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.