Among the various ultrasonographic measurements studied herein, we observed increases in the SWV values of the nerve and the ratio of SWV values of the nerve to muscle in association with the nerve block. Our findings also demonstrated the high diagnostic performance of these SWV values for detecting the success of the nerve block, suggesting that SWV measurements of nerve and nerve/muscle ratio could be used as indicators of the success of regional nerve blocks. To the best of our knowledge, this is the first study to investigate the potential use of ultrasonographic parameters to identify the quality of regional nerve blocks.
The results of this study show that successful nerve blocks are associated with an increase in the SWVN value compared with the pre-block value, beginning from 30 min after the injection of the local anesthetic and remaining for 90 min. Despite the lack of statistical significance, we strongly speculate that the increase in the SWVN value is directly related to the nerve blockade rather than the serum levels of the local anesthetic or a secondary change due to the liquid injection and electric nerve stimulation, based on the following observations: (1) there was no obvious change in the value of SWVN in the sham-block group, (2) the SWVN values at 90 min post-block in the nerve-block group were significantly higher than those in the sham-block group, and (3) we assigned the legs of the same dog to the sham-block and nerve-block groups. Moreover, the change rate of the SWVN in the nerve-block group was significantly higher than that in the sham-block group at all time points. The lack of a statistically significant difference in actual SWVN values is probably due to fluctuations in this value among the individual dogs. Indeed, the CV values of the actual SWVN were relatively high (> 10%) at all time points in both the nerve-block and sham-block groups. It has been demonstrated that the tissue elasticity of the peripheral nerve changes with various factors, including limb position and movement, laterality, and age37–41. On measuring the SWV of the dogs in prone position, it is possible that each dog had a different limb position, which may have affected this result.
In the present study, SWVNMR significantly increased after nerve block and showed a higher diagnostic performance for detecting the success of the nerve block compared to SWVN. Since the CV values of SWVNMR were lower than those of SWVN and there was no obvious change in SWVM from baseline in both the nerve-block and sham-block groups, we speculated the high diagnostic ability of SWNMR due to the normalization of SWV variation among individuals by calculating the nerve/muscle ratio in each individual. In SWE, differences in the measured values might have occurred due to the degree of probe compression even if the operator pays careful attention. Additionally, previous studies have reported that the SWE values were influenced by the depth of target regions44. To reduce the measurement variation due to these factors, it has been proposed to evaluate the ratio of the SWE value of the target tissue to that of the surrounding non target tissue45,46. Our results are consistent with these reports.
Our data also showed that the diagnostic performances of the change rates of SWVN and SWVNMR were higher than those of the actual measured values. Notably, the diagnostic accuracy of the change rate of SWVNMR for successful nerve block was 100%, which was the highest among all the examined ultrasonic parameters. These change rates of SWVN and SWVNMR showing higher diagnostic abilities may be attributed to further normalization of the differences in SWVs between individuals compared with the actual SWVN and SWVNMR values through ratio calculations before and after nerve block This speculation is strongly supported by the results that the CV values of the change rates of SWVN and SWVNMR were lower than those of the respective actual measurements. Similar to our results, the findings of a previous study demonstrated that the relative change rate of the peripheral blood flow index from the baseline values has a higher diagnostic ability than actual values in determining the effect of the peripheral nerve block because of the large individual variation in baseline value47. Collectively, it is suggested that evaluating the ratio of SWV values of nerve to muscle and change rate in the SWV value, rather than measuring the actual SWV value itself, is an important factor for assessing nerve block success.
We observed no significant change in CSA in association with the nerve block. This result is consistent with a study that demonstrated that there was no morphological change in the sciatic nerve due to the continuous administration of local anesthetics in a rat mode32. The small diameter of the sciatic nerve of dogs, that is, ≤ 2 mm, may also have affected this result. In addition to CSA, our results revealed no significant change in the NBF of the sciatic nerve in association with the nerve block, contrary to our expectations. Other investigations have demonstrated that local anesthetics, including levobupivacaine, ropivacaine, and bupivacaine, induce a reduction in nerve blood flow in animal models32,33. The discrepancy between these studies and our present results may be explained by differences in the measurement methods used. In previous studies, nerve blood flow was directly measured adjacent to the target nerve using a laser Doppler flowmeter47, which theoretically has a higher detection sensitivity of blood flow compared to SMI. We speculate that the change in the nerve blood flow induced by the local anesthetic is very faint and lower than the detection sensitivity of SMI, and as a result, the SMI failed to identify this blood flow change in the present study. Although the possibility may remain that SMI would detect the change in blood flow in human nerves (with larger diameters compared to dogs), our data suggest that the evaluation of nerve blood flow by SMI may be insufficient as an indicator of nerve block success. Additionally, the fact that the quantification of nerve blood flow by SMI is more complicated without the dedicated software compared to other examinations such as CSA and SWE is a considerable disadvantage of SMI for routine clinical applications.
The mechanism underlying the change in SWV values within the nerve associated with a nerve block is unclear, but several physiological mechanisms have been considered. The decrease in intraneural blood flow, reduction in the metabolic rate, and cytotoxic effect of local anesthetics may affect the change in SWV values. A reduction in blood flow within the nerve after the administration of local anesthetics has been demonstrated, and Crosby48 established that local anesthetics reduce the metabolic rate of the spinal cord, probably as a result of the profound sensory and motor block after spinal application. In addition, local anesthetics have various cytotoxic effects in cell cultures, including inhibition of cell growth, motility, and survival49–51. This may not apply directly to a peripheral nerve block, as these results are from experimental studies; however, it can be speculated that the combination of these effects causes transient ischemia and edema within the nerve, which finally leads to an increase in nerve stiffness. In support of this hypothesis, a relationship between nerve ischemia and an increase in nerve elasticity has been reported in diabetic neuropathy and compressive neuropathy15,16,19,20. Other direct and/or indirect anesthetic effects, such as the membrane-expanding effect of local anesthetics52 and the change in the elasticity of the surrounding tissues induced by a nerve block may also affect the change in the SWV values of a nerve.
An assessment using ultrasonography has several advantages over the currently used block assessment techniques. For example, US measurements, particularly an examination of SWE, are an objective means of assessing the outcome of a nerve block, unlike both the pinprick and cold sensation techniques, which require patients to report the precise sensation felt upon the application of a given stimulus. With US measurements, the patients were not subjected to the potential discomfort of a pinprick or icepack test. The US method can also be applied to patients undergoing general anesthesia. In addition, since US is used for nerve block, a one-stop assessment is possible. However, there are some drawbacks of an assessment using US elastography compared to the currently used block assessment techniques. The US method requires dedicated equipment. The measurement of the SWV is also relatively complex compared to the pinprick and cold sensation techniques.
This study has several limitations. First, the sample size was small, which may limit the power of the assessment of the efficacy of nerve block success by US measurements. Second, US elastography examination is limited by several technical difficulties, including the depth of the lesion and the proficiency of the operator, which limit its clinical application. Third, we did not directly assess sensory nerve block using a skin sensation test. However, this would not affect the outcome of the present experiment because the effect of a nerve block occurs on the sensory nerve, followed by the motor nerve53; thus, if the motor nerve block is achieved, it can be interpreted that the sensory-nerve block is also achieved. Finally, we evaluated the US measurements only from 30 min after the nerve block, and thus, we cannot make a definitive conclusion whether the SWV values can be a reliable predictor of the success of a nerve block at early time points after a nerve block. Further studies are needed to determine the applicability and reliability of SWV values to assess the success of a nerve block.