Anesthetic in this block can be given as a single dose, can be partnered with an epidural catheter as combined spinalepidural analgesia, or can be administered as a continuous infusion. Injection of a local anesthetic into the subarachnoid space to efect analgesia has long been used for delivery. Advantages include rapid analgesia onset, short duration of action, and high success rate. he subarachnoid space during pregnancy is smaller, which likely results from internal vertebral venous plexus engorgement. Thus, in parturients, the same amount of anesthetic agent in the same volume of solution produces a much higher blockade than in nonpregnant women.
The irst stage of labor requires a sensory block to the level of the umbilicus (Tlo)' During the second stage of labor and for operative vaginal delivery, a sensory block of 52 through S4 is usually adequate to cover pain from perineal stretching and/ or instrumentation. Analgesic options include continuous lumbar epidural analgesia, combined spinal-epidural, continuous spinal analgesia, and other blocks such as pudendal and paracervical blocks.
Local anesthetic agents are usually given to establish a sensory block to the desired dermatome level. They are almost exclusively used in conjunction with neuraxial opioids. he mechanism of action is a function of the administration route and lipid solubility. Analgesia is induced by absorption into the vascular system (supraspinal), actions on the dorsal horns, and direct spread in the cerebrospinal fluid to the brainstem. Highly-soluble lipid opioids such as fentanyl and sufentanil have a rapid onset of action. But, because they are absorbed into lipid membranes and the epidural vasculature, their duration of action is short. Hydrophilic solutions such as morphine, on the other hand, provide extended analgesia (Lavoie, 2013). he major advantages of using such a combination are the rapid onset of pain relie, a decrease in shivering, and less dense motor blockade. Side efects are common and include pruritus and urinary retention. Nalbuphine, 2.5 to 5 mg intravenously, can be used to treat pruritis without diminishing the analgesic efect.
A level of sensory blockade extending to the T 4 dermatome is desired for cesarean delivery. Depending on maternal size, 10 to 12 mg of bupivacaine in a hyperbaric solution or 50 to 75 mg of lidocaine hyperbaric solution is administered. The addition of opioid increases the rapidity of blockade onset, reduces shivering, and minimizes referred pain and other symptoms such as nausea and vomiting. The addition of a preservative-free morphine (Duramorph or Astramorph), 0.1 to 0.3 mg intrathecal or 2 to 4 mg epidural, provides pain control up to 24 hours postoperatively.
Hypotension. Shown in Table 25-5 are some of the more common adverse events associated with neuraxial analgesia. Importantly, obese women have signiicantly impaired ventilation, and thus close clinical monitoring is imperative (Vricella, 2011).
TABLE 25-5. Complications of Regional Analgesia
Uncommon Inadvertant intrathecal, subdural, or intravascular injection of local anesthetic drugs Neurologic injury
Hypotension is a common complication that may develop soon after injection of the local anesthetic agent. It is the consequence of vasodilatation from sympathetic blockade and is compounded by obstructed venous return due to uter ine compression of the great vessels. In the supine position, even in the absence of maternal hypotension measured in the brachial artery, placental blood flow may still be significantly reduced. Treatment includes uterine displacement by left lat eral patient positioning, intravenous crystalloid hydration, and intravenous bolus injections of ephedrine or phenyleph rine.