The activity sensor is an accelerometer embedded in the device, which detects the patient’s body motion.
Since the activity sensed varies among patients, the motion sensitivity can be adjusted by reprogramming the activity threshold parameter.
- by lowering the threshold, the pacing rate is modulated by less prominent motion
- an increase in threshold requires more marked body motion to modulate the pacing rate
- if the threshold is increased, the body movements must be greater to have an effect on the pacing rate
- the activity counter used to calculate the sensor rate is regulated by the rate and amplitude of the accelerometer signal
- the activities of daily living rate (ADL rate) is the approximate rate that the patient must reach during moderate exercise (walking or household chores)
- the upper sensor rate is the upper sensor-driven rate limit reached during vigorous exercise
- an independent control of the rate response is ensured in the range of ADL rates and in the range of maximum exertion rates
- the pacemaker maintains a linear relationship between the signal of the activity sensor and the response rate
- the rate optimization system has an effect on the slope of the linear relationship in both rate ranges
Rate response set points
The setpoints define the 2 characteristic slopes of a dual slope rate response.
- a low setpoint indicates that relatively low intensity activities are needed to reach the maximum rate
- the ADL setpoint determines the pacing rate toward the ADL rate
- the upper rate (UR) setpoint determines the pacing rate toward the maximum rate response
Automatic rate response
With the automatic rate response, the optimization of the rates histogram continues to adjust the rate curve by automatically varying the setpoints.
- the rate response is adjusted on the basis of the programming of the ADL response and exertion response parameters
- the ADL response controls the first slope, which determines the steepness of the increase in pacing rate between the lower rate and the ADL rate
- the exertion response controls the second slope, which determines the steepness of the increase in pacing rate as it approaches the maximum response
- the rate response function is active for only 30 minutes after implantation
Manual rate responsiveness
With the use of the manual rate response, a rate profile is obtained during a patient session, at which time the rates and set points are programmed. This rate profile remains constant until the parameters are reprogrammed.
Optimization of the rate histogram
The optimization of rates histogram adjusts automatically the patient’s rate response between office visits.
- the device collects and records daily averages and, on the long term, the percentage of time during which the sensor-indicated rate is at various pacing rates
- it then uses the ADL and the exertion response parameters to analyze the percentage of time during which the pacing rate remains in the ADL versus the exertion ranges
- based on daily comparisons, the device adjusts the ADL, the UR, or both setpoints
Adjustment of the activity threshold
- when a patient presents with an insufficient rate response during exercise, the activity threshold can be reprogrammed to a lower (more sensitive) level; the most sensitive level is “Low”
- if the pacing rate is high at rest, the activity threshold might need to be reprogrammed to a higher (less sensitive) level; the least sensitive level is “High”
Rate profile optimization
Before programming the other rate response parameters, one must first verify that the lower rate, ADL and upper sensor rates settings are appropriate for each patient.
- if the reprogramming of the rates has not optimized the rate profile, it might be necessary to reprogram the ADL and Exertion Response settings; the reprogramming of these parameters can design a rate profile that corresponds to the individual life style or activity levels within each range of rates
- the ADL response is adjusted in order to set the speed at which the patient reaches the ADL rate and the Exertion Response for the speed at which the patient reaches the exertion rate
- in both cases, a lower value decreases, while a higher value increases the rate response
Manual adjustment of the set points
The rate profile optimization can be programmed OFF, allowing the manual programming of the setpoints.
- the ADL and UR points determine the pacing rate profile
- the rate response calculations continue as programmed