Evaluation of risk factors

The overall failure rate (defined as NRS>3) found in this study was 26%. This failure rate is similar to previous data reported by Michael et al. for 23% and Ghislaine et al. for 20%^{25, 42}. Using a multivariate analysis, we are able to determine factors which are associated with ineffectiveness of epidural painless labor. Three significant factors

are identified.

1. Cervical dilatation velocity

Labor pain during first stage results from stimuli arising from mechanical distension of the lower uterine and cervical dilatation⁴³. The increasing intensity of pain accompany with progression of cervical dilatation^{2, 44}. The mechanism may be a lower activation threshold in the mechanoreceptors, and the chemoreceptor stimulation produced by the repeated stimuli of uterine contractions³. As expected, the faster cervical dilatation, the more painful sensation was detected. Timing of epidural catheter insertion is no different in two groups. The result advocates that pain is subjective and depends on the person’s past experience of pain. Every parturients thought their pain is the “most pain imaginable” when ask for epidural painless.

2. Loading drugs and continuing drugs

In previous study about the risk factors related to ineffectiveness painless labor, the regimen of loading and continue drugs were the same in all cases and were not been analysis^{9, 25}. In our study, different regimens with fixed concentration were used.

Ropivacaine is an amino amide local anesthetic that is structurally similar to bupivacaine. In comparison with Bupivacaine, Ropivacaine is equally effective for epidural block for surgery, obstetric procedures and postoperative analgesia⁴⁵. There are no significant differences in pain VAS between 0.1% Ropivacaine, and 0.125%

Bupivacaine given for labor epidural analgesia.⁴⁶ Recommended epidural doses of Ropivacaine for postoperative or labor pain are 20-40 mg as bolus with an interval of

>or=30 minutes⁴⁷. But there are different results in our study. Our study showed Bupivacaine is associated with ineffectiveness but not Ropivacaine in both loading and continue use. It’s difficult to explain. The possible reason is the correlation between drug and volume or performer is not investigated. Interaction between drugs and volume, drugs and performers may exist and not analyzed in our study.

Factors associated with ineffectiveness of epidural painless labor in univariate analysis but not significant in multivariate analyses are described as follows.

1. Loading with Fentanyl

Loading without Fentanyl is associated with ineffectiveness of epidural painless labor in our univariate analysis. The use of epidural administered opioids to control postoperative pain is a well established and widely accepted technique⁴⁸. And so did in labor pain. Lee et al. reported that epidural infusion of 0.1% Ropivacaine alone provided adequate analgesia in the first stage of labor, and that of additionally 2μg/mL Fentanyl improved analgesia to a quality similar to 0.2% Ropivacaine alone. But the VAS was higher in parturients with 0.1% Ropivacaine⁴⁹.

2. Duration of phase I

Our result concluded that parturients experience poor pain relief had shorter duration of phase I. This result is compatible with that faster cervical dilatation velocity is a factor associated with ineffectiveness epidural analgesia. In spite of Le Coq et al.

demonstrated that epidural duration shorter than one hour or longer than six hours are both risk factors to inadequate painrelief²⁵. Our study showed different results. Although the result was not significant different in multivariate analysis.

Other factors related to inadequate pain relieve found in previous studies but not significant in our study including follows.

1. Nulliparity

Nulliparous women experience greater pain than multiparous women in early labor but the difference is lesser as labor progression⁶. Hess et al. concluded that nulliparity was independently associated with recurrent breakthrough pain during labor epidural analgesia⁹. But Le Coq et al. reported no differences between nulliparities and multiparities in inadequate pain relief using epidural. Our results support the latter one.

And this issue needs to be further investigated.

2. Fetal weight

Hess et al. concluded that heavier fetal weight was independently associated with recurrent breakthrough pain during labor epidural analgesia⁹. But others had the

opposite opinion that there was no relation between birth weight and pain scale in natural vaginal delivery^{21, 25, 50}. Our results agree with that fetal weight is not related to ineffectiveness of epidural painless labor. The possible explanation is that our study focused on the analgesia effect during the first phase of labor. The pain during first stage of labor mainly arises from uterus contracture and cervical dilatation. And bigger fetus may cause more traction on pelvic structure and perineum which mainly occurs on late first stage and second stage. More evidence is needed to approve it.

3. Fetal occipital posterior presentation

The incidence of fetus occipital posterior (OP) presentation is ranging from 4.6%

and 5.5% by Yancey and Sizer, to 6% by Ponkey⁵¹. OP presentation are definitely associated with a marked increase in the risk of Caesarean section delivery, To W. et al.

reported the odds ratio for the OP group was 30.2 (95% CI 25.6-35.5) for Caesarean section then occipital anterior group⁴¹. Sizer et al. reported a higher incidence of emergency Cesarean deliveries in OP compared with occipital anterior labors (41.7%

versus 13.7%, p<0.001)²⁴. It means many fetuses with OP presentation were delivered by Cesarean section and the presentations didn’t be recorded in our study. So the incidence of OP presentation in may be lower estimated. The standard diagnosis tool of fetal presentation is ultrasound. In our study there were no routine examinations of fetal presentation when parturients in labor. The presentation was recorded in the moment of

delivery and may underestimate the incidence of OP presentation too.

The incidence of OP presentation in our study is 2.6% in training group. There were no different between failure group and successful group. Due to the low incidence of OP presentation, we are in doubt on this conclusion and more data should be collected to confirm it.

4. The epidural catheter implantation in early cervical dilatation

Most study concerned about the effects of early epidural analgesia on Cesarean or instrumental delivery in parturients. Massimo et al.’s systematic review concluded that cervical dilatation is not a reliable means of determining when epidural analgesia should be initiated⁵³. Few concerned about early epidural analgesia and effectiveness of pain relief. Hess et al. found earlier cervical dilatation was positive related to breakthrough pain during epidural painless labor. But Le Coq et al. had the opposite finding. Our result showed no difference between two groups in timing of epidural initiation. More studies will be needed in this issue.