HEAD-TO-CERVIX FORCE - AN IMPORTANT PHYSIOLOGICAL VARIABLE IN LABOR .1. THE TEMPORAL RELATION BETWEEN HEAD-TO-CERVIX FORCE AND INTRAUTERINEPRESSURE DURING LABOR

Objective To investigate the relation between the rise in intrauterine pressure and rise in fetal head to cervix force in normal, slow and i nduced labour. Design Prospective observational study. Setting The lab our ward of a London teaching hospital. Participants Forty patients we re recruited from the antenatal clinic and labour ward of a West Londo n Hospital. Five had normal onset and progression of labour, 14 had sl ow progression of labour and 21 had induced onset of labour. Method In trauterine pressure and head-to-cervix force was measured simultaneous ly using an intrauterine pressure catheter and a specially designed fo ur sensor head-to-cervix force probe. Results For each contraction of each labour, scattergrams of force by pressure were plotted. Three pat terns were observed. When the rise in pressure preceded the rise in fo rce, a positive 'loop' was generated. When the rise in pressure and fo rce occurred simultaneously a linear pattern was generated (a neutral 'loop'). When the rise in pressure lags the rise in force, a negative 'loop' was generated. In normally progressive labour the distribution of loops was 29.1%, 22.6% and 48.3%, respectively, in slow labour the distribution was 2.61%, 14.1% and 59.8% and in induced labour the dist ribution was 33.8%, 14.4% and 51.8%. These distributions were not stat istically different. However, a higher proportion of negative loops wa s observed in labours augmented with oxytocin compared to those receiv ing no oxytocin (MW-U = 87, P = 0.036). No differences were observed c omparing parity, use of PGE(8), epidural analgesia, or mode of deliver y. Contraction frequency (number/10 minutes) was inversely correlated to the percentage of negative loops (r(s) = -0.34, P = 0.033) and posi tively correlated with percentage of positive loops (r(s) = 0.36, P = 0.027). Conclusions This is the first report of the temporal relation between intrauterine pressure and head-to-cervix force in labour. The most common pattern is that the rise in pressure lags the rise in forc e, suggesting that a seal has to be created between the fetal head and cervix before a rise in pressure can occur. When oxytocin is given in labour, a higher proportion of loops are negative indicating that the re is poor application of the fetal head and cervix in a greater propo rtion of contractions.