[A new mechanical sensor for activity controlled and body position dependent frequency adaptive pacemaker stimulation]. / Ein neuer mechanischer Sensor für aktivitätsgesteuerte und körperpositionsabhängige frequenzadaptive Schrittmacherstimulation.

In the past, the most important feature in the development of rate-responsive pacemaker systems was ensuring an adequate rate response to physical exercise. But there are other variables that modulate the rate of the healthy heart; examples include postural changes in heart rate in order to prevent orthostatic reactions, and the nightly decrease in intrinsic pacemaker rate. Therefore, we developed a sensor that could distinguish, not only rest and activity, but also discriminate between a supine and an upright position. This sensor is a multicontact tilt switch, which contains a small mercury ball. By measuring the number of openings and closures within the sensor as the ball touches the numerous sensor contacts it is possible to discriminate between different levels of exercise; the contact pins are circled around the base plate. There is an almost linear increase in the number of changes per time with increasing exercise. Besides measuring body activity from the number of contact changes per time, it is also possible to determine the body's posture from the count and positioning of the closed contacts within the casing. Based upon the results of 12 volunteers and nine pacemaker patients, a new algorithm is being prepared which enables the pacemaker to decrease stimulation rate in the supine position, to increase it in the upright position, and to correlate pacing rate with the increase in physical exercise. There is a good correlation between number of contact changes and the intrinsic heart rate (r = 0.87).

A new mechanical sensor for detecting body activity and posture, suitable for rate responsive pacing.

In the past, thought about rate responsive pacing mainly focused on rate increase with exercise but did not consider that a rate increase with postural changes also is mandatory in order to prevent orthostatic reactions. A nightly decrease in pacemaker rate when the body is at rest and in a supine position is a further advantage for the patient's sleep and recovery. Therefore, we developed a sensor that could detect not only rest and body activity but also discriminate between a supine and an upright position. This sensor is a multicontact tilt switch containing a small mercury ball, as shown in the left panel of the figure below. The principle of discrimination between rest and low and high body activity is realized by the movement of the mercury ball resulting from body motion, which causes openings and closures within the sensor as the ball touches the numerous sensor contacts. In the upright position, a distinct number of contacts at the bottom of the tilt switch are closed. In the supine position, there is no closure of the bottom contacts and a postural discrimination can be achieved. We studied 12 volunteers and 10 pacemaker patients with this new device both at rest and during physical exercise. The right panel of the figure illustrates that the contacts per second correlate to the increase of physical exercise, such as walking on the treadmill. Further studies with an external pacemaker containing a small sensor suitable to fit into the pacemaker are in preparation.

A new rate-modulated pacemaker system optimized by combination of two sensors.

A new rate-modulated pacemaker system optimized by combination of two sensors is described. The parameter body activity and central venous blood temperature control the pacemaker rate. The specific characteristic of each parameter determines its role within the algorithm. While the motion sensor yields a fast reaction following the onset or a change of stress intensity, central venous blood temperature corresponds better to body metabolism. An indication of increased exercise from the motion sensor results in an accordingly rapid increase in the pacing rate. Unless this increased exercise is confirmed by an increase in central venous blood temperature within 2 or 3 minutes, the new motion level will be assumed to be the new baseline motion value and the pace rate will return to a basic pacing rate. Prolonged inappropriate responses are therefore avoided. Longer lasting exercise, fever and nonphysiological signals are recognized and handled safely. Exercise tests with five volunteers under various conditions showed pacing rate behavior that was close to normal.