Regional Anesthetic Techniques: Spinal Opioids Alone

REGIONAL ANESTHETIC TECHNIQUES

Epidural or intrathecal techniques, alone or in com-bination, are currently the most popular methods of pain relief during labor and delivery. They can pro-vide excellent analgesia while allowing the mother to be awake and cooperative during labor. Although spinal opioids or local anesthetics alone can provide satisfactory analgesia, techniques that combine the two have proved to be the most satisfactory in mos parturients. Moreover, the synergy between opioids and local anesthetics decreases doserequirements and provides excellent analgesia with few maternal side effects and little or no neonatal depression.

Spinal Opioids Alone

Opioids may be given intrathecally as a single injec-tion or intermittently via an epidural or intrathe-cal catheter (Table 41–2). Relatively large doses are required for analgesia during labor when epidural or intrathecal opioids are used alone. For example, the ED₅₀ during labor is 124 mcg for epidural fentanyl and 21 mcg for epidural sufentanil. The higher doses may be associated with a high risk of side ef fects, most importantly respiratory depression. For that reason combinations of local anesthetics and opioids are most commonly used . Pure opioid techniques are most useful for high-risk patients who may not tolerate the functional sympathec-tomy associated with spinal or epidural anesthesia . This group includes patients with hypovolemia or significant cardiovascular disease such as moderate to severe aortic stenosis, tetralogy of Fallot, Eisenmenger’s syndrome, or pulmonary hypertension. With the exception of meperidine, which has local anesthetic properties, spinal opi-oids alone do not produce motor blockade or sym-pathectomy. Thus, they do not impair the ability of the parturient to “push.” Disadvantages include

less complete analgesia, lack of perineal relaxation, and side effects such as pruritus, nausea, vomiting, sedation, and respiratory depression. Side effects may be ameliorated with low doses of naloxone (0.1–0.2 mg/h intravenously).

Intrathecal Opioids

Intrathecal morphine in doses of 0.1–0.5 mg may produce satisfactory and prolonged (4–6 h) anal-gesia during the first stage of labor. Unfortunately, the onset of analgesia is slow (45–60 min), and these doses may not be sufficient in many patients. Higher doses are associated with a relatively high incidence of side effects. Morphine is therefore rarely used alone. The combination of morphine, 0.1–0.25 mg, and fentanyl, 12.5 mcg (or sufentanil, 5 mcg), may result in a more rapid onset of analgesia (5 min). Intermittent boluses of 10–15 mg of meperidine, 12.5–25 mcg of fentanyl, or 3–10 mcg of sufentanil via an intrathecal catheter can also provide satisfac-tory analgesia for labor. Early reports of fetal bra-dycardia following intrathecal opioid injections (eg, sufentanil) have not been confirmed by subsequent studies. Hypotension following administration of intrathecal opioids for labor is likely related to the resultant analgesia and decreased circulating cat-echolamine levels.

Epidural Opioids

Relatively large doses (≥7.5 mg) of epidural mor-phine are required for satisfactory labor analgesia, but doses larger than 5 mg are not recommended because of the increased risk of delayed respiratory depression and because the resultant analgesia is effective only in the early first stage of labor. Onset may take 30–60 min but analgesia lasts up to 12–24 h (as does the risk of delayed respiratory depression). Epidural meperidine, 50–100 mg, provides good, but relatively brief, analgesia (1–3 h). Epidural fen-tanyl, 50–150 mcg, or sufentanil, 10–20 mcg, usu-ally produces analgesia within 5–10 min with few side effects, but it has a short duration (1–2 h). Although “single-shot” epidural opioids do not appear to cause significant neonatal depression, caution should be exercised following repeated administrations. Combinations of a lower dose of morphine, 2.5 mg, with fentanyl, 25–50 mcg (or sufentanil, 7.5–10 mcg), may result in a more rapid onset and prolongation of analgesia (4–5 h) with fewer side effects.