Direct diaphragm stimulation

Diaphragm pacing for respiratory insufficiency

Diaphragm pacing (DP) by electrical stimulation of the phrenic nerve offers important advantages to a highly select group of patients with respiratory paralysis. The patient wears an external radiofrequency (RF) transmitter over an implanted receiver, and a stimulating current is induced without the need for any transcutaneous wires. We review the conditions and requirements of patients who may benefit most from DP. We outline the preoperative evaluation and procedures for surgical implantation. We discuss the risk of diaphragmatic fatigue posed by initiation of DP and the use of gradual conditioning to limit this problem. Other problems encountered by patients in the course of DP can be minimized by well-instructed home caregivers and by systematic medical follow-up. Although a few patients derive considerable benefit from DP, many patients with respiratory paralysis are better treated by less invasive means such as nasal bilevel positive airway pressure or intermittent positive pressure ventilation, which we also review.

Initial clinical experience with rate adaptive cardiac pacing using two sensors simultaneously

In the rate adaptive pacemakers, all presently available sensors show one or more drawbacks. Combining two sensors in a single pacemaker, we tried to optimize its rate responsive characteristics. In this study, we present the rate adaptive behavior of a two sensor pacemaker system, using both QT interval and activity sensing. In addition, we compared the rate response with that of each sensor alone. Nine patients with an implanted QT interval sensing pacemaker, and an externally attached activity sensing pacemaker performed three exercise stress tests on treadmill. The QT interval, measured by the implanted pacemaker, and the activity level, were transmitted to an external computer. This computer contained the two sensor rate adaptive algorithm, and reprogrammed the implanted pacemaker on beat-to-beat basis.

CONCLUSION

In the two sensor mode the rate increases immediately at the onset of exercise, caused by the prompt response of the activity sensor. Further rate increase is driven by the QT interval sensor and therefore proportional to the level of exercise. Furthermore, the rate decay during the recovery phase is more physiological.

Electrical techniques for stimulation of the phrenic nerve to pace the diaphragm: inductive coupling and battery powered total implant in asynchronous and demand modes

A review of the electrical design of a rf inductively coupled phrenic nerve stimulator for the diaphragm developed in our laboratories will be discussed. Modifications of the original circuit are based on long-term laboratory and clinical studies. A total implant battery powered stimulator was designed exclusively for animal studies to evaluate the effects of several stimulating parameters on diaphragm fatigue and neuromuscular structure. On the basis of these studies the optimum current level, stimulus frequency, respiratory rate, electrode configuration, and waveform were selected for clinical use to pace the diaphragm. A multiprogrammable dual output stimulator responsive to interrogation has been constructed and used in the experimental laboratory in anticipation of clinical application. There was an insignificant difference between the effect on neural structure or diaphragm function of stimulation with pulse width modulated constant voltage or with amplitude-modulated constant current. Demand pacing: maintenance of normal PACO2 by monitoring ET PACO2 with feedback to the diaphragm pacemaker to adjust the pacing rate has been successful in the experimental animal.

Treadmill assessment of an activity-modulated pacemaker: the importance of individual programming

Maximum benefit from a rate-modulated pacemaker requires individualized programming of rate response settings. We tested an externally strapped activity-sensing pacemaker (Activitrax-Medtronic 8400) in eight healthy volunteers, to assess the pacing responses of the different rate response and activity threshold settings. Five males and three females, aged 20 to 70 years (mean 40), performed a total of 67 treadmill exercise tests, using a specific protocol designed to assess the activity-sensing unit. The external unit was compared to implanted units in four patients, to validate its accuracy. A reproducible sinus response to the treadmill protocol was observed, against which pacing responses were compared. The activity threshold determines the degree of activity required to elicit a pacing rate response, whereas the rate response setting determines the rate attained. Rates of 140 bpm were rarely achieved, despite vigorous exercise. The sensor responds rapidly to activity, not to physiologic demand; to increase in speed, not grade. Four patients performed repeated limited treadmill tests to determine their optimum program setting, with symptomatic status and the healthy volunteer sinus response as guides. These results, and those from the external Activitrax unit, suggest that LOW 6 and MEDIUM 6-10 settings will prove optimum for most patients.