2 sensors are available: accelerometer (G) and minute ventilation (MV):
1. Accelerometer
Adaptive Rate Pacing with Accelerometer Sensor provides an increase in heart rate by detecting body movement associated with physical activity. The accelerometer is a mass suspended from the integrated circuit in a diving board orientation along the X-axis.
- the accelerometer sensor measures the amplitude (force of motion) and frequency of body motion
- the movement of the accelerometer within the PG is then converted to an electrical signal which is used by the sensor circuitry to determine rate changes above the lower rate limit
- the sensor responds to activity in the frequency range of 1-10 Hz where physiologic activity signals predominantly occur
Activity threshold
Determines the minimum amount of activity needed to cause an increase in the sensor-driven pacing rate.
- a lower setting requires less motion to increase the pacing rate
- a higher setting requires more motion to increase the pacing rate
- for example, programming from the Medium setting to the Low setting would allow a lower amount of activity to cause a sensor-driven rate increase
- the Activity Threshold should be set low enough to allow a rate increase with minor activity such as walking but be high enough to prevent rate increases due to lower intensity motion
Reaction time
Determines how quickly the pacing rate will rise to a new level once an increase in activity above the Activity Threshold is detected.
- the value selected indicates the time required to increase the pacing rate from the lower rate limit to the maximal sensor rate at a maximum level of activity
- the rate will increase in steps and the device will pace for multiple cycles at each step before increasing to the next higher rate to avoid abrupt increases in the pacing rate
- shorter reaction times result in faster increases in the pacing rate
- longer reaction times result in slower increases in the pacing rate
Response factor
Determines the aggressiveness of the sensor at all levels of activity and functions independently from the reaction time and recovery time parameters.
- a high response factor setting will require less activity to reach the maximum sensor rate and a low response factor setting will require more activity to reach the maximum sensor rate
- each value increase in the response factor setting will result in a 10-15 beat per minute increase in heart rate at a given activity level
- if programmed too low, there will be an insufficient increase in heart rate with activity
- if programmed too high, the patient may reach the MSR at moderate levels of activity
- reprogramming the maximum sensor rate or lower rate limit does not change the slope (overall aggressiveness) of the response factor
Revovery time
Determines how quickly the pacing rate will decrease to a new level once a decrease in activity is detected.
- the value selected indicates the time required to decrease the pacing rate from the maximum sensor rate to the lower rate limit in the absence of activity
- the rate will decrease in steps and the device will pace for multiple cycles at each step before decreasing to the next lower rate to avoid abrupt decreases in the pacing rate
- shorter recovery times result in faster decreases in pacing rate and longer recovery times result in slower decreases in the pacing rate
- the recovery time is also applied when returning to the lower rate limit after temporarily pacing at a higher temporary lower rate limit
2. Minute ventilation (MV)
The pulse generator uses transthoracic impedance to measure minute ventilation (MV), which is the product of respiration rate and tidal volume. Based on the MV measurement, the pulse generator calculates the sensor-indicated rate.
- transthoracic impedance increases on inspiration as the lungs fill with air (since air has high impedance) and decreases on expiration
- to measure the respiratory changes in impedance, a current (I) is driven between the RA Ring and Can and the voltage (V) is measured from the RA Tip to Can
- R (impedance) = V (voltage) / I (current)
- the relationship between the detected increase in MV and the resulting increase in the sensor-indicated rate is established by the MV Response Factor
- the Ventilatory Threshold is a physiologic term describing the point during exercise when the breathing rate increases faster than the heart rate (sometimes referred to as Anaerobic or Lactate Threshold)
- the Response Factor controls the MV rate response for sensor rates between the lower rate limit and the Ventilatory Threshold
- the Ventilatory Threshold Response Factor controls the MV rate response when the sensor rate is above the Ventilatory Threshold
