A surfeit of cesarean sections are performed annually worldwide. In 2016, the cesarean section (CS) rate was 31.9% (Martin et al., 2017) and 41.1% (Liang et al., 2018) in the United States and China, respectively. Despite the numerous measures used to manage pain, the incidence of inadequate analgesia after CS remains nearly 50% (Patel et al., 2017). Pain relief after CS is crucial, as poor pain relief can interfere with breast feeding and delay infant weight gain (Hirose et al., 1996). Inadequate analgesia can impair functions such as ambulation and dietary intake, and lead to complications such as thromboembolism (Leung, 2004) and postoperative ileus (inhibition of gastrointestinal motility after surgery that prevents oral intake) (Cho et al., 2017). Poor pain relief may even lead to chronic pain (Jin et al., 2016) and increased postpartum depression (Shen et al., 2020). Therefore, effective pain management after CS continues to be a relevant public health issue.

The search for the ideal method to manage postoperative pain is ongoing. Although the use of opioids is the gold standard (Kerai et al., 2017), the effects of opioids on lactation have limited intravenous opioid use—especially long-term use. Wu et al. found that patient-controlled epidural analgesia (PCEA) provided significantly superior analgesia compared with intravenous patient-controlled analgesia (PCIA) (Wu et al., 2005). Epidural morphine is commonly used for analgesia after CS (Carvalho and Butwick, 2017), but can produce a high incidence of pruritus, nausea, and vomiting (Youssef et al., 2014). These adverse drug reactions (ADRs) are difficult to prevent, and therefore modified regimens such as low concentrations of local anesthetics together with other opioids have been advocated to reduce opioid dosage and thereby decrease the incidence of bothersome ADRs. Epidural opioids also reduce the dosage of local anesthetics. A study by Gogarten et al. found that when combined with ropivacaine, sufentanil can reduce the concentration of ropivacaine required and the degree of motor blockade (Gogarten et al., 2004).

Ropivacaine has been used in labor for epidural analgesia and post-cesarean analgesia because of its lower potential for accumulation, greater sensorimotor-differential block, and increased cardiovascular safety profile (Bullingham et al., 2018; Wang et al., 2019). The breast milk/plasma concentration ratio of ropivacaine was lower than bupivacaine when administered epidurally after CS (Matsota et al., 2009), which is essential for neonatal safety. Although ropivacaine co-administered with sufentanil successfully reduced postoperative pain, few investigators have thus far evaluated the optimal concentration necessary. Therefore, we evaluated the analgesic effects by using two different concentrations (0.1% and 0.15%) of ropivacaine with or without 0.5 μg/ml of sufentanil to relieve post-cesarean pain.

This trial was conducted at the Department of Anesthesiology, Xuanwu Hospital of Capital Medical University in China. Ethics approval (ea1/185/10) was obtained from the ethics committee of the Xuanwu Hospital of Capital Medical University. The trial was registered with the Chinese clinical trial registry (ChiCTR 1900 021740), and written informed consent was obtained on the day before surgery.

Our primary aim was to compare the analgesic efficacy of the four groups at the first 24 h after CS by a prospective, randomized, double-blinded clinical trial. Our secondary objective was to compare patient satisfaction, gastrointestinal symptoms, postsurgical recovery, and ADRs among the four groups.

Of the 303 patients screened for eligibility, we enrolled 240 patients, and 42 patients were excluded prior to randomization (Figure 1). One hundred and ninety-eight patients were subsequently randomly allocated to four groups. After randomization, 24 patients were excluded (Figure 1). We ultimately analyzed 43 patients in the R₁ group, 45 in the R₂ group, 43 in the R₁S group, and 43 in the R₂S group (Figure 1).

Postoperative analgesia after CS is challenging since we need not only to consider maternal comfort, but the anesthetic technique should also have no ADRs on the newborn. Epidural morphine is commonly used for analgesia after CS (Carvalho and Butwick, 2017), but there is a concomitantly higher incidence of pruritus, nausea and vomiting (Youssef et al., 2014). As these ADRs diminish overall patient satisfaction, we advocate the use of low concentrations of local anesthetics with other opioids so as to reduce opioid dosage and thereby decrease the incidence of ADRs.

The present study indicated that co-administration 0.15% ropivacaine with 0.5 μg/ml of sufentanil epidurally alleviated CS pain at rest, during movement, and when massaging the uterus, and it reduced the number of self-administered doses of the PCA compared to 0.1% ropivacaine or 0.15% ropivacaine at 6 h. Moreover, it improved overall patient satisfaction scores and reduced the time to first flatus compared to 0.1% ropivacaine. The time to first ambulation was not delayed. However, co-administered with sufentanil increased the incidence of pruritus compared to ropivacaine only at 6 h. Higher concentration of ropivacaine increased the incidence of numbness compared to lower concentration at both 6 and 24 h, whether or not it was co-administered with sufentanil. We believe that although a higher incidence of pruritus and numbness was demonstrated, co-administration of 0.15% ropivacaine with 0.5 μg/ml of sufentanil epidurally may be the better choice for pain relief relative to the other groups due to the lower NRS-R, NRS-M, and NRS-U scores, shorter time to first flatus, and higher patient-satisfaction scores.

Published guidelines advocate for fentanyl and sufentanil for epidural analgesia (Plante and Gaiser, 2017). The authors of a systematic review found that compared with fentanyl, sufentanil was more effective in extending the duration of analgesia when used in CSEA during labor (Zhi et al., 2020). Ropivacaine blocks conduction in the sensory and motor nerves, while sufentanil interrupts pain transmission in the dorsal horn. Therefore, adding sufentanil to ropivacaine may synergistically prolong the duration of sensory block (Parpaglioni et al., 2009). In the present study, ropivacaine co-administered with sufentanil relieved the pain not only at rest but also during movement compared to ropivacaine only at 6 h. However, when massage the uterus, 0.15% ropivacaine with sufentanil showed an improved analgesic effect, while 0.1% ropivacaine with sufentanil did not show improvement compared with 0.1% ropivacaine only at 6 h. All these results indicated that a higher concentration of ropivacaine (0.15%) with sufentanil produced a stronger analgesic effect.

Although NRS scores were significantly lower in the R₂S group relative to the other groups, the analgesic effect was not satisfactory in any group when massage the uterus. Carvalho et al. found that patients tolerated moderate pain rather than exposing their baby to any possible harmful effects of analgesics (Carvalho et al., 2005), and others found that pain was not the only factor that determined overall patient satisfaction (Schyns-Van Den Berg et al., 2015); these may be reasons for the high overall satisfaction scores we received. In our study, the rare use of supplementary analgesia may be also attributed—at least partially—to the patients’ fear of exposing their baby to any possible harmful analgesics (Carvalho et al., 2005).

Postoperative pain increases the secretion of catecholamines, and sympathetic stimulation prevents the release of acetylcholine (Cho et al., 2017), thus inhibiting the recovery of gastrointestinal function; this may be the reason that time to first flatus in the R₁, R₂, and R₁S groups was longer than that in the R₂S group.

Early movement hastens postpartum recovery (Macones et al., 2019). Epidural analgesia with local anesthetics may delay ambulation of the mother, but when administered at low concentrations, the probability is small (Chen et al., 2014). In our study, the time to first ambulation in the R₂S group was shorter than R₁S group, this maybe attribute to the better analgesic effect in the R₂S group than in the R₁S group. The time to first ambulation in the R₂S group was not delayed compared with the other groups. We attribute this to better analgesic effect and the administration of low-concentration ropivacaine (0.1% or 0.15%). Other investigators also reported similar results, Liu et al. found that 0.2% ropivacaine induced a 30% incidence in motor block, but not in those individuals who received 0.05% or 0.1% ropivacaine (Liu et al., 1999).

Complications of neuraxial drugs are the results of drug absorption into the blood circulation or presence in cerebrospinal fluid, or both. The principal ADRs observed in the present study were numbness and pruritus. The incidence of numbness was higher in the R₂ and R₂S groups when compared with the R₁ group at both 6 h and 24 h, indicating that higher concentration of ropivacaine (0.15%) increased numbness.

The incidence of pruritus was higher in the R₂S group relative to the R₂ group, and that was higher in the R₁S group compared to the R₁ and R₂ groups at 6 h. But the incidence of pruritus in the present study was overall low (9.3% and 7.0% at 6 and 24 h, respectively, in the R₂S group; and 9.3% and 7.0% at 6 and 24 h, respectively, in the R₁S group). By contrast, the incidence of epidural morphine (2.5 mg)-induced pruritus was 44% (Youssef et al., 2014). Our use of a small effective dose of sufentanil and the addition of ropivacaine might explain the low incidence of pruritus. Cai et al. reported that when ropivacaine was combined with sufentanil for labor analgesia, the incidence of pruritus was 3.3% (lower than what we reported herein), which may be due to the lower concentration of sufentanil (0.3 μg/ml) the authors used (Cai et al., 2020). Pruritus may, at times, be more unpleasant for patients than pain, from this perspective, epidural sufentanil appears to be better than morphine.

With respect to other ADRs such as nausea and vomiting, headache, dizziness, hypotension, and respiratory depression, there were no significant differences among the four groups.

There were some limitations to our study. First, we did not assess the possible preoperative confounding factors for pain after CS (Pan et al., 2013). However, the previous pain experience due to a preceding CS was evenly distributed among the four groups. Second, the observation period was the initial 24 h, and we recommend that future studies lengthen the observation time to 48 h or even longer. Third, in the present study, the incidence of urinary retention was not recorded, as the urinary catheter was inserted prior to surgery and removed after patients could walk with assistance in the ward.

Although the ADRs (pruritus and numbness) were greater in the R₂S group, by significantly reducing the pain scores and the time to first flatus, patients reported higher satisfaction scores in the R₂S group relative to other groups. In conclusion, the epidural co-administration of 0.15% ropivacaine and 0.5 μg/ml of sufentanil (at a 4 ml/h background infusion with a bolus dosage of 4 ml/20 min as needed) may be the better choice for post-cesarean analgesia compared to the other three regimens investigated in this study.