- Activity sensor
- Programmable rates
- Rate response set points
- Automatic rate response
- Manual rate responsiveness
- Optimization of the rate histogram
- Adjustment of the activity threshold
- Rate profile optimization
- Manual adjustment of the set points
The rate response of Medtronic pacemakers includes a) an activity sensor that measures the patient’s motion, b) a rate calculation tool that converts the level of physical activity to a pacing rate, and c) an optimization function of the rates histogram to automatically adjust the rate response parameters over time and the acceleration and deceleration functions in order to smooth the pacing rate. The algorithm offers a dual slope rate response, which can be automatic or manual.
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, whereas 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 pacing rate is determined by the patient’s level of physical activity and by the rate response parameters. At rest, for example when the patient is sitting, the pacing rate is near the backup rate. During increasing activity, for example when the patient is walking, the pacing rate increases.
The main advantages of accelerometers are their simplicity, reliability, low energy consumption, use of standard leads, high sensitivity at the onset of exercise, and high correlation between physical performance and rate acceleration, observed under various conditions, including walk and daily life activity. Furthermore, the response is more physiologic than with a piezoelectric sensor, because it is less sensitive to vibrations and is more closely correlated with the level of exercise.
The lower rate corresponds to the slowest rate at which pacing occurs in absence of a sinus rate or physical activity.
The activities of daily living rate (ADL rate) is the approximate rate that the patient must reach during moderate exercise, defining a level that helps maintaining a stable pacing rate during changes in moderate activities, such as 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 1) between lower and ADL rates, and 2) between back-up and the maximum exertion rates. The rate optimization system has an effect on the slope of the linear relationship in both rate ranges.
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.
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 aim of the rates histogram optimization is to ensure that the rate response remains appropriate for the entire range of individual patient’s activities. 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
The activity threshold parameter does not need to be reprogrammed in a high proportion of patients.
However, 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". Conversely, 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, in which case a) the ADL and UR points determine the pacing rate profile, and b) the rate response calculations continue as programmed.
