Efficacy of the Epidural Positioning Device (EPD) on Optimising the Acoustic Target Window for Neuraxial Needle Placement

Thirty term pregnant women will be recruited. Signed informed consent will be obtained from all subjects and demographic details, including age, weight, height and body mass index (BMI) will be documented. Randomization and blinding: Randomization will not be performed in this study, because each patient is their own control Interventions: Two experienced operators who have previously performed several hundred landmark-guided and ultrasound-assisted neuraxial blocks will perform the clinical and ultrasound examination.. The patient will then be placed in each of the positions described below. While in each of the positions an ultrasound (US) probe will be placed at level L3- L4 at the paramedian position to visualize the paravertebral longitudinal ligament (PLL) . These measurements will be recorded by an observer. In both phases (first US operator and second US operator ), 2 ultrasound measurements will be made for each of the following positions at 1 lumbar level in the paramedian view. These measurements will be made with the patient in the following positions: • Sitting position, feet supported with back flexed. • Sitting position, feet supported and arms supported through the application of an epidural positioning device. All images will be assigned a number based on a random number generator to remove any identifying data, including volunteer number, date, time, and position. The PLL will be remeasured on every saved image by two separate anesthesiologists ('readers') who are not involved with any previous data collection but are members of the study team. Both will be blinded to the number allocation and each other's measured values. This aim to remove any bias by the 'scanning' anesthesiologists and test for reproducibility in the measurement of the PLL, between 'readers'. A statistician calculated a minimum sample size of 30 subjects, in order to demonstrate significant change in the mean length of the PLL (acoustic target area), in any of the two positions. This estimation was based on data derived from a similar study, which had used a minimum sample size of 16 subjects, with a 1 mm mean change in 'needle target area' dimensions, a 1 mm standard deviation (SD), and an alpha of 0.05 with a power of 90 % [1, 2].