In this randomized controlled study, we found that the first puncture success rate in the paramedian approach group was 92%, which was significantly higher than the 86% in the median approach group (P < 0.05). In the paramedian approach, the superior and interspinous ligaments were avoided so that the epidural space was entered directly from the ligamentum flavum. There were fewer ligaments and other anatomical structures; therefore, the first puncture success rate was higher in the paramedian approach group than in the median approach group. In addition, based on the analysis of the anatomical structure of the spine, the paramedian approach is not limited by the inclination of the spinous process and the bone structure. When entering the epidural space, the end of the puncture needle is more inclined to form an angle on the side of the head, the cerebrospinal fluid returns smoothly after the insertion of the lumbar anesthesia needle, and it is easier to place the epidural catheter after the completion of the lumbar anesthesia. Some studies have confirmed that the puncture interspace of the paramedian approach is wider than that of median approach, which reduces the difficulty of puncture, avoids repeated puncture and increases the success rate of puncture.
At the same time, the controversy over the use of ultrasound to assess the distance between the skin and the epidural space should be considered. In theory, the actual puncture depth is deeper than that predicted by ultrasound. Our study found that the actual depth of extradural puncture was deeper than that predicted by ultrasound, but there was no significant difference between groups (P < 0.05). The reason may be that when the operator evaluates the success of epidural puncture, the needle insertion is stopped immediately when negative pressure is felt during the puncture to avoid placing the puncture needle too deeply, especially for anesthesiologists with more puncture experience. There was no significant difference between the predicted puncture depth and the actual puncture depth in the median approach group, which suggested that ultrasound could effectively predict the epidural puncture depth in different approaches of combined spinal epidural anesthesia. However, it should be pointed out that the actual puncture depth of both groups of data was deeper than that predicted by ultrasound. Considering the possible influence of fat thickness or tissue edema on the backs of obese pregnant women, our study compressed the maternal skin to avoid its influence when ultrasound placed the puncture point.
In this study, a high-frequency convex array probe was used to accurately determine the best puncture point of anesthesia in the median approach group. The selection of puncture point in the paramedian approach group was based on the lateral paracentesis of 1.5 cm at the median approach puncture point, and on this basis, ultrasound was used to predict the puncture depth. Therefore, it was found that the positioning time of the paramedian group was higher than that of the median group (P < 0.05). However, there was no significant difference in the total operation time between the two groups (P > 0.05). Considering that the first puncture success rate of the two groups was high, the number of attempts was small, and the skilled operation of anesthesiologists had a certain relationship.
Anesthesia safety is also an important factor in this study. Among the one hundred obese pregnant women included in this study, there were adverse effects of anesthesia in both groups, such as epidural catheter bleeding, nerve stimulation signs and the occurrence of low back pain after anesthesia, and there were significant differences between the two groups in the occurrence of low back pain (P < 0.05). We considered that the paramedian approach group could avoid the supraspinous ligament and part of the interspinous ligament and allow entrance into the epidural space through the ligamentum flavum in the process of puncture. The analysis shows that the main reason for the difference in the incidence of low back pain between the two groups is the difference in ligament injury caused by the dural puncture needle. There was no significant difference in nerve stimulation between the two groups. It is worth mentioning that there were no cases of unexpected dural puncture in the either of the groups, further suggesting the advantage of ultrasound in obese women. There was a higher degree of satisfaction in the paramedian approach group than in the median approach group (P < 0.05).
Previous studies have found that there is no significant difference in the success rate of traditional intraspinal puncture when ultrasound is used and when it is not in normal pregnant patient, which does not reflect the application advantages of ultrasound in epidural puncture. Therefore, this study is more valuable for the application of ultrasound-guided epidural puncture in obese cesarean section women. In addition, it should be pointed out that the technology of ultrasound intervention in epidural puncture can be divided into prepuncture positioning and real-time guiding operation, but the real-time guiding requires aseptic treatment of the probe, the operation process is complex, and the advantage is not obvious compared with the prepuncture ultrasound positioning; additionally, in the operation process, elimination of air/liquid resistance is still used to determine whether the tip of the needle reaches the epidural cavity. Therefore, in this study, we chose to place the epidural catheter before ultrasound-guided puncture rather than under real-time ultrasound-guided puncture. Of course, there are some limitations in our research. First, although we used the same anesthesiologist with much experience in using spinal ultrasound, the results are still controversial. Second, we had a relatively small number of cases and fewer positive results. More samples are needed for further study.