Determinants of Successful Induction
Being parous, having favorable Bishops score at initiation of oxytocin and performing ARM were significantly associated with increased success of induction by 2 times, 4 times and 8 times compared to nulliparous, unfavorable Bishop score and not performing ARM respectively. These results were in line with other study reports from Ethiopia [13-16]. This is because it is a well-established science that being parous, favorable cervical status and elective amniotomy or ARM were good predictors of successful induction of labor. Performing ARM strengthens the cascade of uterine contractions thus hastens labor and increase successful vaginal delivery. It was found that nulliparity had increased risk of failed induction by 1.5-3 times in other studies as well [13-14, 17-18].
Similarly, delivering to normal birth weight neonate compared to macrosomic neonate has increased success by 4 times. This might be justified by the fact that macrosomia is associated with labor dystocia and cephalo-pelvic disproportion thus ending in cesarean delivery. Our finding however, was not consistent with different literatures of the similar settings in Ethiopia that showed no association between neonatal birth weight outcome and induction success [13-14, 16].
However, induction on elective basis compared to induction on emergency basis has reduced the induction success by 80%. This doesn’t show association with failed induction in study conducted by Woubishet et al [13]. We expect successful induction of labor with elective induction than emergency induction. Because with elective induction one can buy time to ripen cervix till it gets favorable before initiating oxytocin thus increasing the success rate. But the finding of our study was opposite to this logic. This might be explained by the fact that majority of study participants (82%) were induced on emergency basis. On other hand, of all remaining elective inductions, 68% were induced for post term pregnancy. Post term was associated with decreased induction success as seen in different literatures [14, 16].
Labor Outcomes
Success of induction was lower among HDG compared to LDG (61.1% vs. 72.2%) while rate of CS was higher among HDG compared to LDG (38.8% vs. 27.8%). These findings were consistent with one meta-analysis that showed higher CS rate among HDG [3] and one cohort study done at Inova Alexandria Hospital (28% vs. 27%) [5]. However, the finding of our study was in contrary to one Cochrane review (18.8 vs 19.8) [12], one double masked randomized oxytocin trial (11.3% vs. 15%) [9], and other two studies (9% vs. 12%) [19] & (10.4% vs. 25.8% ) [7] that showed higher CS rate among LDG. Although CS for failed induction occurred less frequently with the high-dose regimen (45.2% vs. 56.7%), CS for NRFHRP was performed more frequently (45.2% vs. 20%) compared to LDG.
In this study higher successful induction and lower CS rate among LDG were observed compared to HDG. We can raise many possible explanations why these occurred unlike other studies. Firstly, 60% of participants in LDG had favorable Bishop Score compared to HDG (only 13%) predicting higher successful induction and lower CS rate. Secondly, high dose oxytocin had statistically significant relation with NRFHRP in this study and mere occurrence of NRFHRP necessitating CS during labor might have reduced the possible number of successful vaginal deliveries if labor has to be continued. The fact that the number of mothers undergoing CS for NRFHRP among HDG was higher by 2.3 times than among the LDG (45% vs. 20%) may explain higher CS & lower successful induction observed among HDG.
Thirdly, although not statistically significant in this study, higher utilization of misoprostol for cervical priming among LDG (52.3% vs. 37%), presence of higher proportion of mothers with previous history of successful induction (10.2% vs. 4.6%) and significantly lower proportion of nulliparous women in LDG (29.6% vs. 51.9%) compared to HDG might have contributed to higher successful induction rate among LDG in our study. Because misoprostol use was standard of management as it increases success of induction. Lastly, the fact that centers with low oxytocin regimen use oral misoprostol for cervical priming before oxytocin induction in contrary to high dose center which initiate direct oxytocin induction for prolonged PROM, and PROM being major indication of induction (60%), might have contributed to higher induction success rate and thus lower CS rate among LDG .
Rate of failed induction was nearly the same among HDG (17.6%) and LDG (15.7%).This similarity in rate among the two groups was also seen in one cohort study comparing the two oxytocin regimen (4.3% & 5.1 %) [5] and in one other double masked randomized oxytocin trial (6.0% & 6.1%) [9]. However, rate of failed induction was generally higher in our study compared to those studies. This might be due to the fact that the studies were following different protocols in relation to total duration of hours waited to diagnose failed induction. In this study failure to acquire either adequate uterine contraction or failed to show favorable cervical changes despite being on oxytocin drip for a period of six to eight hours was used to diagnose failed induction. But other centers in literatures used to give more time ranging from 12 to 24 hours as latent phase can usually be prolonged but ended in vaginal delivery [1].
Mean Induction to Delivery Time & Mean Oxytocin to Vaginal Delivery
Mean “Induction to delivery” time for study participants were 5.9 hours and 6.3 hours for participants of HDG and LDG respectively while mean time elapsed from initiation of oxytocin to vaginal Delivery were 5.1 hours and 6 hours among HDG and LDG respectively. Mothers receiving high dose oxytocin regimen had slightly shorter duration of labor. This finding was similar to many literatures although majority of them showed significant shortening of induction to delivery time (2-3 hours) compared to our study [3, 5, 6, 9, 10, 19]. Similar effect was found when using oxytocin for augmentation [4]. This might be due to frequent escalation of oxytocin dose among HDG compared to LDG (every 20 minutes Vs 30 minutes) till getting adequate uterine contraction or favorable cervical outcome.
The limitation to this study was confounders like parity, Bishop score, cervical ripening agent & fetal weight were not controlled so as to look the real effect of oxytocin regimen on induction success.