Exploring the Analgesic Efficacy and mechanisms of low-dose esketamine in pregnant women undergoing cesarean section: A randomized controlled trial

Background Postoperative pain is a prevalent concern following a cesarean section. This study aimed to investigate the effect and mechanism of low-dose (0.1 mg/kg) esketamine on postoperative pain management in pregnant women undergoing cesarean sections, specifically in cases where both patient-controlled intravenous analgesia (PCIA) and patient-controlled epidural analgesia (PCEA) were employed. Methods Pregnant women intending to undergo elective cesarean section were divided into four subgroups based on the intravenous administration of esketamine and the specific analgesia methods employed: E1 (0.1 mg/kg esketamine + PCEA), E2 (0.1 mg/kg esketamine + PCIA), C1 (saline + PCEA), and C2 (saline + PCIA). The primary outcome was the maximum pain score within 24 h postoperatively. Secondary outcomes included the pressure pain threshold and tolerance at 30 min and 24 h postoperatively, along with the inflammation and adverse event index scores. Results A total of 118 pregnant women were assigned to the four groups: E1 (n = 29), E2 (n = 29), C1 (n = 30), and C2 (n = 30). Compared with those in the control groups (C1 + C2), the maximum postoperative pain scores within 24 h in the esketamine groups (E1 + E2) were significantly lower (4 [2–5] vs. 4 [4–6], P = 0.002), and the E1 group exhibited superior analgesic effects compared with other groups. No significant differences were observed in postoperative hyperalgesia or inflammation across the four groups. Notably, esketamine combined with PCIA increased the incidence of postoperative nausea and vomiting (7 [25 %] vs. 0 [0 %]; P = 0.005). Conclusion The administration of low-dose (0.1 mg/kg) esketamine effectively alleviates pain following cesarean section, and the analgesic effect is notably enhanced in combination with PCEA. Importantly, these effects do not appear to be mediated through anti-inflammatory mechanisms or the inhibition of hyperalgesia. Clinical trial registration number NCT05414006.


Introduction
Cesarean section (CS) is the most commonly performed surgical procedure [1], and inadequate pain relief following CS is a global concern [2].Some studies on the severity of acute postoperative pain following CS have reported a high incidence of moderate-to-severe postoperative pain of 39-78 % [3], with severe pain in 20 % of cases [4,5].Notably, chronic postoperative pain occurs in 18.3 % and 6.8 % of cases at 3 months and 1 year post-CS, respectively [6].In addition, the mismanagement of perioperative pain is associated with increased opioid use, chronic pain, impaired maternal-fetal bonding, delayed functional recovery, and increased postpartum depression [7].Patient-controlled epidural analgesia (PCEA) and patient-controlled intravenous analgesia (PCIA) are currently the most commonly used analgesic approaches.High-dose opioids in PCIA exert various adverse effects, such as ileus, constipation, and respiratory depression.Whereas PCEA is limited by complications including motor block and lower extremity venous thrombosis [8].Consequently, exploring new analgesic approaches and improving the previously established strategies are imperative.
Esketamine, an isomer of ketamine, reduces pain signals induced by nociceptive stimuli in the cortex by inhibiting N-methyl-Daspartate (NMDA) receptor activation.Several trials have shown that esketamine can relieve pain post-CS [9][10][11][12] and attenuate postoperative inflammation [13].The nociceptive stimuli from the surgical wound are transmitted to the dorsal horn of the spinal cord via the dorsal root ganglion and activate the NMDA receptors to transmit the pain signal to the neural center, resulting in decreased pain tolerance and hyperalgesia [14].Otherwise, surgical trauma can break down the tissue-organ barrier, causing the release of inflammatory factors and an inflammatory response in the body [15].Notably, inflammation and hyperalgesia are major causes of postoperative pain.Therefore, based on the mechanisms of inflammation and hyperalgesia and the pharmacology of esketamine, esketamine could be a promising option for postoperative pain following CS.
This randomized controlled trial aimed to explore whether low-dose esketamine can relieve postoperative pain after CS in women who underwent CS with PCIA and PCEA analgesia and compare the effects of the combined application of these two different postoperative analgesia approaches.Additionally, hyperalgesia and inflammatory index evaluations were performed to explore the potential mechanisms.

Research participants
This prospective, randomized, placebo-controlled, triple-blind clinical trial was approved by the Ethics Committee of the Second Affiliated Hospital of Chongqing Medical University (2022-77).All procedures were performed in accordance with the Declaration of Helsinki guidelines (revised in 2013) and Consolidated Standards of Reporting Trials (CONSORT) reporting guidelines.This trial was registered with clinicaltrials.gov(Identifier: NCT05414006) and conducted at the Second Affiliated Hospital, Chongqing Medical University, a comprehensive teaching hospital.The protocol was explained to all participants, and informed consent was obtained before trial initiation.The clinical data and protocols used in this study are available upon reasonable request from the corresponding author.

Inclusion and exclusion criteria
The trial enrolled patients that met the following criteria: ASA status I-III, age 20-45 years, gestational age 37-42 weeks, and undergoing elective CS with subarachnoid anesthesia.We excluded patients with contraindications for CS, contraindications of combined spinal and epidural anesthesia, severe systemic disease, alcoholism and long-term use of anti-inflammatory and analgesic drugs, contraindications to esketamine or hydromorphone, and those who were unable to cooperate or refused to participate in the trial.

Randomization and masking
In this study, simple randomization with a sealed envelope method was used.A biostatistician who was not responsible for the data management and statistical analysis generated random numbers for the four different groups using SPSS software.Another independent assistant prepared and placed random numbers in opaque, numbered, and sealed envelopes.Pregnant women were enrolled sequentially during the study period and randomly divided into four groups according to the random numbers in the envelopes.A researcher independent of data collection and analysis prepared the interventional drugs and analgesic pumps based on the random numbers.After the intervention, the envelopes were resealed and preserved until the end of the study.The participants, outcome evaluators, and data analysts were blinded throughout the study.The study drugs were prepared preoperatively and assigned based on random computer-generated numbers.Only "study drugs" and the patients' random numbers were marked on the syringes and PCA devices injected after umbilical rupture.The participants, care providers, and outcome assessors were blinded to the allocation.The researchers who analyzed the data were not involved in any treatment or evaluation.

Preoperative and intraoperative management
In this study, all CSs were performed by the same experienced surgical team, with an experience of >1000 CSs.Anesthesia was performed by the same experienced and skilled group of anesthesiologists.After establishing the venous infusion channel of the upper limb, all women received 500 mL of normal saline via an intravenous cannula at a rate of 1 mL/min to keep the vein open.Anesthesiologists monitored the pulse oxygen saturation (SpO 2 ), electrocardiogram, and blood pressure.Combined epidural anesthesia was performed at the L2-L3 space with 1.5 mL of 0.75 % bupivacaine injection + cerebrospinal fluid, totaling 2.5 mL.The block level was tested intermittently.If T6 was not reached, 5 mL of 2 % lidocaine was administered epidurally.Close monitoring of maternal vital signs was maintained, and 6 μg of norepinephrine was administered in cases of maternal hypotension (MAP ≤80 % of the MAP base value or an MAP <60 mmHg).In cases of bradycardia (ventricular rate <50 BPM), 0.3 mg of atropine was administered, and in cases of maternal SpO 2 < 90 % or respiratory distress, 60 % oxygen was given via a mask.A Pfannenstiel incision with peritoneal closure was performed in all CSs.Immediately after umbilical rupture, 20 UI of oxytocin was injected (500 mL, 5 % glucose solution) at a rate of 100 mL/h.Dexamethasone (10 mg), metoclopramide (5 mg), and ondansetron (8 mg) were routinely administered to prevent postoperative nausea and vomiting.

Intervention and allocation
All participants were randomly divided into four groups at a ratio of 1:1:1:1 as follows: the E1 group received esketamine at a dose of 0.1 mg/kg intravenously after delivery, followed by PCEA (200 mL containing 10 μg/mL of hydromorphone + 0.11 % ropivacaine).
For PCEA, patients received an initial dose of 6 mL, delivered in pulses of 6 mL/h thereafter, with intermittent boluses of 6 mL.
Lockout time was set at 30 min, with a limit of 18 mL/h.For PCIA, patients received an initial dose of 20 μg/kg, delivered slowly over 10 min.The PCIA pump was programmed for continuous background infusion at a rate of 3 μg/kg/h, with an additional 3 μg/kg provided with each button press.A lockout interval was set at 10 min, with a maximum limit of 20 μg/kg/h.In instances where analgesic consumption reached the maximum hourly dose and the Numerical Rating Scale (NRS) score was ≥4, intravenous tramadol at a dose of 50 mg was administered for rescue analgesia.

Outcome measurement
The demographic characteristics (age, height, weight, and gestational age) and comorbidities of the patients were recorded.The primary outcome of this study was the maximum pain NRS score within 24 h postoperatively, expressed as the NRS score (0 = no pain and 10 = most painful), evaluated by a researcher blinded to the group allocation.Maximum rest incision pain, moving incision pain, and visceral pain scores were evaluated at 0-6 h, 6-12 h, and 12-24 h postoperatively.Further, the area under the curve (AUC) of the NRS pain score at different time points was also calculated.
Upon admission, all patients underwent assessments using the Generalized Anxiety Disorder-7 (GAD-7) scale and the Edinburgh Postnatal Depression Scale (EPDS) to evaluate the presence of anxiety and depression.Preoperative measurements included the neutrophil count (N), neutrophil-to-lymphocyte ratio (NLR), and white blood cell count (WBC).After entering the operating room, pressure pain threshold and tolerance were measured using an algometer placed on the forearm of the dominant hand.Each measurement was separated by a period of at least 3 min, and the average of these three measurements was considered the baseline value.
Secondary outcome measures of this study included the pressure pain threshold and tolerance at 30 min and 24 h postoperatively; number of patient-controlled analgesia pumps pressed; patient-controlled analgesia pump consumption; side effects of drugs, including dizziness, postoperative nausea and vomiting (PONV), drowsiness, irritability, itching, hallucinations, nightmares, diplopia, hypertension, and tachycardia; serious adverse events such as respiratory failure requiring oxygen therapy or ventilator support; severe hemodynamic disorders (heart rate <45 beats per min, systolic blood pressure <80 mmHg, or cardiac arrest); and severe neurological impairment (convulsions and coma).
In this study, the hyperalgesia assessments were conducted using an FDIX 25 algometer (Zhiji, DS2-100N, Zhiji Precision Instruments Co., Ltd., Dongguan, China [in kgf]).The procedure involved vertical contact with the skin, with pressure gradually increasing until the patient reported pain, at which point the pressure pain threshold was recorded.Pressure was further increased until the patient could no longer tolerate it, and the pressure pain tolerance was recorded.The average value was measured on the skin on the inner forearm at the front crease of the elbow at 3, 6, and 9 cm.The preoperative (T0) value was used as the baseline, and the values were measured at 30 min (T1) and 24 h (T2) postoperatively.The extent of change relative to the T0 value indicated the degree of hyperalgesia.

Statistical analysis
The maximum postoperative pain NRS score within 24 h based on the pre-experimental study was 5.2 ± 2.0 when esketamine was not administered, and the NRS score of esketamine prophylactics was 3.9 ± 1.8.The required sample size of 46 cases for each group was determined using the PASS sample size calculation software at a significance level of 0.05 and a power of 90 % according to the 1:1 parallel control difference test design.The final sample size was calculated as 58 patients for each group, considering a 20 % attrition rate.Consequently, this study included 60 patients in each group, with 30 patients in each subgroup, accounting for 120 patients in total.
Data analysis was performed using IBM SPSS Statistics version 22.0 (IBM Corp., Armonk, N.Y., USA).A general descriptive method was adopted based on the type of data.Continuous variables with a normal distribution were presented as the mean ± standard deviation, whereas quantitative data with a non-normal distribution were presented as the median (interquartile interval), and the quantitative percentage was used for qualitative variables.According to the data type, baseline data between patients in groups C and E were compared using an independent sample t-test, Mann-Whitney U test, or Chi-square test.Maximum pain NRS, rest incision pain, moving incision pain, and visceral pain score AUCs within 24 h postoperatively were compared using an independent sample t-test or Mann-Whitney U test.Adverse events between groups C and E were compared using a chi-square test.Exploratory subgroup analysis between groups E1 and C1 and E2 and C2 was also performed using an independent sample t-test or Mann-Whitney U test.Additionally, a multiple linear regression analysis was performed to investigate the influence of esketamine use (yes or no) and the choice of analgesic method (PCIA or PCEA) on postoperative pain intensity, including maximum pain NRS, rest incision pain, moving incision pain, and visceral pain score AUCs within 24 h postoperatively.A two-sided P < 0.05 was considered statistically significant.

Results
In this study, 158 pregnant women were screened for eligibility.Of these, 120 women who met the inclusion criteria were successfully enrolled.The final visit for the last randomized participant was concluded on August 2, 2022.Throughout the study duration, the blinding process remained uninterrupted.Two women were ultimately excluded from the trial, with one withdrawing voluntarily Fig. 1.CONSORT flow chart of the study.
J. Zhang et al. and another being excluded due to pain resulting from urinary stones.Consequently, 118 pregnant women were randomly assigned to groups E1 (n = 29), E2 (n = 29), C1 (n = 30), or C2 (n = 30) (Fig. 1), and all were included in the final per-group analyses (Fig. 1).No significant differences were observed in the demographic characteristics of the four groups (Table 1).
Compared with patients in group C (C1 and C2), those in group E (E1 and E2) exhibited a statistically significant reduction in the maximum postoperative pain score within 24 h, the AUCs for rest incision pain, moving incision pain, and visceral pain (Table 2).The maximum postoperative pain score within 24 h and the AUCs for rest incision pain and moving incision pain were higher in group C1 than in group E1.Furthermore, the maximum postoperative pain score within 24 h and the AUC for visceral pain were higher in group E2 compared with group C2 (Table 2).
The NRS scores of rest incision, visceral, and moving incision pain were significantly lower in group E (E1 and E2) than in group C (C1 and C2) at 6-12 h and 12-24 h postoperatively (Fig. 2A).In the intergroup analysis of PCEA, moving incision pain at 6-12 h and rest incision and moving pain at 12-24 h postoperatively were higher in group C (Fig. 2B).In the intergroup analysis of PCIA, 6-12 h and 12-24 h postoperative visceral pain were higher in group C than in group E (Fig. 2C).
Various factors were examined in the intergroup analyses of PCEA (Table 3) and PCIA (Table 4).Notably, no significant difference was observed in the hyperalgesia index, including preoperative pressure pain threshold, preoperative pressure pain tolerance, inflammation index, including N, NLR, and WBC count, and in terms of adverse reactions such as dizziness and drowsiness (Table 5).

Discussion
The primary outcomes of this investigation offer several key insights.First, a low dose of esketamine (0.1 mg/kg) plays a significant role in alleviating pain post-CS.Second, esketamine in combination with PCEA is a more effective approach for managing acute postoperative pain.Third, the mechanism underlying the analgesic effect of this low esketamine dosage does not appear to be rooted in the inhibition of hyperalgesia or its anti-inflammatory properties.Lastly, esketamine in combination with PCIA does lead to an increased incidence of PONV.PCEA and PCIA with opioids are common analgesic methods used after a CS.High-dose opioids cause several reactions, such as a high pain tolerance and hyperalgesia, weakening the analgesic effect.PCEA reduces pain and stress reactions caused by trauma and may promote early recovery.Therefore, PCEA is significantly superior to PCIA, and this hypothesis has also been tested in other abdominal surgeries [16,17].Esketamine can exert complementary analgesic effects, emerging as a superior choice in combination with PCEA for acute pain management.An increasing body of evidence suggests that perioperative esketamine administration during CS can effectively relieve postoperative pain [9,10,[18][19][20][21][22][23][24].In most previous studies, the dose of esketamine was ≥0.2 mg/kg, with some inevitable adverse reactions [19,25].Therefore, in this study, we selected a single low-dose intravenous injection of 0.1 mg/kg to explore its effects in relieving pain post-CS.The exact mechanism through which low-dose esketamine Data are presented as the mean (SD) or number (proportion).Abbreviations: WBC, white blood cell; N, neutrophil; NLR, neutrophil-to-lymphocyte ratio; PPT-T0, preoperative pressure pain threshold; PTO-T0, preoperative pressure pain tolerance.
alleviates post-CS pain remains uncertain.Surgical trauma induces a robust inflammatory response, causing inflammatory pain and noxious stimulation that activates NMDA receptors through the dorsal root ganglion, amplifying pain signals reaching the cerebral cortex and leading to hyperalgesia.Consequently, we hypothesized that esketamine may attenuate pain signal transduction and inhibit hyperalgesia by noncompetitively antagonizing NMDA receptors, thereby reducing postoperative pain.A prior study demonstrated the ability of esketamine to inhibit hyperalgesia following thyroidectomy.However, in thyroidectomy, hyperalgesia results from both   surgical trauma and perioperative opioid use.

Table 4
Comparisons of other outcomes between groups E2 and C2 using PCIA.Data are presented as the mean (SD), number (proportion), or mean (interquartile range).PPT ratio-T1 = PPT-T1/PPT-T0; PPT ratio-T2 = PPT-T2/PPT-T0; PTO ratio-T1 = PTO-T1/PTO-T0; PTO ratio-T2 = PTO-T2/PTO-T0.Abbreviations: WBC, white blood cell; N, neutrophil; NLR, neutrophil-to-lymphocyte ratio; PPT-T0, preoperative pressure pain threshold; PTO-T0, preoperative pressure pain tolerance.Data are presented as the number (proportion).occurrence of hyperalgesia.Nevertheless, no statistical difference was found in the intergroup analysis, which may be explained as follows: 1) The analgesic effect of low-dose esketamine may not be attributed to the inhibition of hyperalgesia, and 2) the inhibitory effect of esketamine on trauma-induced hyperalgesia might not be as pronounced as that of opioids.Ketamine and esketamine reduce perioperative inflammation [13,[26][27][28][29][30], and the NLR ratio may reflect systemic inflammation and stress [31][32][33].In our analysis, we compared NLR, N, and WBC levels among the various subgroups both preoperatively and at 2 days postoperatively.However, no significant differences among the subgroups indicated that the potential mechanism of the analgesic effect of low-dose esketamine was not anti-inflammatory.
In addition, our findings revealed that PCEA is significantly better than PCIA, which is consistent with the results of previous studies [34][35][36][37][38][39].Gauger et al., in a randomized controlled trial on the analgesic effects of hydromorphine combined with PCEA and PCIA after posterior spinal cord fusion, reported that PCEA was significantly better than PCIA [40].In our study, the PCEA drugs were similar to those used by Gauger et al., further validating the superior effects of PCEA to PCIA for analgesia after cesarean delivery.
This study had some limitations.First, we did not conduct a follow-up assessment of depression and anxiety among the women in puerperia in the postoperative period, which should be explored in the future.Second, we did not explore different doses of esketamine to determine the optimal dose.Third, we did not record additional analgesia, and the effects of esketamine on the incidence of additional analgesia should be further explored in future studies.Finally, this study only evaluated acute pain in pregnant women within 24 h postoperatively, and the effect of esketamine on the development of chronic pain after CS remains unexplored.

Conclusions
Intraoperatively administered 0.1 mg/kg esketamine could effectively relieve pain after CS, with the superior analgesic effect of esketamine combined with PCEA.However, the underlying mechanism is not related to the inhibition of hyperalgesia and antiinflammation and needs to be confirmed in further studies.

Ethics statement
This clinical trial was approved by the Ethics Committee of the Second Affiliated Hospital of Chongqing Medical University (2022-77).All procedures were performed in accordance with the Declaration of Helsinki guidelines (revised in 2013) and Consolidated Standards of Reporting Trials (CONSORT) reporting guidelines.

Clinical trial
The clinical trial described in this paper was registered at clinicaltrials.govunder the registration number NCT05414006.
as the mean (±) standard deviation and interquartile range.Abbreviation: AUC: Area under curve.

Fig. 2 .
Fig. 2. Comparisons of postoperative pain NRS between esketamine and control groups.A: Comparison between the esketamine groups (E1 and E2) and the control groups (C1 and C2).B: Comparison between groups E1 and C1 in patients administered PCEA.C: Comparison between groups E2 and C2 in patients administered PCIA.(NRS: number rating scale).

Fig. 3 .
Fig. 3. Regression analysis of influencing factors of maximum pain NRS (A) AUC of visceral pain NRS (B), AUC of rest incision pain NRS (C), and moving incision pain NRS (D).(AUC, area under the curve; NRS, numerical rating scale; CI, confidence interval).

Table 1
Baseline demographic characteristics of patients in different groups.

Table 2
Comparisons of primary outcomes related to pain between different groups.

Table 3
Comparisons of other outcomes between groups E1 and C1 using PCEA.

Table 5
Comparisons of adverse reactions between groups E and C.