Some patients present with chronotropic insufficiency, with a heart rate that does not adapt to changes in their physical activity. This inability to increase their heart rate on exertion may be associated with symptoms such as breathlessness, fatigue or reduced exercise capacity.

To compensate for the inability to accelerate physiologically during exercise, modern pacemakers are equipped with sensors capable of recording an indicator of metabolic activity and accelerating the stimulation rate in response. The operation of a servo-controlled pacemaker adds a dynamic behavior to the pacing/exhaust interval: the base frequency is variable according to the effort indication provided by a specific sensor. Programming a servo-control system is indicated for patients suffering from chronotropic insufficiency, the aim being to ensure the most physiological increase in frequency possible, corresponding to the level of metabolic needs imposed by the current effort.

A servo-controlled pacemaker is identified by the 4th letter of the international code (R): SSIR, DDDR, DDIR…

Each company has its own particularities in terms of sensor and servo programming. Several types of system have been developed since the 1970s, but only three are used in current devices, depending on the brand: measurement of physical activity using a piezoelectric quartz or an accelerometer, minute ventilation using the bio-impedance technique, and intracardiac impedance.

The quality criteria for a servo sensor are as follows:

• the information provided must be directly related to the sympathetic system or to the patient’s physical activity.
• the relationship between the amplitude of the sensor signal and the level of effort must be linear.
• the sensitivity of the sensor must be optimal for rapid system reactivity.
• the range of variation of the parameter measured must be sufficiently wide to be specific.
• the information provided must be reproducible.
• the sensor must be as small as possible to be included in
• the stimulator without increasing its size, and have minimal power consumption.

The most important advantages of accelerometers are simplicity, reliability, low power consumption, use of traditional probes, excellent sensitivity at the start of effort, good correlation between physical performance and frequency acceleration, obtained under various conditions (walking, everyday effort). The response is also more physiological than with a piezoelectric sensor, as it is less sensitive to vibrations, with a better relationship to effort level. On the other hand, there is no acceleration under mental stress.

The specific advantages of minute ventilation and intracardiac impedance are the use of traditional (non-dedicated) probes, better reactivity than the accelerometer for mental stress and a good correlation between physical performance and frequency acceleration for average efforts.