Predictive value of traction force measurement in vacuum extraction: Development of a multivariate prognostic model

Objective
To enable early prediction of strong traction force vacuum extraction.
Design
Observational cohort.
Setting
Karolinska University Hospital delivery ward, tertiary unit.
Population and sample size
Term mid and low metal cup vacuum extraction deliveries June 2012—February 2015, n = 277.
Methods
Traction forces during vacuum extraction were collected prospectively using an intelligent handle. Levels of traction force were analysed pairwise by subjective category strong versus non-strong extraction, in order to define an objective predictive value for strong extraction.
Statistical analysis
A logistic regression model based on the shrinkage and selection method lasso was used to identify the predictive capacity of the different traction force variables.
Predictors
Total (time force integral, Newton minutes) and peak traction (Newton) force in the first to third pull; difference in traction force between the second and first pull, as well as the third and first pull respectively. Accumulated traction force at the second and third pull.
Outcome
Subjectively categorized extraction as strong versus non-strong.
Results
The prevalence of strong extraction was 26%. Prediction including the first and second pull: AUC 0,85 (CI 0,80–0,90); specificity 0,76; sensitivity 0,87; PPV 0,56; NPV 0,94. Prediction including the first to third pull: AUC 0,86 (CI 0,80–0,91); specificity 0,87; sensitivity 0,70; PPV 0,65; NPV 0,89.
Conclusion
Traction force measurement during vacuum extraction can help exclude strong category extraction from the second pull. From the third pull, two-thirds of strong extractions can be predicted.