynamics of the patient’s heart due to the heart’s inotropic response to exercise
The sensor gain defines the slope of the linear function between exertion and pacing rate.
- the optimum setting is achieved when the desired maximum pacing rate during exertion is reached during maximum exercise levels
- if the sensor-driven rate is not sufficient at high levels of exertion, the sensor gain setting should be increased
- the sensor gain should be reduced if high pacing rates are obtained at low levels of exertion
When the Automatic Sensor Gain setting is programmed, the Sensor Gain parameter is adjusted automatically.
- the device samples the sensor-indicated rate
- the sensor gain is increased by 10%, if the activity rate does not reach or exceed the programmed activity rate (fixed to 90% of maximum sensor rate) for 30 minutes each day over 7 consecutive days; an increase in gain cannot occur more often than every 7 days
- if, during the 24 hour period beginning at midnight, the activity rate reaches or exceeds the programmed activity rate (90% of maximum sensor rate) for one hour, the sensor gain setting is reduced by 10%
- a change in the sensor gain only occurs at midnight
The Automatic Sensor Gain function is primarily influenced by the Maximum Sensor Rate setting. Therefore the Maximum Sensor Rate must be appropriately selected.
The effects of rate adaptation are limited to sensor signals exceeding the programmable sensor threshold. Sensor signals below this threshold do not affect rate response.
- the programmable sensor threshold ensures that a stable rate at rest can be achieved by ignoring sensor signals of low amplitude that are not related to exertion
- if the pacing rate at rest is unstable, or tends to stay above the lower rate without activity, the sensor threshold should be increased
- the sensor threshold should be reduced if a sufficient rate increase is not observed at a given level of exertion
The rate increase parameter determines the maximum rate of change in the pacing rate if the sensor signal indicates increasing exertion.
The rate decrease parameter determines the maximum rate of change in the pacing rate, if the sensor signal indicates decreasing exertion.
Maximum Activity (Sensor) Rate
Regardless of the sensor signal strength, the pacing rate during sensor-driven operation cannot exceed the programmed maximum sensor rate.
Closed Loop Rate
The device can be programmed to use a unique rate-adaptive principle called Closed Loop Stimulation (CLS) to adapt the patient’s pacing rate. The DDD-CLS and VVI-CLS mode use the CLS concept to determine the pacing rate variations.
- the device measures electrical impedance by injecting a small AC current between the pacemaker case and the ventricular electrode tip
- the induced voltage (which is proportional to the intracardiac impedance) is measured between pacemaker case and ventricular electrode tip
When the pacemaker is programmed to CLS rate adaptation, the pacing rate during CLS-driven operation will never exceed the programmed maximum closed loop rate.