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.