LABOR PAIN: ANATOMIC CONSIDERATIONS
In the first stage of labor, pain is carried on visceral afferent type C fibers that travel with sympathetic nerve fibers to the cervical and uterine plexuses, then the hypogastric and aortic plexuses, before entering the spinal cord from T10 to L1. Pain originates from stretching of the lower uterine segment and dilation of the cervix. At this point in her labor, the mother will generally feel pain referred to her lower abdomen, but as labor progresses it may be referred to the lumbosacral area, gluteal region, and thighs. Later in the first and into the second stage of labor, pain is also transmitted by somatic nerves, namely the pudendal nerves, which enter the spinal cord from S2 to S4. Thus, adequate analgesia for the second stage of labor requires blockade of T10–S4. Pain at this point is the result of stretching the pelvic floor, vagina, and perineum. The pain of labor is usually described as moderate to severe, with the intensity of pain corresponding with increasing cervical dilation and more frequent and intense contractions.
Uterine enlargement and compression of the vena cava causes engorgement of epidural veins. This change increases the incidence of unintentional intravascular positioning of the epidural catheter in the pregnant patient. Along with higher intraabdominal pressure, epidural vein engorgement decreases the cerebrospinal fluid (CSF) volume in the thoracolumbar region of pregnant women, which partially explains decreased dosing requirements in intrathecal analgesia. As in nonpregnant adults, the spinal cord ends at L1–L2, and a horizontal line from iliac crest to iliac crest is approximately at the level of L4. As the introducer needle is advanced it will subsequently encounter skin, subcutaneous fat, the supraspinous ligament, interspinous ligament, the ligamentum flavum, beyond which lies the epidural space. If the introducer needle is accidentally advanced beyond this point, dural puncture will occur and the needle will enter the intrathecal space. CSF flow should be expected if this occurs, but does not always happen.
Regional techniques are employed more than any other means of providing analgesia in the obstetric population. It has the advantage of providing excellent pain relief with minimal depressant effects on the mother and fetus, allowing the mother to participate fully in what is usually one of the happiest experiences of her life. Most commonly used in the United States and other developed countries are central neuraxial blocks (epidural, spinal, and combined spinal/epidural), paracervical blocks, and pudendal blocks. Less frequently employed are lumbar sympathetic blocks. While these are generally safe procedures for a vast majority of parturients, absolute contraindications to regional techniques include infection at the site of needle/catheter insertion, coagulopathy or thrombocytopenia, the inability to cooperate during the procedure, patient refusal, allergy to the medications used, uncorrected hypovolemia, and unfamiliarity with the procedure.
Epidural analgesia is an excellent choice in regional ...