Implantable Sensors for Rate Adaptation and Hemodynamic Monitoring

Abstract

In this group, sensor-driven CRT pacing improved maximum oxygen consumption and work capacity compared with CRT pacing without rate adaption. This increment is independent of A-V interval shortening during exercise. These data suggest that rate modulation plays a critical role in some patients with impaired LV function who require pacing therapy, even in those with CRT. HEART RATE MODULATION FOR NONEXERCISE NEEDSExercise is only one of the many physiologic requirements for variation in heart rate. Other changes, such as mental stress, can lead to a rate increase. The sinus rate is also higher when a person assumes the upright posture. Isometric exercise also results in an increase in cardiac output and heart rate in most people. The changes in heart rate that occur during various physiologic maneuvers (e.g., Valsalva) and baroreceptor reflexes may also be important. An appropriate compensatory heart rate response may be especially important in pathophysiologic conditions such as anemia, acute blood loss, or other causes of hypovolemia, and during febrile illness. Ideal Sensor CharacteristicsBased on the physiology of the normal sinus node, a sensor system to overcome chronotropic incompetence needs to be sensitive to both exercise and nonexercise needs. It also should be specific, unaffected by internal or external changes that can cause an inappropriate rate change. The sensor should achieve rate modulation at an appropriate speed, with its response proportional to the level of exercise load. The sensor system should be easy to implement (preferably with standard device casing and lead system), should be stable in the body's internal environment, and should not significantly increase battery consumption. CLASSIFICATION OF SENSORSIn a sensor-driven pacing system, a sensor (or combination of sensors) must first detect a physical or physiologic parameter that is related to metabolic demand 5 ( Fig. 5-1). Second, the rate-modulating circuit in the pulse generator must have an algorithm that relates changes in the sensed parameter to a change in pacing rate. Third, because the magnitude of the physical or physiologic changes monitored by a sensor may differ between patients, clinician input may be necessary to adjust the algorithm, generally by programming one or more rate-responsive variables, to achieve the clinically desired rate response. This requirement for adjustment has decreased with automatic optimization of the rate-responsive settings. Most sensors operate in an open-loop algorithm; that is, the induced rate changes do not induce a negative feedback on the sensed parameter. In a closed-loop sensor system, the induced hemodynamic changes will induce an opposite change in the level of the sensed parameter that is responsible for the initial rate adaptation (negative feedback loop). Theoretically, minimal programming is required in a closed-loop system (see Fig. 5-1). Furthermore, the actual rate response is calculated using rate-response curves after programming a threshold of sensor detection (Fig. 5-2).Im...