The present study demonstrated that the table height (at the umbilicus, lowest rib margin, and xiphoid process level of the residents administering CSEA) did not influence the success rates of CESA and epidural space puncture. The table height also did not influence the incidence of complications of CESA, including paresthesia and blood intravascular cannulation. Paresthesia was more likely to occur on the left side than the right side when the catheter was inserted into the epidural space. The table height at which the needle insertion point was level with the operator’s lowest rib margin was preferred by most residents; this height was also best during training to administer epidural space puncture.
In our study, the success rate of epidural space puncture administered by residents was 68.3%, consist with Drake reported a success rate of 63%-90% 4. In our study, the success rate of CSEA administered by residents was 53.4%, and the success rate of dural puncture after successfull epidural space puncture was 78.2%. One reason for failure of CSEA is that the epidural needle may be angled away from the midline, and so the spinal needle passes to the side of the dural sac 5. A smaller height difference between the puncture points and the anesthesiologist’s eyes may lead to better visualization, as the line from the anesthesiologist’s eyes to the puncture site is more horizontal. Therefore, as the angle formed between the epidural needle and the skin on the patient’s back in the coronal plane is reportedly greater in groups U and R than in group X 3, this may increase the risk of the epidural needle being angled away from the midline, causing the spinal needle to pass to the side of the dural sac. However, there was no significant difference in the success rate of CSEA between groups U, R, and X in our study. This may be because the difference between the angles formed between the epidural needle and the skin in the coronal plane in groups U, R, and X was too small to influence whether the spinal needle punctured the dural sac. Another potential reason for this lack of significance is that the residents were allowed to regulate the direction of the spinal needles by adjusting the angle between the epidural needle and the patient’s back. However, the incidence of directional change did not significantly differ between the three groups, and the success rates of dural puncture did not significantly differ in accordance with the angle between the epidural needle and the patient’s back.
Paresthesia was more likely to appear on the left side than the right side when the catheter was inserted into the epidural space, but there was no difference between sides in the incidence of paresthesia during the advancement of the spinal needle. This may be because the residents were more likely to insert the epidural needle near the left side of the patient, and so the nerve roots on the left side of the epidural space were more likely to be irritated and cause paresthesia when the catheter was inserted. The catheters used in our study were stiff, which may have induced greater incidence of paresthesia and intravascular cannulation than wire-embedded catheters 7, 8. A previous study reported that the angle formed between the patient’s back skin and the needle in the coronal plane was greater than 90° in groups U and R 3; this might increase the risk of the spinal needle being inserted closer to the right side compared with the epidural needle. Therefore, there was no difference in the incidence of paresthesia on the left versus the right side during the process of spinal needle advancement. (Figure 3)
There was no relationship between the success rates of CESA and the residents’ age, height, BMI, sex, experience level, and myopia. Similarly, these factors had no influence on the incidence of paresthesia. This suggests that the demographic characteristics of the residents did not influence their ability to administer CSEA during training; the effectiveness of their training was only influenced by their degree of comfort during the process of performing epidural anesthesia. In contrast, a previous study showed that the resident’s experience level significantly affected the incidence of paresthesia during the advancement of a thoracic epidural catheter7. However, this previous study divided the residents into those with less than 4 years of experience and those with more than 4 years of experience, while all of the residents in our study had less than 3 years of experience.
The table height most favored by residents was that at which the puncture point was at the same height as their lower rib margin, as this height was considered more comfortable and thus favorable for the performance of successful CESA. A working surface height that ranges from 10 cm below to 5 cm above the elbow is optimal for standing workers 9, 10. The table height at which the puncture point was level with the residents’ lower rib margins were within this range. At this table height, the residents’ upper arms could rest on their chest walls, and the forearms were more stable and less tired when performing CSEA. A previous study reported that the anesthesiologists preferred higher table heights than the residents 3. Compared with the anesthesiologists, the residents were more vulnerable to upper arm tiredness because of the longer duration of the procedure 3.
The present study had some limitations. First, the number of CSEA procedures performed varied between residents. This was because each resident was assigned to a different surgery room, and the number of CSEA procedures performed by each resident was dependent on the surgeries scheduled on that day. At least 20–25 epidural blocks are reportedly necessary to achieve consistency during residency training in anesthesiology 11. However, although each resident in our study administered epidural anesthesia less than 20 times during training, we focused on the training process rather than the training results. Second, the present study did not evaluate the administration of CSEA while the patients were in a sitting position. Finally, we did not evaluate the administration of CSEA while the operating table height was above the xiphoid process of the residents. This was because some patients were afraid on operating tables with a height elevated for tall residents.