The approval from the institutional review board was not required because the clinical data were presented anonymously and the written informed consent was obtained from the patient.
The total doses of 1% mepivacaine and 0.75% ropivacaine used for the 2 sessions of the CFSNBs were 460 and 345 mg, respectively. However, the total doses were administered at an interval of 115 minutes between the end of the local anesthetic injection for the 1st and 2nd CFSNBs (225 mg of 1% mepivacaine and 168.75 mg of 0.75% ropivacaine for the 1st CFSNBs and 235 mg of 1% mepivacaine and 176.25 mg of 0.75% ropivacaine for the 2nd CFSNBs). Despite the separate administration of the local anesthetics at an interval, the possibility of LAST remained. In previous studies, when 700 and 667.2 ± 61.2 mg (mean ± standard deviation) of mepivacaine were administered via a single-shot technique for CFSNBs, no signs or symptoms of LAST were detected. However, an average of 667.2 mg of mepivacaine increased the maximum plasma concentration of mepivacaine (5.1 μg/mL on average) to the threshold needed for toxic symptoms to appear (5–6 μg/mL), whereas 700 mg of mepivacaine did not (3.91 ± 0.95 μg/mL) have the same effect. In addition, a threshold concentration of ropivacaine for LAST (1–2 μg/mL) was achieved in the absence of suggestive signs and symptoms of LAST following the administration of 300 mg of 0.75% ropivacaine mixed with 1:200,000 epinephrine for CFSNBs. Therefore, a dose of ropivacaine higher than that used in the above study and the additive effect of mepivacaine (although the dose of the mepivacaine was lower than those in the above studies[17,18]) would considerably increase the possibility of LAST. Moreover, the interval between the 2 block sessions (115 minutes) did not appear to help prevent the development of LAST because the local anesthetic concentration was maintained near the peak value until 90 to 120 minutes after the end of local anesthetic injection.[17,20] Fortunately, our patient did not experience signs and symptoms of LAST.
Additional factors may have contributed to the prevention of LAST. For example, the use of a local anesthetic mixture has advantages over a single local anesthetic because the mixture compensates for the short durations of action of rapidly acting anesthetics (mepivacaine in this case) and the long latencies of longer acting anesthetics (ropivacaine in this case). The mixture could also decrease the incidence of cardiac events by decreasing the doses of more potent local anesthetics, the convulsion doses of which can easily cause a cardiac event. In addition, real-time ultrasound guidance might prevent vascular injury, which commonly leads to intravascular injection of a local anesthetic. Ultrasound imaging could also detect inadvertent intravascular entry of a block needle, thereby minimizing the amount of local anesthetic injected into a blood vessel.[22,23] Furthermore, the compression of veins resulting from ultrasound transducer use reduces the risk of vascular puncture and local anesthetic injection into vessels. When a fixed dose of mepivacaine was administered for various types of nerve blocks, CFSNBs resulted in the lowest maximum plasma concentration of the anesthetic and prolonged the time to reaching the maximum plasma concentration than other types of regional anesthesia, such as lumbar epidural, caudal, intercostal-nerve (bilateral), and supraclavicular-brachial-plexus blocks. Finally, dexmedetomidine used for intraoperative sedation has an anticonvulsive effect by increasing the convulsive dose of local anesthetics. Therefore, it appears that our patient, who did not experience LAST, benefited from the above factors contributing to the prevention of LAST despite the excessive local anesthetic dose.
Even if a concern existed regarding tourniquet pain due to the omission of the obturator and lateral femoral cutaneous nerve blocks for the prevention of LAST, the patient did not complain of tourniquet pain during the 2 consecutive surgeries. This was presumably due to the intravenous administration of dexmedetomidine before the beginning of the surgery, as dexmedetomidine reduces sympathoadrenal responses to tourniquet pain as well as opioid requirements to relieve the pain.
In conclusion, the timely placement of bilateral CFSNBs immediately before corresponding limb surgery that lasts for less than 2 hours provides successful surgical anesthesia in the bilateral lower limbs without resulting in LAST, hemodynamic instability, or tourniquet pain and avoids the regression of CFSNBs performed earlier during the surgery.
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