Correct epidural catheter (EC) placement is essential for effective epidural analgesia (EA) during labour, yet confirmation remains indirect. B-mode ultrasound facilitates EA placement but does not reliably visualise the EC. Alternative modalities such as M-mode and colour Doppler (cD) ultrasonography may improve EC localisation, though evidence in obstetrics remains limited. This study evaluated the feasibility of detecting the EC within the epidural space (ES) using M-mode, with secondary objectives of comparing the diagnostic performance of both modes.

We conducted a prospective single-centre observational study including 100 parturients requesting EA. After EC insertion, parasagittal oblique and transverse ultrasound views were obtained. For EC location, M-mode detected a granular pattern at the catheter depth in the ES after saline injection, while cD detected saline flow as colour aliasing. EA effectiveness was assessed 20 min post-insertion.

M-mode localised the EC in 48% of cases, all with effective EA (sensitivity 49%, specificity 100%, positive predictive value (PPV) 100%, negative predictive value (NPV) 4%). Using cD, the EC was visualised in 76% of cases (sensitivity 78%, specificity and PPV 100%, NPV 8%) with faster detection ( p  = 0.001). Considering either modality increased sensitivity to 87%, with specificity and PPV 100% and NPV 13%. Among 15 cases undetected by either mode, EA failed in two, requiring catheter repositioning.

cD is more sensitive and faster than M-mode for EC localisation in obstetrics. Neither technique alone suffices for systematic screening, but a combined approach markedly improves detection. Absence of EC visualisation could not be interpreted as incorrect placement.