The purpose of this study is to observe the appropriate indwelling depth of epidural catheter when labor analgesia is performed in the mode of DPE combined with PIEB, and then shorten the onset time of labor analgesia and reduce the occurrence of catheter-related adverse outcomes.

In clinical practice, neuraxial analgesia is the most effective method for labor analgesia, including combined spinal-epidural (CSE) analgesia, dural puncture epidural (DPE) analgesia, and conventional epidural (EP) analgesia. Among them, DPE technology is an improvement of CSE technology. It means using a spinal needle to puncture the dura mater but not injecting drugs into the intrathecal space. Then, the epidural catheter is inserted into the epidural space. The punctured dura mater creates a new pathway for drug displacement from the epidural space to the subarachnoid space. In theory, compared with EP, DPE technology makes drugs enter the subarachnoid space more easily and can achieve a faster onset time of analgesia. At the same time, compared with CSE, the amount of drugs entering the subarachnoid space is not excessive, which can reduce the occurrence of adverse reactions such as itching and hypotension. At present, although there is controversy over the clinical advantages of DPE, its effectiveness has been confirmed in the article published by Gunaydin B et al. in 2019. In a series of RCT trials conducted by Professor Xu Zhendong's team in China, it is believed that the effect of DPE is related to the drug injection mode in the epidural space, the thickness of the puncture needle, and the distance between the drug outlet end and the puncture hole. At the same time, DPE technology combined with programmed intermittent epidural bolus (PIEB) mode seems to be a better choice in labor analgesia. Because the single drug bolus dose generated under the PIEB mode creates greater pressure in the epidural space, which is more conducive to drugs entering the subarachnoid space through the puncture hole. Based on the above research, when performing labor analgesia in the DPE combined with PIEB mode, we believe that the closer the opening of the epidural catheter is to the puncture hole, the easier it is for drugs to generate local pressure around the puncture hole and thus enter the subarachnoid space. This is related to the indwelling depth of the epidural catheter. The shorter the indwelling depth, the more drugs may enter the puncture hole, but it will also increase the probability of catheter detachment. The longer the indwelling depth, the farther the local pressure generated is from the puncture hole, and the advantage of the PIEB mode may be masked. At the same time, an excessively long indwelling depth will increase the incidence of unilateral block.

Inclusion Criteria:

1. Those aged 18 to 45 years old;
2. Pregnancy lasts for 37 to 42 weeks;
3. Single healthy pregnant women with cervical dilation < 3.0 cm who wish to use epidural labor analgesia;
4. The NRS score of uterine contraction pain is > 3;
5. ASA grade I-II;
6. BMI ≤ 40 kg/m².

Exclusion Criteria:

1. Those with contraindications to intraspinal anesthesia;
2. Those with a history of allergy to local anesthetics and opioids;
3. Pregnancy diseases (such as pregnancy-induced hypertension, preeclampsia or gestational diabetes, etc.);
4. Known fetal abnormalities or situations associated with an increased risk of cesarean section;
5. Patients who have taken opioids or sedatives within 4 hours before epidural analgesia.