Spinal somatosensory evoked potentials after epidural isoproterenol in awake sheep

An active electronic, high-density epidural paddle array for chronic spinal cord neuromodulation

Objective: Epidural electrical stimulation (EES) has shown promise as both a clinical therapy and research tool for studying nervous system function. However, available clinical EES paddles are limited to using a small number of contacts due to the burden of wires necessary to connect each contact to the therapeutic delivery device, limiting the treatment area or density of epidural electrode arrays. We aimed to eliminate this burden using advanced on-paddle electronics. Approach: We developed a smart EES paddle with a 60-electrode programmable array, addressable using an active electronic multiplexer embedded within the electrode paddle body. The electronics are sealed in novel, ultra-low profile hermetic packaging. We conducted extensive reliability testing on the novel array, including a battery of ISO 10993-1 biocompatibility tests and determination of the hermetic package leak rate. We then evaluated the EES device in vivo, placed on the epidural surface of the ovine lumbosacral spinal cord for 15 months. Main results: The active paddle array performed nominally when implanted in sheep for over 15 months and no device-related malfunctions were observed. The onboard multiplexer enabled bespoke electrode arrangements across, and within, experimental sessions. We identified stereotyped responses to stimulation in lower extremity musculature, and examined local field potential responses to EES using high-density recording bipoles. Finally, spatial electrode encoding enabled machine learning models to accurately perform EES parameter inference for unseen stimulation electrodes, reducing the need for extensive training data in future deep models. Significance: We report the development and chronic large animal in vivo evaluation of a high-density EES paddle array containing active electronics. Our results provide a foundation for more advanced computation and processing to be integrated directly into devices implanted at the neural interface, opening new avenues for the study of nervous system function and new therapies to treat neural injury and dysfunction.

Permanent middle cerebral artery occlusion in sheep: a novel large animal model of focal cerebral ischemia

As effective stroke treatment by thrombolysis is bound to a narrow time window excluding most patients, numerous experimental treatment strategies have been developed to gain new options for stroke treatment. However, all approaches using neuroprotective agents that have been successfully evaluated in rodents have subsequently failed in clinical trials. Existing large animal models are of significant scientific value, but sometimes limited by ethical drawbacks and mostly do not allow for long-term observation. In this study, we are introducing a simple, but reliable stroke model using permanent middle cerebral artery occlusion in sheep. This model allows for control of ischemic lesion size and subsequent neurofunctional impact, and it is monitored by behavioral phenotyping, magnetic resonance imaging, and positron emission tomography. Neuropathologic and (immuno)histologic investigations showed typical ischemic lesion patterns whereas commercially available antibodies against vascular, neuronal, astroglial, and microglial antigens were feasible for ovine brain specimens. Based on absent mortality in this study and uncomplicated species-appropriate housing, long-term studies can be realized with comparatively low expenditures. This model could be used as an alternative to existing large animal models, especially for longitudinal analyses of the safety and therapeutic impact of novel therapies in the field of translational stroke research.

Evaluating hemodynamic and T wave criteria of simulated intravascular test doses using bupivacaine or isoproterenol in anesthetized children

An increase in T wave amplitude > or =25% is a reliable indicator for detecting intravascular injection of lidocaine-epinephrine test dose in anesthetized children. We examined whether a simulated IV test dose containing bupivacaine instead of lidocaine, and isoproterenol instead of epinephrine, produces reliable changes in heart rate (HR) and T wave morphology. One hundred healthy infants and children (6-72 mo) were randomized to one of five groups (n = 20 each) during 1.0 minimum alveolar anesthetic concentration sevoflurane and 67% nitrous oxide in oxygen: atropine pretreatment (0.01 mg/kg IV) followed by 0.25% bupivacaine containing epinephrine 0.5 microg/kg IV, atropine followed by normal saline, atropine followed by 1% lidocaine containing isoproterenol 0.1 microg/kg, saline pretreatment followed by the lidocaine-isoproterenol test dose, and saline followed by saline. HR was recorded every 20 s and T wave amplitude of lead II was continuously recorded. All patients receiving the bupivacaine-epinephrine test dose and none receiving saline met the HR (positive if > or =10 bpm increase) and T wave criteria (positive if > or =25% increase in amplitude). The isoproterenol-containing test dose produced positive responses based only on the HR criterion with or without atropine pretreatment. Our results indicate that HR and T wave changes are useful if a bupivacaine-epinephrine test dose is used and that HR is the only useful indicator if an isoproterenol-containing test dose is used in sevoflurane-anesthetized children.

IMPLICATIONS

To determine if an epidurally administered local anesthetic has been unintentionally injected into a blood vessel, a small dose of epinephrine or isoproterenol may be added to a local anesthetic. We found that an increase in heart rate > or =10 bpm and an increase in T wave amplitude of lead II >or =25% are useful indicators for detecting accidental intravascular injection of an epinephrine-containing test dose in sevoflurane-anesthetized children, whereas only a heart rate change is a reliable diagnostic tool if an isoproterenol-containing test dose is used.

Simulation of an epidural test dose with intravenous isoproterenol in sevoflurane- and halothane-anesthetized children

Isoproterenol has been suggested as an alternative marker for epidural test dosing in children receiving halothane anesthesia. The purpose of this prospective, randomized, double-blind study was to determine the chronotropic response to IV isoproterenol in sevoflurane-anesthetized children. Thirty-six ASA physical status I children (0.5-8 yr) were anesthetized with either halothane or sevoflurane at 1 minimum alveolar anesthetic concentration adjusted for age in 70% nitrous oxide. Patients received incremental IV injections of isoproterenol until their heart rate increased > or = 20 bpm above baseline. The minimal effective dose of isoproterenol required to produce an increase of > or = 20 bpm was 55 ng/kg (42-72 ng/kg; 95% confidence interval) in sevoflurane-anesthetized children and 32 ng/kg (26-38 ng/kg; 95% confidence interval) in halothane-anesthetized children (P < 0.05). This dose-response study suggests that sevoflurane antagonizes beta-adrenergic-mediated chronotropic responses to isoproterenol more than halothane. These observations also suggest that larger doses of isoproterenol will be necessary for epidural test dosing in children receiving sevoflurane rather than halothane anesthesia.

IMPLICATIONS

Isoproterenol has been suggested as an alternative marker for epidural test dosing in children receiving halothane anesthesia. This isoproterenol dose-response study indicates that larger doses of isoproterenol will be necessary for epidural test dosing in children undergoing sevoflurane rather than halothane anesthesia.

The effects of adding isoproterenol to 0.125% bupivacaine on the quality and duration of epidural analgesia in laboring parturients

This study was conducted to determine the effects of adding isoproterenol to epidural bupivacaine and sufentanil on the quality and duration of analgesia during labor. In a double blind, randomized study, 80 women were divided into two groups, receiving three doses of 0.125% bupivacaine with 7.5 microg of sufentanil and either 12.5 microg of epinephrine (EPI group) or 5 microg of isoproterenol (ISO group). Contraction pain was measured using a 100-mm visual analog scale (VAS) before epidural analgesia, at 5-min intervals for 15 min after each epidural injection, and hourly thereafter. Overall, no significant differences were observed in VAS scores between the groups. However, in the ISO group, VAS scores at 10 and 15 min after the first and second administration were significantly lower than those in the EPI group. Analgesia after each administration lasted significantly longer in patients who received epinephrine. Because of the limited duration of analgesia in the ISO group, more patients in this group received a fourth epidural administration of 0.125% bupivacaine with epinephrine 1:800,000. In conclusion, the addition of isoproterenol to bupivacaine and sufentanil induces a faster onset of analgesia and reduces the duration of analgesia compared with bupivacaine with sufentanil and epinephrine. Therefore, it is preferable to use isoproterenol only once, as a test dose, after the placement of the epidural catheter.

IMPLICATIONS

We analyzed the quality and duration of analgesia in laboring women after they received bupivacaine and sufentanil combined with isoproterenol or epinephrine epidurally. We found that the addition of isoproterenol to bupivacaine and sufentanil induces a faster onset of analgesia and reduces the duration of analgesia.

Hemodynamic effects of intravenous isoproterenol versus saline in the parturient

The use of epinephrine as a test dose for epidural analgesia in obstetrics remains controversial. Isoproterenol as a test dose may be efficacious in the parturient. However, the effects of isoproterenol on the uterine blood flow (UBF) and umbilical blood flow (UMB) in the parturient are unknown. In a randomized, double-blind study, the hemodynamic variables in 60 nonlaboring women at term were studied 5 min before and for 10 min after an intravenous injection of either 5 micrograms isoproterenol or 5 micrograms saline. The UBF was assessed in 35 women and the UMB in 25 women using a color Doppler technique. The results of 50 women were used for further analysis. Maternal heart rate (MHR) was measured continuously, and maternal mean arterial pressure was measured every minute. MHR did not change after saline but increased significantly after injection of isoproterenol. UBF also increased significantly after isoproterenol during the same time interval. UMB did not change. Other hemodynamic variables did not change. We conclude that isoproterenol, 5 micrograms, may be a suitable test dose for epidural analgesia in obstetrics.