Predicting successful lumbar plexus block using blood flow index measured by laser speckle contrast imaging

Background: Laser speckle contrast imaging (LSCI) is a powerful optical imaging technique for real-time and dynamic measurement of regional blood flow. The objective of this prospective observational study was to investigate blood flow after changes blockade of lumbar plexus or its three major branches respectively, with LSCI technique. Methods: This study included 47 adult patients scheduled for elective lower limb surgery. For the selective blockade of lumbar plexus, femoral nerve, obturator nerve, and lateral femoral cutaneous nerve, blood flow images and pinprick sensory scores of the blocked lower limb were recorded 5 min pre block and every 5 min for 30 min post block. Blood flow index (BFI) values of toes were calculated by LSCI software. Results: In this study, we have 21 cases of successful lumbar plexus blocks, 2 cases of failed lumbar plexus block, 8 cases of successful femoral nerve blocks, 8 cases of successful obturator nerve blocks, and 8 cases of successful lateral femoral cutaneous nerve blocks. The BFI values of all five toes were significantly increased as early as 5 min after successful lumbar plexus block, whereas no signiﬁcant difference was found in BFI values after failed lumbar plexus block. BFI changes after successful selective blockade of femoral nerve, obturator nerve, or lateral femoral cutaneous nerve were negligible. BFI value of the big toe at 5 min after the successful lumbar plexus block was increased by 2.57 fold compared with the baseline value, which represented the highest increase among five tested toes. BFI value of the big toe at 10 min after lumbar plexus block showed great power to predict block outcome with a sensitivity of 100% and a speciﬁcity of 100%. The optimal cut-off value given by ROC analysis was 22.11 PU. Conclusions: Increased blood flow index measured by laser speckle contrast imaging is a reliable indicator of successful lumbar plexus block, but cannot indicate successful selective blocks of three major branches of the lumbar plexus. BFI value of the big toe at (medial midthigh), and femoral cutaneous midthigh). 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. plexus block, whereas no signiﬁcant in BFI values after failed lumbar plexus block. BFI value of the big toe at 10 min after lumbar plexus great power to predict block with a sensitivity of 100% and a speciﬁcity of 100%. Successful selective blockade of femoral obturator nerve, femoral cutaneous not cause BFI changes.


Background
Lumbar plexus block is a useful technique in the management of pain after lower extremity surgery. [1][2][3] Traditional assessment methods of block success, such as pinprick and cold sensory tests, rely on patient compliance and individual subjective judgment.
There is a need for objective methods to evaluate the quality of nerve block. Successful peripheral nerve block could induce local vasodilation and increase blood flow via sympathetic blockade. 45 The success of lumbar sympathetic block can be assessed by various measurements, such as laser Doppler perfusion imager, thermometry, infrared thermography, and pulse oximetry. [6][7][8][9][10] However, these tests are subject to poor temporal resolution and low spatial resolution because of long time intervals of data acquisition and limited observation field. Laser speckle contrast imaging (LSCI) is an optical imaging technique that allows real-time and full-field assessment of blood flow with high spatial and temporal resolution. [11][12][13] LSCI has been widely applied in monitoring cerebral blood flow, skin perfusion, and retinal blood flow. [14][15][16][17][18][19] Our previous study demonstrated that LSCI might be an early, objective, quantitative, and reliable indicator of successful sciatic block. 20 The primary aim of this study was to investigate blood flow changes after lumbar plexus block using LSCI technique.
The three major branches of the lumbar plexus are the femoral nerve, the obturator nerve, and the lateral femoral cutaneous nerve. It was reported that changes of skin temperature could predict the success of infraclavicular brachial plexus block. 21 Selective blockade of the ulnar and median nerve resulted in substantial increases in skin temperature of the 4 hand, whereas selective blocking of the musculocutaneous or radial nerve did not increase skin temperature in any area. 22 This may imply that blockade of the proximal trunk and distal branches of the nerve may have different impact on sympathetic fibers. The blood flow changes after selective blockade of the femoral nerve, the obturator nerve, or the lateral femoral cutaneous nerve have never been studied. The secondary aim of this study was to explore the changes of blood flow after selective blockade of three major branches of the lumbar plexus. 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 24 o C. Blood flow 5 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 highfrequency (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.).

Methods
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 6 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). 2526 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 system 15  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 betweengroup 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.

9
During the study period, 47 patients completed the study. Lumbar plexus block was successful in 21 patients and failed in 2 patients. There were 8 cases of successful FNB, 8 cases of successful ONB, and 8 cases of successful LFCNB in our study. Surgeries performed included knee arthroscopy, total knee arthroplasty, internal fixation of femoral fracture, femoral plate removal, femur lesion excision, total hip replacement, femoral drilling decompression, and adductor myotomy. Patient characteristics are shown in Table   1. There were no significant demographic differences in patients with successful and failed lumbar plexus block.
As shown in Fig. 2 For successful selective FNB, ONB, and LFCNB, BFI value at each toe was not significantly increased after the block compared with the baseline value (Fig. 3).. There were no significant differences in mean arterial pressure before and after the block ( Table 2)..

Discussion
This study investigated the blood flow changes determined by LSCI technique after lumbar plexus block, or successful selective FNB, ONB, and LFCNB, respectively. Our results demonstrated that the BFI values were significantly increased as early as 5 min after successful lumbar plexus block, whereas no significant difference was found in BFI values after failed lumbar plexus block. BFI value of the big toe at 10 min after lumbar plexus block showed great power to predict block outcome with a sensitivity of 100% and a specificity of 100%. Successful selective blockade of femoral nerve, obturator nerve, or lateral femoral cutaneous nerve did not cause BFI changes. To our knowledge, this is the first study using LSCI technique to identify the blood flow changes after blockade of the lumbar plexus or selective blockade of its three major branches.

Successful lumbar sympathetic block had been assessed by various objective methods.
Infrared thermographic imaging was reported to be a simple and safe method for assessing successful lumbar sympathetic block. Cutaneous temperature changes at the most distal parts of the lower extremity by thermocouple probe were reported to predict the efficacy of lumbar sympathetic block in the early stage. 78 However, skin temperature monitoring can be easily influenced by environmental temperature. The pulse oxymetry was introduced to measure change in pulse transit time in the lower extremity, which had been considered as an early indicator of successful lumbar sympathetic block. 10 Skin conductance monitor was proved to indicate successful lumbar sympathetic block for all procedures. 27 Nevertheless, the pulse oxymetry and skin conductance monitor are restricted to single-point measurement and unable to quantify overall perfusion in the involved capillary bed. Compared with these assessment methods, the superiority of LSCI technique lies in high temporal and spatial resolution in a wide field of view. For LSCI measurements, region of interests could be freely selected and the BFI values were realtime acquired. It was reported that the best place to observe peripheral vascular responsiveness was fingertip. 22 As we have previously reported, the BFI values of toes were significantly increased after successful sciatic nerve block. 20 Therefore, the nail bed of each toe was chosen as region of interests in this study.
In the current study, the BFI value with the highest increase was at the big toe after successful lumbar plexus block. It seems that the big toe was the ideal position to observe blood flow changes after lumbar plexus block. Accordingly, one previous study showed that the maximum change of skin temperature after successful lumbar sympathetic block was attained at the big toe when compared with the other regions in the lower extremity. 7 Another study demonstrated that the greatest rise and most rapid changes in skin temperature occurred in the big toe, irrespective of the applied regional-anesthesia techniques of the lower extremity (combined femoral-nerve and sciatic-nerve block, epidural anesthesia, and spinal anesthesia). 28 Areas of skin temperature increase were not totally determined by the segmental innervation area 2228 , which indicated that the sympathetic block territory was not corresponding to the cutaneous innervation of the blocked nerve. It could be advantageous to perform LSCI measurements at the big toe because this region was abundant in arteriovenous anastomoses and richly innervated by sympathetic vasoconstrictor fibers. 29 Our results revealed that BFI value at each toe was not significantly increased after successful selective femoral nerve block. This observation supported the prior finding that changes of skin temperature after femoral nerve block were negligible and late. 30 Sympathetic nerve fibers descend to join the lumbar plexus from the sympathetic trunk in a complex way, which has not been known in detail. 31 It was reported that the femoral 12 artery blood flow responses were not mediated by the lumbar sympathetic nerves. 32 In our study, BFI value at each toe was not significantly changed after selective block of three major branches of lumbar plexus separately. This may suggest that selective block of the separate nerves at more peripheral sites have relatively little impact on sympathetic blockade.
LSCI has been widely applied in clinical practice to better quantify the microcirculation perfusion in real time. [33][34][35][36] Compared with the traditional assessment methods such as pinprick or cold sensory tests, LSCI measurement was objective, simple, and powerful.
Traditional block assessing methods depend on patients' subjective judgement to the pinprick or cold stimuli, which is not applicable in special patient population (such as elderly patients with preoperative cognitive dysfunction, children, patients with mental problems, and patients who received peripheral nerve block after general anesthesia).
LSCI technique is an alternative measurement to predict the block success of lumbar plexus with high sensitivity and specificity, but cannot indicate the success for selective FNB, ONB, and LFCNB.
Laser speckle images are generated as soon as the target is illuminated by laser source.
The LSCI system takes full advantage of intuitive interface, fast speed and high accuracy of data acquisition, real-time image processing, and efficient data analysis. The system offers a friendly user interface which simplifies user operation without the need of special training. The simple structure and easy-to-understand interface for LSCI make the users achieve the desired output with minimal input. LSCI has become a valuable and powerful tool to investigate the fast change in microcirculation blood flow because of safety, wide measurement range, good reliability, high stability, and strong practicability. [37][38][39][40] Patient populations, such as those with peripheral vascular disease, those receiving vasodilator drugs prior to surgery, or those with infected toenails might have altered the BFI values and thus require separate determination.
This study has several limitations. Firstly, the sample size was small. Secondly, we did not perform motor block assessment for obturator nerve. Sensory block assessment for obturator nerve was reported unreliable. 41 Lastly, blood flow images were obtained at 5min intervals post block in this study and continuous measurement would be better.

Conclusions
In conclusion, this study demonstrates that increased blood flow index measured by laser speckle contrast imaging is a reliable indicator of successful lumbar plexus block, but cannot indicate successful selective blocks of three major branches of the lumbar plexus.

Consent for publication
The written informed consent was obtained from patients for publication of the individual blood flow images.

Availability of data and materials
The datasets used and analysed during the current study are available from the corresponding author on reasonable request.

Competing interests
The authors declare that they have no competing interests. Authors' contributions XW helped design the study, collect the data, analyze the results, and write the manuscript. PL helped design the study, analyze the results, and revise the manuscript. XL and LM helped design the study and prepare the manuscript. DW helped select the participants and collect the data. YH helped analyze the results and revise the manuscript.
WM helped design the study, analyze the results, revise the manuscript, and supervise the study. All authors read and approved the final manuscript.