Principal objective
Predictors of failed mid-cavity VAD were assessed by calculating odds ratios (ORs).
In the multivariate analysis, seven variables were shown to be statistically significant independent risk factors for failed mid-cavity VAD (Table 2).
ROC curves for the multivariate risk prediction were calculated. The performance of the prediction model for the training set was compared to that of its ability to predict failed mid-cavity VAD for the test set.
The model was reliable (Hosmer-Lemeshow test = 8.5; p = 0.39) and accurate (concordance index = 0.74).
The equation corresponding to this model is as follows:
Odds =
exp [-2.5651
+ (0.5198 if caput succedaneum = medium-sized or 1.4332 if caput succedaneum = bulky)
+ (0.2380 if duration of expulsive efforts = 11 to 20 or -0.6549 if duration of expulsive efforts = 21 to 30 or 0.2611 if duration of expulsive efforts ≥31)
+ (0.4668 if head-perineum distance = 51 to 60 or 1.8968 if head-perineum distance ≥ 61)
+ (1.0722 if maternal height < 1.55 or -0.1937 if maternal height = 1.55 to 1.64 or -0.4910 if maternal height ≥1.75)
+ (0.5438 if duration from five centimetres to complete cervical dilation ≥ 3)
+ (0.6719 if term ≥ 42)
+ (0.3938 if fetal head position = occiput-posterior or occiput-transverse)].
The odds for the predicted probability of mid-cavity VAD failure were calculated as follows: probability = odds/(1 -odds).
Using the Youden index, the threshold to predict a failed mid-cavity VAD was set at 0.163. For internal validation, a resampling of 5000 datasets was performed. The mean concordance index was 0.75 (95 % confidence interval = 0.71 - 0.79), the mean specificity was 67.4 (95 % confidence interval = 64.3 – 70.6), and the mean sensitivity was 64.4 (95 % confidence interval = 57.5 – 71.1).