Diaphragmatic Pacer Placement: Anesthetic Management (DP)

Patients included are part of a program developed by the Spinal Cord Injury Unit of the Institut Guttmann of placement and strengthening of the diaphragm with NeuRx® Diaphragm Pacing Stimulation (DPS) System (Synapse Biomedical, Oberlin, OH, USA) device for patients suffering from neuromuscular disorders or upper spinal cord injuries dependent on MV. All of them were preselected by a multidisciplinary expert committee after the assessment of their clinical history and the evaluation of the phrenic nerve function using two complementary techniques like phrenic nerve stimulation and fluoroscopic evaluation of diaphragm movement. Pediatric patients were the exception, since they underwent surgery in order to obtain a definitive diagnosis on the phrenic nerve functionality. Patients were admitted in the center 24 hours prior to the intervention.

The surgical procedure of DP implantation consists in placing four intramuscular electrodes, two in each hemidiaphragm, using a conventional abdominal laparoscopy with carbon dioxide insufflation at 10 L/minute speed and pressure up to 15 mmHg. Four ports are inserted: one for the optical equipment, two for the mapping electrode and electrode insertion instruments and a smaller sized port as an exit site for the wires of the electrodes. Reverse Trendelenburg position is required for the procedure. It consists in locating the optimum point for electrode insertion. The process involves mapping between 30 and 50 different points in each hemidiaphragm by applying an electric stimulus of 2-24 mA at 100 µsec pulse widths11. It results in both qualitative and quantitative assessment of diaphragmatic movement: qualitative through laparoscopy and quantitative using the external assessment of the intraabdominal pressure during stimulation with temporal mapping electrode. Site of main electrode is identified as the location of each hemidiaphragm's change of maximum pressure and site of secondary electrode as replica of main site. Once the sites are identified in each hemidiaphragm, intramuscular electrode placement phase is initiated. Response to desired stimulation is checked subsequently. Finally the electrodes are tunneled out to the corresponding percutaneous exit site and an electrocardiogram strip is recorded with all electrodes active in order to confirm there is no capture of cardiac rhythm. In the case of pediatric age patients equipment and incisions were adapted to patient's weight and size. However the surgical procedure was the same.

Upon arrival to the operating room, heart rate (HR), non-invasive arterial pressure (NIAP), pulse oximetry oxygen saturation (SaO2) and capnography (Carescape Monitor B850, GE Healthcare, Finland) were standardly monitored. In patients with previous history of ischemic cardiopathology and/or difficult to manage autonomic dysreflexia crisis, invasive arterial pressure was monitored through radial artery catheterization using the Seldinger technique (Leathercath Arterial; Vygon Ecoven, France). In most cases no premedication was provided. In pediatric patients prophylactic atropine was required for induction, it was given in doses of 0.01 mg/kg. Induction was carried out either intravenously with propofol in doses raging 1.5-2 mg/kg in adult patients and 3 mg/kg in pediatric patients or by inhalation with sevoflurane in pediatric patients. After a prior priming of the circuit, a series of three forced inspirations were carried out using reservoir bag at concentrations of 6% of the anesthetic until reaching a minimal alveolar concentration (MAC) of 2.0-3.5%. Anesthetic maintenance was carried out following anesthetist's criterion, with sevoflurane of 2-2.5% MAC or continuous intravenous infusion with propofol for Total Intravenous Anesthesia (TIVA) maintaining the infusion at 10 mg/kg/h for 30 minutes, followed by 8 mg/kg/h for another 30 minutes and 6 mg/kg/h until the end of the intervention, maintaining bispectral index (BIS) value of 40-60. Patients were disconnected from their usual ventilation system and transferred to the GE Datex-Ohmeda Aespire 3000 (GE Healthcare, Finland) system on volume control mode, with tidal volume of 6 mL/kg, respiratory rate between 10 and 15 breaths per minute and PEEP +7 cmH₂O with a mixture of O2/air at 50% in order to reach oxygen saturation superior to 95% before initiating the surgical procedure. Given the type of procedure, it was attempted to minimize the use of muscle relaxants, since these are not required in patients with tracheostomy. Rocuronium at 0.4 mg/kg at the anesthetist's criterion was used for managing the airway in patients who needed orotracheal intubation (OTI). In these cases, neuromuscular relaxation was monitored (NMT Neuromuscular Transmission MechanoSensor GE Healthcare, Finland) and a Train of Four (TOF) measurement of 100% was obtained prior to administering muscle relaxants. OTI was performed when TOF answer was 0 and diaphragmatic mapping phase initiated solely if TOF answer was 4 and with a percentage superior to 90%. When these levels of response were not achieved, sugammadex 2 mg/kg was used to reverse the blocking in case of induction with rocuronium or neostigmine 0.04-0.07 mg/kg.

Intraoperative analgesia was carried out using continuous infusion of remifentanil (0.5-1 µg/kg/min) until the end of the intervention. Surgeons infiltrated laparoscopic ports with bupivacaine 0.5% with vasoconstrictor before incision. Patients requiring OTI received a fentanyl bolus (3 µg/kg) in the induction and in all cases at the end of the surgery a fentanyl bolus (1.5 µg/kg) was administered in order to avoid postoperative hyperalgesia associated to remifentanil.

Isolated boluses of propofol (0.5-1 mg/kg) or fentanyl (1-1.5 µg/kg) were administered when patients required anesthetic deepening for a correct tolerance of the laparoscopic technique without using muscle relaxants. Patients received bolus of 5 mg ephedrine repeatedly until normal values were reached if intraoperative arterial hypotension appeared, defined as a 20% decrease of baseline or mean arterial pressure (MAP) inferior to 60 mmHg.