Rate responsive pacing; sensors and application algorithms

In patients in whom a pacemaker is implanted solely to correct defects in cardiac conduction it is frequently possible to place a lead in the right atrium and thus detect physiological variations in the sinus rate. This signal can be used to trigger the pacemaker's output via a second lead to the right ventricle, thereby ensuring that an appropriate heart rate is always maintained. If this form of truly physiological rate control is impossible (e.g. in the presence of sinoatrial dysfunction), alternative methods for varying the heart rate can be employed. The last ten years has seen the development of a number of different sensors that indirectly determine the need to vary the heart rate. These sensors fall into a number of distinct categories. The simplest are merely mechanical devices that sense body movement and are thus able to detect that the patient is taking physical exercise. There are also two groups of so-called physiological sensors in current use. One group responds to metabolic changes (e.g. the oxygen saturation of venous blood) and the other is sensitive to the electrical or mechanical effects of adrenergic stimulation of the ventricular myocardium (e.g. local acceleration of wall movement). Intensive research has so far established that no individual sensor possesses ideal performance characteristics. Efforts continue to develop new sensor technology and to combine existing sensors in such a way as to optimise their clinical function.