Discussion
There is an increasing appreciation of SAPB for pain management in the perioperative period, and SAPB has been reported to be widely used in rib fractures, thoracotomy, breast cancer surgery, and shoulder arthroplasty[19]. The success of SAPB is highly related to the volume and optimal concentration of LAs used[8]. In theory, a greater concentration of LAs may produce a better analgesic effect. However, LAs also have a certain toxic effect, and the safety of ropivacaine for SAPB is still not discussed, especially in the condition of continuous administration of LAs. Therefore, more studies are necessary to determine the optimal dosing regimen to achieve the desired analgesic effect while avoiding potentially toxic side effects. The purpose of this study was to compare the pharmacokinetic characteristics and analgesic efficacy of continuous SAPB by using high and low concentrations of ropivacaine in patients who received VATS.
In terms of postoperative pain control, the VAS scores in Group H at 12, 24, and 48 hours postoperatively were significantly lower than those in Group L, whether at rest or on coughing, and the results were statistically significant. However, the clinically important difference in VAS scores was defined as 1.0~1.3 cm for a single measurement[15]. Therefore, in our trial, there were no clinically significant results for the difference in mean VAS scores between the two groups. In this study, rescue analgesia was administered with 50 mg tramadol if the patient had a VAS score ≥4. However, there was no significant difference in rescue analgesia between the two groups, probably because of the good analgesic effect already achieved with continuous SAPB. Collectively, the analgesic effect of Group L was not inferior to that of Group H.
The toxicity of LAs agents is commonly determined by studying the plasma concentration following intravenous infusions in healthy volunteers[18, 20, 21]. Ropivacaine is a local anesthetic commonly used for preoperative or postoperative nerve blocks[22]. Knudsen and colleagues evaluated the plasma toxicity concentrations of ropivacaine in healthy volunteers after intravenous infusion and found that plasma concentrations cause toxicity during intravenous infusion and thus might differ from plasma levels observed during extravascular infusion; symptoms attributable to toxicity commenced in the sampled range of 3.4-5.3 mg/L[18]. The pharmacokinetics of LAs, however, vary depending on the site of injection, and plasma concentrations of ropivacaine have been reported to peak at 2.2μg·mL−1at 30 minutes and remain high for approximately 6 hours after ultrasound-guided TAP[9, 10]. E. C. Hessianet al studied the safety of ropivacaine by continuous TAP[23]. Recent studies have shown that continuous SAPB analgesia is more effective and helps improve patient satisfaction and postoperative recovery [2, 5, 6]. There are no studies evaluating the plasma concentration of ropivacaine during continuous infusion of SAPB thus far. Our findings collectively revealed that the peak concentration of total plasma ropivacaine during continuous SAPB was 2.93 μg·mL−1 for Group H and 2.01 μg·mL−1 for Group L. The results showed that the maximum plasma in Group H remained far below the theoretical toxicity threshold of 3.4 μg·mL−1, and the blood concentration was lower in Group L. By pharmacokinetic studies, the concentration of ropivacaine used was well below the concentration threshold for intoxication. No hypoxemia or incision infection occurred in either group in the postoperative period. At the same time, we also followed up the patients for postoperative nausea or vomiting, with no patients in Group L and one patient in Group H, which was consistent with the results of previous studies[5, 6]. This also indicates that the two groups of ropivacaine concentrations were safe in continuous SAPB. Therefore, continuous SAPB with 0.2% ropivacaine was both effective and safe.