This prospective observational study was approved by Ethics Committee of Tongji Hospital (TJ-IRB20170401) and written informed consent was obtained from all subjects participating in the trial. The trial was registered prior to patient enrollment at clinicaltrials.gov (NCT03169517, Principal investigator: Wei Mei, Date of registration: May 25, 2017). In this study, 47 patients scheduled for elective lower limb surgery between May 2017 and September 2017 at Tongji Hospital in Wuhan, China were enrolled. Inclusion criteria were American Society of Anesthesiologists (ASA) physical status I-II and age 18 to 75 years. Exclusion criteria were peripheral vascular disease, toe injury or deficiency, colored or infected toenails, vasodilator medications prior to surgery, preoperative analgesic intake, body mass index >35, or contraindication to peripheral nerve block (including patient refusal, allergy to local anesthetics, peripheral neuropathy, or coagulopathy).
Upon arrival at the anesthesia induction room, intravenous access and routine monitoring with electrocardiography, noninvasive blood pressure, and pulse oximetry were performed. All patients rest quietly in supine position and were allowed 20 min to adapt to the surroundings before measurement. Room temperature was kept at 24oC. Blood flow images of the toes in the operated limb were obtained by LSCI and pinprick sensory tests were performed 5 min before nerve block (t=0) as the baseline value. Ultrasound guidance was performed using a low-frequency (2 to 5 MHz) curved array transducer (SonoSite M-Turbo; SonoSite Inc, Bothell, Washington) for lumbar plexus block and a high-frequency (6 to 13 MHz) linear array probe (SonoSite M-Turbo; SonoSite Inc, Bothell, Washington) for blockade of the femoral nerve, the obturator nerve, and the lateral femoral cutaneous nerve. An 21-gauge, 10-cm Tuohy needle (UniPlex Nanoline; Pajunk Inc, Geisingen, Germany) was used for nerve block in our study. All the blocks were performed by a dedicated investigator (W.M.).
For lumbar plexus block, patients were turned to the lateral decubitus position with the side to be blocked upward. Ultrasound-guided lumbar plexus block was performed by a paramedian transverse scan (PMTS) of the lumbar paravertebral region with the ultrasound beam being insonated through the intertransverse space (ITS) (PMTS-ITS).23 The lumbar plexus nerve was identified as a hyperechoic structure in the posterior aspect of the psoas muscle. After inﬁltration with 1% lidocaine, the needle connected to a nerve stimulator (Stimuplex, HNS 12, Braun Medical, Melsungen, Germany) was placed lateral to the probe and slowly advanced toward the lumbar plexus nerve using an in-plane technique. Correct needle position was confirmed when ipsilateral quadriceps muscle contraction was elicited at a current of 0.5-0.8 mA. Subsequently, 30 ml of 0.4% ropivacaine was administered with repeated negative aspiration. After the completion of lumbar plexus block, patients were returned to the supine position.
For femoral nerve block (FNB), patients were placed in supine position. The ultrasound probe was placed inferior to the inguinal ligament and the femoral nerve was appeared as an oval hyperechoic structure which was lateral to femoral artery.24 Insert the needle from the lateral edge of the probe with an in-plane approach. The trace and the tip of the needle could be well visualized as it approached the target nerve. Correct needle position was confirmed when 2 ml of saline spread as an expanding hypoechoic area surrounding the nerve. Subsequently, 15 ml of 0.4% ropivacaine was injected with repeated negative aspiration.
For obturator nerve block (ONB), patients lay supine with the hip externally rotated. The transducer was positioned at the proximal thigh inferior to the inguinal ligament. The anterior division was seen as a hyperechoic and oval structure between the adductor longus and brevis muscles. The posterior division was between the adductor brevis and magnus muscles.24 With in-plane technique, the needle was ﬁrst directed into the fascial plane between the adductor brevis and magnus muscles and 5 ml of 0.4% ropivacaine was injected at this location. Then the needle was withdrawn to the superficial plane between the adductor longus and brevis muscles and 5 ml of 0.4% ropivacaine was administered.
For lateral femoral cutaneous nerve block (LFCNB), patients were placed supine. The linear probe was placed on the inguinal crease and moved towards the anterior superior iliac spine. The lateral femoral cutaneous nerve was appeared as hyperechoic structure between the fascia lata and iliaca.24 Insert the needle in a lateral-to-medial direction with in-plane approach. Subsequently, 5 ml of 0.4% ropivacaine was injected with repeated negative aspiration.
Blood flow images by LSCI and pinprick sensory scores were recorded every 5 min (t=5, t=10, t=15, t=20, t=25, t=30) for 30 min starting from the completion of local anesthetics injection. At each time point, LSCI monitoring was performed prior to pinprick sensory tests. Pinprick sensory tests were performed with a 22 gauge blunt needle on the skin innervated by the femoral nerve (anterior aspect of the midthigh), obturator nerve (medial aspect of the midthigh), and lateral femoral cutaneous nerve (lateral aspect of the midthigh).25 26 The pinprick sensory measurement was performed bilaterally with the same pinprick stimulus. The rating scale for pinprick sensory blockade was 1 (sensation) and 0 (no sensation). Blood flow images and pinprick sensory test scores were obtained individually by two investigators for each patient (LSCI: X.W.; pinprick sensory tests: D.W.). The two investigators were blinded to each other’s assessments. Haemodynamic data including non-invasive blood pressure, heart rate, and oxygen saturation were also recorded at each time point.
Blood flow index (BFI) was measured by a laser speckle blood flow imaging system15 (SIM BFI-WF, SIM Opto-Technology Co., Ltd., Wuhan, China). The equipment mainly consists of a wheeled stage, a laser speckle imaging head, and a connection arm that enables the imaging head moving flexibly in horizontal and vertical directions. The system uses a laser beam with the wavelength of 785 nm to illuminate the target area and employs a 12-bit charge-coupled device (CCD) camera to acquire the speckle pattern. The CCD exposure time was set to 20 ms. The light was perpendicular to the patient’s foot at a measurement distance of 20-25 cm. Thirty consecutive images were acquired at an interval of 1 s and Region of interests (ROIs) could be selected in the blood flow image. Mean BFI values of the ROIs were calculated by LSCI software (SIM BFI Software, SIM Opto-Technology Co., Ltd., Wuhan, China), which expresses values in perfusion unit (PU). In our study, five separate ROIs (circular areas of 1 cm in diameter on the nail bed of each toe) were established by the circular-ROI selecting tool in the software (Fig. 1).
Lumbar plexus block was defined as successful if complete loss of sensation was observed in all 3 nerve distribution areas at 30 min after the block. In contrast, lumbar plexus block was identified as failed when pinprick test in any of the 3 nerve innervation areas was recorded as “sensation” at 30 min post block. Successful FNB, ONB, and LFCNB were respectively defined as “no sensation” in the anterior, medial, and lateral aspect of the midthigh at 30 min after the block. On the contrary, failed FNB, ONB, and LFCNB were identified as sensation still existing in the nerve innervating area at 30 min post block.
Sample size was calculated by MedCalc Software version 15.2 (MedCalc Software, Ostend, Belgium). This study was originally designed to detect differences in BFI values after nerve block compared with the pre-block baseline value. Based on our previous study, the mean difference of BFI value was 42.77 PU and the standard deviation was 21.18 PU. Setting the α-level to be 0.05 and the power to be 90%, the sample size required for the comparison of two paired samples was calculated as 5. To make the results more convincing, we enrolled at least 8 patients for each type of nerve block. For the accuracy of a diagnostic test, the pre-determined value of the area under the curve was 0.9 and the null hypothesis value was set at 0.5. Previously unpublished data at our hospital showed that the success rate for lumbar plexus block was estimated to be 90%. With the α-level of 0.1 and the β-level of 0.20, a total of 23 cases are required with 21 cases in the successful group and 2 cases in the failed group. We performed statistical analyses by SPSS 22.0 (IBM Corp., New York, USA) and made all line graphics with GraphPad Prism version 6.01 for Windows (Graph Pad Software, San Diego, CA). Continuous data are expressed as mean ± SD or median (IQR). Categorical data are reported as number (percentage). The independent sample t test was used for between-group comparisons of normally distributed data. Categorical data were compared using the Fisher exact test. Repeated-measures analysis of variance (ANOVA) was applied to compare measurements over time. If significant, a two-sided paired t test was performed to examine differences at individual time points within each group. A P value of less than 0.05 was considered significant. A receiver operator characteristic (ROC) curve analysis was used for discriminating the power of the increased blood flow index to detect a successful block.