Long-term intramuscular electrical activation of the phrenic nerve: efficacy as a ventilatory prosthesis.

The efficacy of a system for long-term intramuscular activation of the phrenic nerve as a ventilatory prosthesis was evaluated in seven dogs. (The safety and reliability of this system is addressed in a companion paper.) Five dogs underwent chronic bilateral intramuscular diaphragm stimulation (IDS) for 61 to 183 days at stimulus parameters selected to evoke at least 120% of the animal's basal ventilation. Two dogs maintained as controls did not undergo chronic stimulation. The ability of IDS to provide long-term ventilation without diaphragm fatigue was evaluated in terms of the ventilatory capacity of IDS, the effects of chronic IDS on diaphragm contractile properties, and the phrenic nerve recruitment properties of chronic IDS electrodes. Hemidiaphragms with electrodes placed within 2 cm of the phrenic nerve trunk could be completely activated by 25 mA pulses having a 100 microsecond pulse width. The tidal volume evoked by IDS in this study was 167% (+/- 48 s.d.) of that required for full-time basal ventilation without diaphragm fatigue. Evoked tidal volume increased after 8 to 9 weeks of chronic IDS for stimulus pulse intervals longer than 50 mS. Electrode recruitment properties were stable for both active and passive implanted electrodes. We conclude from these studies that with properly placed electrodes IDS is capable of providing reliable full-time ventilatory support without fatiguing the diaphragm.