The growth curve from rat number 1 is shown in panel c, with a subset of the individual ECAP recordings in panel a. The growth curve regression lines used to calculate the ECAPT are shown in black. For the other three growth curves, the estimated confidence intervals were 10 μA (6% of the ECAPT, rat number 9 with 50-μs PW), 12.7 μA (22% of the ECAPT, rat number 5 with 200-μs PW), and 21.1 μA (19% of the ECAPT, rat number 5 with 50-μs PW).įigure 6 Polyphasic ECAPs and growth curves with ECAPT, ECMAPT, and vMT in response to 50-Hz, 50-μs PW SCS in exemplar rats. The 95% confidence interval was <5 μA for 33 of 36 growth curves. The noise floor for the ECAP amplitudes was 0.7 ± 0.5 μV (mean ± SD) across all animals. The ECAPTs, EMAPTs, and vMTs are presented as a function of PW in Table 1. Shown in Figure 7 is the complete set of growth curves for all nine included rats, plotted as a function of stimulation charge/phase (that is, the stimulation current multiplied by the PW) the plots are continued to 5 μA beyond the vMT.
These results also may be noted in the example plots of Figure 6, in which we show both the growth curves and a subset of individual ECAP recordings elicited with 50-Hz, 50-μs PW SCS in rats number 1 and number 2. In all but one instance, we observed the occurrence of the vMT ( Fig. 5, right panel) at the same stimulation amplitude, or slightly above, the one that resulted in the ECMAPT. As we increased the stimulation amplitude further, ECMAPs were observed with latencies of approximately 3 to 6 ms after stimulation ( Fig. 5, middle panel). Once above the ECAPT, we observed ECAPs within the first 1.5 ms after stimulation ( Fig. 5, left panel). In the remaining nine animals-and as appreciated in the example recordings of Figure 5-a narrow stimulation artifact presented simultaneously on both the ECMAP and ECAP recording channels, coincident with the delivery of the stimulation pulse. Because damage of the sensing lead was suspected in this animal, this rat was excluded from further analysis. In one animal (rat number 10), however, the ECAPs were extremely small and noisy. We observed ECAPs in all ten rats for all PWs tested (50, 100, 150, and 200 μs). The animals were then frozen, and x-rays were taken to image the location of the leads. After the experiment, we stapled the leads onto the animal to secure them for imaging purposes, and the animals were euthanized. We placed these needles in the muscle group in which contraction was first usually noted (the erector spinae). We determined MTs both visually-by noting the stimulation amplitude at which a muscle twitch was first observed in response to test stimulation-and through bipolar electromyography (EMG) needles (Model number 8227304, Medtronic plc). Although midline placements were targeted, a strict placement scheme was not enforced to better encompass the lead placement variability seen in typical clinical and preclinical use cases. We placed an amplifier reference needle (Model number 8227103, Medtronic plc) subcutaneously near the base of the tail. This lead was then advanced through the epidural space to T11–T12. We then made a second incision over the thoracic spine, followed by a microlaminectomy at T8 for retrograde implantation of the recording lead. We introduced this lead into the epidural space between 元 and L4 and positioned the distal tip at L1. Next, we made an incision over the lumbar spine, followed by a microlaminectomy at L4 for anterograde implantation of the stimulation lead.
Briefly, we induced anesthesia in each animal with 3% isoflurane and oxygen as a carrier gas, maintained at 3% to 3.5% isoflurane throughout the procedure.
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