Extraction force and tissue change during removal of a tined intramuscular electrode from rat gastrocnemius.

Many electrical stimulation protocols employ intramuscular electrodes for the activation of targeted muscles. Electrode displacement from the initial implant site can result in degradation of optimal stimulus parameters. Electrodes with tined tips were developed to reduce electrode migration. In the study reported here, intramuscular electrodes with polypropylene tines at the tip were implanted aseptically in the gastrocnemii of adult rats. Test electrodes were explanted immediately following implant in one group and after periods of 1, 3, 7, 14 and 28 days in others. Force as a function of displacement was recorded during removal of the electrodes. Analysis of the results showed that the electrodes were most vulnerable to movement during the first five days. Between 5 and 7 days after implantation there was significant increase in the force required to dislodge the electrode tip. Histology of muscles from which electrodes had been explanted did not show any increase in the area showing tissue changes as compared to control muscles in which the electrode remained in situ. These results indicated that electrode removal caused disruption of encapsulation tissues, with the surrounding muscle mainly unaffected by the explant process.

Phrenic nerve pacing via intramuscular diaphragm electrodes in tetraplegic subjects.

CONTEXT: Diaphragm pacing in ventilator-dependent tetraplegic subjects is usually achieved by the placement of phrenic nerve electrodes via thoracotomy. However, this technique may be accomplished less invasively via laparoscopic placement of IM electrodes, at a lower cost and with less risk of injury to the phrenic nerve. OBJECTIVE: To assess the feasibility of laparascopic placement of IM diaphragm electrodes to achieve long-term ventilatory support in ventilator-dependent tetraplegic subjects. DESIGN, SETTING, AND PARTICIPANTS: Two IM diaphragm electrodes were placed laparoscopically in each hemidiaphragm in five subjects with ventilator-dependent tetraplegia. Studies were performed either on an outpatient basis or with a single overnight hospitalization. Ventilator-dependent tetraplegic subjects were identified in whom bilateral phrenic nerve function was present, as determined by phrenic nerve conduction studies. Following electrode placement, subjects participated in a conditioning program to improve the strength and endurance of the diaphragm over a period of 15 to 25 weeks. The duration of the study was variable depending on the time necessary to determine the maximum duration that individuals could be maintained without mechanical ventilation support. MAIN OUTCOME MEASURES: Magnitude of inspired volume generation and duration of ventilatory support with bilateral diaphragm pacing alone. RESULTS: In four of the five subjects studied, initial bilateral diaphragm stimulation resulted in inspired volumes between 430 and 1,060 mL. Reconditioning of the diaphragm over several weeks resulted in substantial increases in inspired volumes to 1,100 to 1,240 mL. These subjects were comfortably maintained without mechanical ventilatory support for prolonged time periods by diaphragm pacing, by full-time ventilatory support in three subjects, and 20 h per day, in the fourth subject. No response to stimulation was observed in one subject, most likely secondary to denervation atrophy. CONCLUSIONS: Diaphragm pacing in ventilator-dependent tetraplegic subjects can be successfully achieved via laparascopic placement of IM electrodes.

Mapping the phrenic nerve motor point: the key to a successful laparoscopic diaphragm pacing system in the first human series.

BACKGROUND: For patients with high spinal cord injury and chronic respiratory insufficiency, electrically induced diaphragm pacing is an alternative to long-term positive pressure ventilation. The goal of this study was to laparoscopically assess the phrenic nerve motor point of the diaphragm and then implant electrodes to produce chronic negative pressure ventilation. METHODS: Patients undergoing elective laparoscopic procedures (volunteer patient group) underwent a series of electrical stimuli (2 to 24 mA at 100-microsecond pulse widths) with a mapping probe to identify the motor point through qualitative visualization of diaphragm motion and quantitative measurement of the abdominal pressure to assess the strength of the contraction. After Food and Drug Administration and Institutional Review Board approval, tetraplegic patients (spinal cord injured patient group) who were ventilator dependent underwent mapping and implantation of electrodes for pacing in both diaphragms. RESULTS: In the volunteer group, 28 patients underwent 3 to 50 stimulations per diaphragm to identify the motor points. Throughout this series the surgical tools and software were improved to allow rapid motor point location with a grid-mapping algorithm. In the spinal cord injured group, 5 of 6 patients had electrodes successfully implanted at the motor point to produce adequate tidal volumes. The one failure caused a change in our inclusion criteria to include fluoroscopic confirmation of diaphragm movement during surface nerve stimulation. Three patients are completely free of the ventilator, and the other 2 are progressively increasing their time off the ventilator with conditioning. CONCLUSIONS: Mapping and implantation of the electrodes can be done laparoscopically, providing for a low-risk, cost-effective, outpatient, diaphragm pacing system that will support the respiratory needs of patients.

Characterization of the human diaphragm muscle with respect to the phrenic nerve motor points for diaphragmatic pacing.

Diaphragm pacing from laparoscopically placed electrodes is an alternative to conventional phrenic pacers that use electrodes placed in direct contact with the nerve in the neck or chest. The challenge with the laparoscopic approach is determining where to implant the electrodes, as the phrenic nerves are not visible from the abdomen. The objective of this study was to locate the phrenic nerve "motor points" in the human diaphragm muscle from an abdominal perspective. Twenty-five cadavers were examined by excising the diaphragm muscle and assessing for the thickness of the muscle, the motor point area, and the accessibility of the motor point from the abdominal approach. The data indicate the average thickness of the muscle in the motor point region was 3.0 mm for the left and 2.9 mm for the right hemidiaphragm. The average motor point area was 73 mm2 for the left and 58.7 mm2 for the right hemidiaphragm. The motor points were accessible from an abdominal approach, but the motor point on the right hemidiaphragm was located on the central tendon in many cases (12 of 25). Thus, although the nerves branch prior to entry into the muscle on the right side, several well-placed electrodes could still activate the entire nerve. In this study, we have characterized the human diaphragm muscle in the motor point region and found that it is feasible to place laparoscopically intramuscular electrodes in the motor point region. This is the foundation for the laparoscopically placed diaphragm pacing device that has been utilized in a small series of patients.

Phrenic nerve pacing in a tetraplegic patient via intramuscular diaphragm electrodes.

In patients with ventilator-dependent tetraplegia, phrenic nerve pacing (PNP) provides significant clinical advantages compared with mechanical ventilation. This technique however generally requires a thoracotomy with its associated risks and in-patient hospital stay and carries some risk of phrenic nerve injury. We have developed a method by which the phrenic nerves can be activated via intramuscular diaphragm electrodes. In one patient with ventilator-dependent tetraplegia, two intramuscular diaphragm electrodes were implanted into each hemidiaphragm near the phrenic nerve motor points via laparoscopic surgery. The motor points were identified employing a previously devised mapping technique. Because inspired volumes were suboptimal on the right, a second laparoscopic procedure was necessary to position electrodes near the anterior and posterior branches of the right phrenic nerve. During bilateral stimulation, inspired volume was 580 ml. After a reconditioning program of progressively increasing diaphragm pacing, maximum inspired volumes on the left and right hemidiaphragms increased significantly. Maximum combined bilateral stimulation was 1120 ml. Importantly, the patient has been able to comfortably tolerate full-time pacing. If confirmed in additional patients, PNP with intramuscular diaphragm electrodes via laparoscopic surgery may provide a less invasive and less costly alternative to conventional PNP.