Optimum Dose of Spinal Ropivacaine with or Without Single Intravenous Bolus of S-Ketamine During Elective Cesarean Delivery: A Randomized, Sequential Dose-Finding Study

Post-spinal hypotension has a high occurrence during cesarean delivery, and can lead to adverse maternal or fetal outcomes. The purpose of the study was to determine the optimal dose of spinal ropivacaine for cesarean section with or without intravenous single bolus of S-ketamine and to observe the incidences of hypotension of both methods. Eighty women undergoing elective cesarean delivery were randomly allocated into Group ropivacaine or Group ropivacaine with intravenous S-ketamine. If the upper sensory level of the patient reached T6 and the visual analogue scale (VAS) scores continuously below 3 before delivery, the next patient had a 1/9th chance of receiving a lower dose or an 8/9th chance of receiving the same of previous dose. If the patient had VAS scores above 2 or needed an extra epidural rescue bolus before delivery, a higher dose was used for the next patient. The primary outcome was the successful use of spinal ropivacaine to maintain VAS of patients below 3 before delivery and the incidence of post-spinal hypotension of both groups. Secondary outcomes included the incidences of hypotension related symptoms and managements, upper sensory level of anesthesia, level of sedation, neonatal outcomes, Edinburgh postnatal depression scale scores on admission and discharging of hospital and post-operative analgesic effect. The 90% effective dose (ED90) and 95% condence interval (95% CI) were estimated by isotonic regression.

uterine contraction pain levels after surgery, women who had previously undergone a cesarean section and multiparas were excluded.
Patients were recruited for the study on the day before surgery in the obstetrical wards, when they were asked to complete the Edinburgh Postnatal Depression Scale (EPDS), which is a useful screening instrument for detection of women at risk for depression in the perinatal period. 14 All patients were instructed to fast from midnight on the day of surgery. After entering the operating room, an IV line was established with an 18G IV catheter in the left forearm, and lactated Ringer's solution was infused at a rate of 1 ml.min − 1 to maintain the vein open.
Routine monitoring was performed continuously, including electrocardiography, non-invasive blood pressure, and pulse oximetry. Systolic blood pressure (SBP), heart rate (HR), and pulse oximetry were assessed every 2 min. The rst two resting SBP and HR measurements obtained in the supine position were recorded, and their average values were treated as the baseline values. If the baseline SBP was of > 140 mmHg, the patient was excluded from the study due to hypertension.
The CSE puncture was performed routinely at the level of L3-4 with the patient in the right lateral decubitus position. A 17G Tuohy needle was used to perform the epidural puncture with a paramedian approach. After identifying an entrance into the epidural space, a 27G Whitacre needle was inserted through the epidural needle. Once the cerebrospinal uid was detected, a research isobaric ropivacaine dose of 0.75% was injected through the Whitacre needle. After injecting the ropivacaine, an epidural catheter was inserted into the epidural space by advancing it 3 cm through the Tuohy needle. The patient was immediately moved to the supine position with the left uterine displacement created by placing a wedge under the right hip.
In addition, an infusion of 1 ml.kg − 1 .h − 1 lactated Ringer's solution was administered until delivery. A rescue 5-ml bolus of 2% lidocaine was administered through the epidural catheter 2 min before the surgery for patients that could still feel the cold sensation of ice being placed on the skin below the level of T6 and those that complained of pain and whose VAS score was of > 2 points during the surgery. A 5ml IV dose of 0.15 mg.kg − 1 S-ketamine with normal saline or 5 ml normal saline solution was administered with a 5-ml syringe 1 min before the surgery, depending on group assignment. Hypotension was de ned as SBP of < 80% of the baseline value. A rescue dose of 50 µg phenylephrine was administered whenever hypotension was detected. Bradycardia was de ned as HR of < 50 beats per minute (bpm) and treated with 0.5 mg atropine. The level of sedation was assessed using a 5-point scale (1 = agitated; 2 = alert; 3 = calm; 4 = drowsy; 5 = asleep). 15 Immediately after delivery, 1 ml of umbilical vein blood was collected by the obstetrician, and blood gas assessments were performed, using a blood gas analyser (iSTAT1 Analyzer MN:300-G, Abbott Point of Care Inc., USA) with an iSTAT CG7 + test cartridge. A patient-controlled intravenous analgesia (PCIA) pump (AM3300, ACE MEDICAL EQUIPMENT INC., Korea) was connected to the patient, and a 2-mL PCIA bolus was given at the end of the surgery; meanwhile, the patient was taught how to use the PCIA pump.
The PCIA uid consisted of 10 mg hydromorphone and 90 ml normal saline at the total volume of 100 ml. A 2-ml background infusion and a 2-ml patient-controlled analgesia (PCA) bolus was set to a 15-min lockout time. The patient was followed-up at bedside for the post-operative analgesic effect and recovery of lower limb mobility 24 and 48 h after surgery. The EPDS was administered at discharge.
In both RS and R groups, a research spinal dose of ropivacaine in a 5-ml syringe was prepared and given to the patient through the Whitacre needle by a specialist anaesthesiologist, who knew the exact administered dose. The remaining anaesthesiologists and all patients were blinded to the administered doses.
The spinal ropivacaine dose of 12 mg was used for the rst patient in both groups, based on ndings from Tang et al. 16 and Mei et al. 17 . The dose administered to each subsequent patient was determined by the response of the immediately preceding patient. Satisfactory anaesthetic effect was de ned as the upper sensory level of spinal anesthesia not below T6 before surgery and the VAS score of ≤ 2 points before delivery. Provided a satisfactory anaesthetic effect was achieved, the spinal dose of ropivacaine was considered suitable, and the next patient was 1/9-times as likely to receive a lower dose (decreased by 1.5 mg) or 8/9-times as likely to receive the same dose as the previous patient. When satisfactory anaesthetic effect was not achieved, the following patient received a dose that was increased by 1.5 mg.
Minimum and maximum doses of spinal ropivacaine were 9 mg and 18.5 mg, respectively. The dose assignment was prepared by a statistician, using the BCUD function in Microsoft Excel 2016; only the specialist anaesthesiologist had access to this information, ensuring the double-blind nature of this study.
The primary outcomes were satisfactory anaesthetic effect until delivery and the incidence of hypotension. Secondary outcomes included maternal hypotension-related symptoms and their management, upper sensory level, total co-load IV uid volume, blood loss volume, level of sedation during surgery, recovery of lower limb mobility, analgesic effect 24 and 48 h after surgery, and EPDS scores at discharge; neonatal outcomes included induction-delivery interval, umbilical vein blood gas values (including pH, pO 2 , pCO 2 , base excess (BE), SO 2 , and lactate levels), neonatal weight, Apgar scores measured at 1 min and 5 min post-delivery, and the incidence of neonatal intensive care unit admission.
Maternal demographic characteristics such as age, weight, height, gestational weeks, gravida, para, baseline SBP, baseline HR, and EPDS scores at admission were recorded. All primary and secondary outcomes were observed by anaesthesiologists blinded to patient group and dose assignments.

Statistical analysis and sample size calculation
This dose nding study was based on BCUD design and simulation studies that suggest that a sample size of 20-40 patients may provide stable estimates of the target dose for most realistic cases. 18 The present study included 40 patients per group.
The ED90 was de ned as the spinal dose of ropivacaine associated with a 90% anaesthetic success rate and was estimated, using the isotonic regression method 18 . The corresponding 95% CI was obtained, using the bias-corrected percentile method with 2000 bootstrap replications. 19 Isotonic regression and bootstrapping analyses were performed in R software version 3. 4

.4. (R Foundation for Statistical
Computing, Vienna, Austria) Demographic characteristics and secondary outcome estimates were reported as means ± standard deviations, medians (interquartile range), and counts and proportions. Parametric data were analysed with the t-test; nonparametric data were analysed with the Mann-Whitney test. Proportions were compared, using the Chi-square and Fisher exact tests, as suitable. The peri-operative haemodynamic parameters were assessed by multiple t-tests and estimated with a two-stage linear step-up procedure of Benjamini, Krieger, and Yekutieli, with Q = 1%. Statistical comparisons were made, using SPSS for Windows version 18.0 (SPSS Inc., Chicago, IL, USA) and GraphPad Prism 8 for windows (GraphPad Software, San Diego, California, USA). Statistical signi cance was de ned as p-values of < 0.05.
Serial changes of SBP and HR values over time after spinal anesthesia administration are presented in Fig. 4. The SBP values at 4, 6, 8, and 10 min (P = 0.004, < 0.001, < 0.001, < 0.001, respectively) were higher in the RS group than in the R group. Although HR values at 2 and 4 min (P = 0.035 and 0.028, respectively) were higher in the R group than in the RS group, two-stage linear step-up procedures revealed no between-group difference in HR values.

Discussion
In this study, the ED90 of spinal ropivacaine administered with S-ketamine was 11.8 mg (95% CI: 11.7 to 12.7); the corresponding value without S-ketamine was 14. used 21 perfusion index of peripheral vessels to predict the risk of hypotension to enable early and individualised interventions. We believe that the fundamental approach to reducing the incidence of postspinal anesthesia hypotension is to control the upper anesthesia level before delivery, while lowering intrathecal drug use and ensuring satisfactory anaesthetic effect. The present ndings suggest that an ultra-low single IV bolus dose of S-ketamine may meet these criteria.
S-ketamine, the pure dextrorotatory enantiomer of ketamine, is used in clinical analgesia and anesthesia.
The main effects of S-ketamine are mediated by non-competitive inhibition of the N-methyl-D-aspartate receptor; the compound also interacts with opioid receptors, monoamine receptors, and adenosine receptors. 22 It is associated with dissociation (ability to communicate with the patient is retained to a certain degree), quick onset, absence of breathing function depression, and circulatory stability in shock patients, making it popular among emergency and intensive care medicine practitioners. 22 In addition, it is relatively safe for infants and children, 13 making S-ketamine a suitable candidate for an adjuvant IV drug during cesarean delivery with spinal anesthesia. The present study ndings suggest that an ultra-low single IV bolus dose of S-ketamine may induce sleep, decrease visceral traction pain from surgical interventions, and eliminate tension in patients that undergo cesarean delivery without increasing the risk of adverse events.
Furthermore, it has been demonstrated that a low infusion rate of S-ketamine may reduce opioid consumption and the extent of postpartum delirium. 23,24 In this study, the total dose of S-ketamine was even lower than that previously examined, 23,24 while the post-operative analgesic effect of S-ketamine and its interactions with opioids were maintained; no case of postpartum delirium or agitation was observed in the rst 48 h. However, the rates of dreaming and dizziness were higher among the patients that received S-ketamine than among those who did not. Most dreams were associated with high-speed movements; patients experienced the feeling of being shuttled in space. Despite short duration of these side effects and their perceived acceptability among the affected patients, we recommend that patients offered this treatment are carefully briefed ahead of surgery, so they can make an informed decision.
The faster recovery of maternal lower limb mobility in the S-ketamine group was an unexpected nding, likely indicating the contribution of the lower spinal dose of ropivacaine. Satisfactory analgesic effect and faster recovery of lower limb mobility may be direct or indirect results of the ultra-low dose of Sketamine, corresponding to enhanced recovery after surgery.
Anti-depressive effects of ketamine and S-ketamine have been reported in both human and animal studies. [25][26][27] However, studies on the impact of S-ketamine on postpartum depression are rare. Xu et al. 28 demonstrated that an ultra-low single bolus dose of ketamine does not prevent postpartum depression.
Brexanolone infusion is currently the standard treatment for post-partum depression. 29 In this study, an ultra-low single IV bolus of S-ketamine was used after spinal anesthesia; the EPDS was administered at admission and discharge. The EPDS scores were similar in both groups at admission; however, at discharge, these scores were lower in the patients that did than in those that did not receive S-ketamine, suggesting S-ketamine's anti-depressive effects in the postpartum population. However, satisfactory postoperative analgesic effect may have contributed to the lower EDPS scores; further studies are required to validate the anti-depressive effect of S-ketamine in this population.
In the present study, the bolus dose of S-ketamine was 0.15 mg.kg − 1 . Sen et al. 30  or full stomach were included in this study; thus, neither re ux nor aspiration were observed in any of the study patients. Although the coughing, gag, and swallowing re exes remained functional after the administration of S-ketamine, 22 the use of S-ketamine in patients with full stomach or gastroesophageal re ux disease is unsuitable. Finally, the rst contact between mother and neonate is important, promoting parent-child bonding and mother's willingness to breast-feed. Although some patients remained drowsy 5 min after delivery, all patients with IV S-ketamine in the present study could be awakened at that time; all patients communicated with the neonates and were clear-headed at the end of surgery.
This study has some limitations. First, in this study, the patients were blinded to group assignment, which was known to the participating anaesthesiologists that could observe the hypnotic effect of S-ketamine. However, the primary outcomes included the optimal spinal dose of ropivacaine and incidence of hypotension after spinal anesthesia in both groups. The study was thus double-blinded. The incidence of hypotension could be observed objectively. Second, the anti-depressive effect of S-ketamine postpartum was evaluated by the EPDS after post-operative days 4-5, by which point most patients had not recovered either physically or mentally from the impact of cesarean section. Future studies should assess the degree of postpartum depression after ≥ 6 weeks in this population to comprehensively evaluate the anti-depressive effects of S-ketamine postpartum. Third, the low incidence of maternal hypotension and its related side effects was likely due to the decreased dose of spinal ropivacaine, associated with the use of S-ketamine. Although the effect of stimulating the sympathetic nervous system by S-ketamine has been shown in several previous studies, herein, we could not distinguish between the effect of spinal ropivacaine dose reduction and that of S-ketamine use. Establishing whether an ultra-low bolus dose of S-ketamine increases maternal BP and HR after spinal anesthesia requires further research. Finally, the study included limited parturients, thus the safety and side effects of IV S-ketamine for parturients underwent caesarean delivery need further large sample observation.

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The ED90 of spinal ropivacaine was 11.8 mg (95% CI: 11.7 to 12. Declarations Ethical approval and consent to participate: The trial was conducted in accordance with the Declaration of Helsinki (as revised in 2013). The study was approved by China Ethics Committee of Registering Clinical Trials, West Hospital, Sichuan University, Sichuan, China on 29 September 2020 (Chairperson Taixiang Wu, Ethical No.: ChiECRCT20200301) and all patients signed consent to participate before allocated to the study. The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Consent for publication:
Not applicable.
Availability of data and materials: All data generated or analysed during this study are included in this published article [and its supplementary information les].

Competing interests
The authors declare no con ict of interest.
Funding: Figure 1 Flow chart of the study.

Figure 2
The patient allocation sequence and the response to the assigned dose of spinal ropivacaine in RS group.
The patient sequence number (X-axis) is the order of patient exposures using the BCUD design. The assigned dose levels are presented on Y-axis. An effective dose is denoted by a solid circle, while an ineffective one is denoted by a hollow circle. BCUD, biased coin up and down.