The pain pathways involved in the process of labour and delivery have been well established. Nociceptive stimuli during first stage of labour are transmitted via the T10 to L1 posterior nerve root ganglia, while the nociceptive stimuli during second stage of labour are transmitted via the L1 and S2 to S4 (pudendal nerve) posterior nerve root ganglia. Although these pain pathways are well established, there is no information in the literature as to what level or depth of sensory block, assessed by current clinical practices, is required for effective labour analgesia. This information is critical for planning and safety of epidural analgesia during labour.~The investigators hypothesize that the LSBL to either cold or pinprick would be below dermatome T10 in patients receiving labour epidural analgesia during first stage of labour and who are experiencing pain. The investigators also hypothesize that upon receiving a top-up and re-establishment of effective analgesia, (a) the LSBL would be at or above dermatome T10 or (b) USBL would increase to T10 or above T10 if USBL was below T10 before receiving a top-up.
Epidural analgesia is the gold standard for pain control during labour and an essential component of delivering effective and safe epidural analgesia is the assessment of the epidural sensory block. There is significant literature on the assessment of sensory block during spinal anesthesia for cesarean section but limited studies exploring the assessment of sensory blockade in labour analgesia.~Prior studies have documented two thresholds of sensory block to both ice and pinprick: one defined as the lower sensory block level (LSBL) where the patient is able to notice a cold or sharp sensation but perceives that is not as sharp or cold as a control area and the other the upper sensory block level (USBL) where the patient perceives the cold or sharp sensation is at the same temperature or sharpness as the control area.~The goal of this study is to contribute to the standardization of the assessment of sensory block levels during labour epidural analgesia by studying patients with labour epidurals who are experiencing pain and determining the LSBL and USBL and how these change as the patients become comfortable following the administration of manual epidural top ups.