Neuraxial analgesia and anesthesia for labor and delivery: Techniques
EPIDURAL CATHETERIZATION —
For epidural catheterization, the tip of a specialized needle (eg, 17 or 18 gauge) is positioned within the epidural space. A thin catheter (eg, 19 or 20 gauge) is then threaded through the needle, and the needle is withdrawn. The catheter is secured to the skin of the parturient's back with an adhesive dressing.
The catheter may be used to administer additional doses of analgesics on an intermittent and/or continuous basis throughout labor and delivery.
In addition, the epidural catheter can be utilized in case a greater dose of local anesthetic is needed for instrumental or cesarean delivery. The catheter may be left in situ postpartum and used to manage pain after delivery.
CONTINUOUS EPIDURAL INFUSION — Continuous infusion of analgesics into the epidural space avoids the peaks and valleys of intermittent administration and results in a smoother analgesic experience for the parturient with fewer provider interventions.
PATIENT-CONTROLLED EPIDURAL ANALGESIA (PCEA) — This modality differs from the continuous infusion technique in that the parturient herself is given the means to fine-tune the dose of analgesic she receives.
PCEA techniques give parturients the psychological advantage of being in control of their own therapy because they are able to titrate the dose to the severity of pain they are experiencing .
Compared to continuous epidural infusions, PCEA results in a lower total dose of local anesthetics used over the course of labor , a decreased need for the physician to administer additional doses of anesthetic, and a lower incidence of motor block .
Some clinicians advocate the use of a continuous background infusion with superimposed PCEA, whereas others use a "pure" demand-only PCEA approach .
Commonly used PCEA regimens include bupivacaine 0.04 to 0.125 percent or ropivacaine 0.0625 to 0.2 percent together with fentanyl 1 to 3 mcg/mL or sufentanil 0.3 to 1 mcg/mL with or without epinephrine 1 to 2 mcg/mL. Background (basal) rates vary from 0 to 15 mL/hour with patient-controlled doses of 3 to 7 mL available at 5- to 20-minute intervals.
The authors of a systematic review of randomized trials of PCEA for labor pain concluded high volume (>5 mL boluses) dilute local anesthetic solutions with a continuous background infusion appeared to be the most successful approach .
INTRATHECAL ADMINISTRATION —
The onset of action for analgesics is more rapid when administered with a spinal rather than epidural approach.
The popularity of the intrathecal route was enhanced by the availability of specially designed "pencil-point" spinal needles. Compared to classic "cutting-point" spinal needles, the pencil-point spinal needles result in a significantly lower incidence of post-spinal headache .
In the United States, intrathecal analgesics are nearly always given as single dose injections, so the duration of analgesia is limited by the duration of action of the drug(s) used.
Small gauge "microcatheters" (28 to 32 gauge) were developed to pass through small-gauge spinal needles, permitting catheterization of the spinal space with a low incidence (approximately 2 percent) of headache.
Continuous spinal techniques with 5 percent hyperbaric lidocaine have been associated with serious neurotoxicity, such as cauda equina syndrome , possibly due to pooling of high concentrations of hyperbaric lidocaine on nerves of the cauda equina. In response to these reports, the FDA banned the use of spinal microcatheters in 1992, although they are still available in other countries.
The inherent fragility of small gauge spinal microcatheters compared to larger gauge epidural catheters make the spinal catheters relatively more prone to fracture during removal.
There is no evidence that injection of other types of analgesics (eg, opioids) results in neurotoxicity.
COMBINED SPINAL-EPIDURAL ANALGESIA —
Combined spinal-epidural analgesia (CSE) refers to a needle-through-needle approach to deposit analgesics (opioid and/or local anesthetic) in the subarachnoid space and then to place a catheter in the epidural space.
The CSE technique is most useful in specific circumstances, such as late first stage or second stage of labor, where the prolonged latency of epidural analgesics is undesirable.
To perform a CSE, the tip of the epidural needle is first inserted into the epidural space. Next, a longer pencil-point spinal needle is introduced through the epidural needle to pierce the dura and deposit analgesics into the cerebrospinal fluid. The spinal needle is then withdrawn and a catheter is threaded into the epidural space to permit epidural administration of analgesics.
A CSE approach provides more rapid onset of analgesia than does an epidural approach alone (by 5 to 10 minutes). This difference is more important when delivery is imminent than in early labor.
Because many different techniques of providing epidural and CSE analgesia are used, it is difficult to make meaningful comparisons. A meta-analysis confirmed that, although the onset of analgesia is more rapid in parturients receiving a CSE, side effects (eg, pruritus) are more common .
The incidence of cesarean delivery does not appear to be affected whether a CSE or epidural technique is used .
WALKING EPIDURAL —
The ideal analgesic regimen for labor would provide complete pain relief without any side effects for the mother or neonate. An advantage of epidural labor analgesia with local anesthetics is that it eliminates opioid side effects that are commonplace when systemic opioids are used. However, local anesthetics are associated with other side effects, including intense motor block of the lower extremities, rendering it difficult for the parturient to move her legs.
This is more than an annoyance; muscle weakness is thought by some to compromise expulsive efforts (pushing) during the second stage of labor.
The evolution of the present-day "walking epidural" was a response to the need for providing analgesia while preserving motor strength.
The phrase "walking epidural" is a generic description of any neuraxial analgesic technique that preserves motor function in a parturient. However, for various reasons 34 to 85 percent of women do not actually ambulate during labor . The intrathecal route may be used to achieve analgesia without motor block, followed by activation of the epidural catheter with a mixture of opioids and local anesthetics.
Motor strength should be assessed at the bedside prior to allowing the parturient to ambulate in order to prevent them from falling. The ability to perform a deep knee bend is considered by some an indication that the parturient has sufficient lower extremity strength to ambulate.
However, it is important to advise parturients to walk with care and with assistance because neuraxial analgesia may produce alterations of proprioception and position sense.
A practical constraint on ambulation is the need to disconnect the parturient from the continuous fetal monitor, unless telemetry is available. The reticence to interrupt fetal monitoring often limits ambulation to brief intervals.
At most institutions, ambulation following initiation of neuraxial analgesia is typically limited to walking to the restroom to use the commode.
STRATEGIES TO MINIMIZE MATERNAL MOTOR BLOCK —
Decreasing the intensity of motor block may be achieved by administering lower concentrations of local anesthetics by either the epidural or spinal route.
Low concentrations of some local anesthetics (eg, bupivacaine), result in differential motor-sensory block, so analgesia is possible with little or no motor block. As an example, epidural administration of bupivacaine 0.5 percent yields intense motor and sensory block, whereas a 0.04 percent concentration is more likely to produce selective sensory block.
Intrathecal bupivacaine 10 mg produces motor block, whereas 2 mg will not usually cause significant motor weakness.
The mechanism for differential block is not well understood. Analgesic adjuvants (fentanyl, sufentanil, epinephrine) are often co-administered with local anesthetics to offset the decreased analgesic intensity resulting from the reduction of local anesthetic dose. The opioids do not produce any motor block, and thus are ideally suited adjuvants for labor analgesia. Epinephrine, in addition to potentiating analgesia, may intensify the local anesthetic-induced motor block .
The co-administration of different classes of analgesics also has an advantage with regard to side effects and safety; a smaller dose of each agent is used so drug-specific side effects are less likely to occur. Furthermore, side effects of different types of agents do not potentiate one another because they are distinct for each class of analgesics.
As an example, cardiac toxicity, a potential risk of epidural administration of a large dose of local anesthetic, is much less likely when lower total doses are used. In addition, a less serious but annoying opioid side effect such as pruritus is not as likely when smaller doses of opioids are injected into the neuraxis. The incidence of other side effects such as hypotension may also be reduced by using lower local anesthetic doses.
SYNERGISTIC EFFECT OF COMBINATIONS OF NEURAXIAL ANALGESICS —
The co-administration of different classes of pain relievers (eg, local anesthetics and opioids) into the neuraxis not only decreases the incidence and severity of unwanted effects, such as motor block, it also potentiates their analgesic effects.
The concept of Minimum Local Analgesic Concentration (MLAC) was introduced to determine the effective concentration of local anesthetics in 50 percent of parturients in labor (EC50) .MLAC has been used to compare the relative potencies of local anesthetics, and also to quantify the local-anesthetic sparing effects of neuraxial analgesic adjuvants.
As an example, in one study, the addition of fentanyl 1, 2, 3, or 4 mcg/mL to bupivacaine by the epidural route resulted in a decrease of MLAC for bupivacaine alone from 0.069 to 0.057, 0.048, 0.031, and 0.015 percent, respectively .
Studies have also found the duration of analgesia after a single intrathecal dose of local anesthetic and opioid is prolonged if the agents are injected together. In one study, average analgesic duration after bupivacaine (2.5 mg) was 70 minutes compared to 114 minutes after sufentanil (10 mcg) and 148 minutes after a combination of both .
Analgesic duration may be increased to 188 minutes by adding 200 mcg epinephrine, but at a cost of producing motor block in 20 percent of parturients . Some investigators have suggested using neuraxial administration of low doses of four different classes of analgesics: local anesthetic, opioid, alpha-2 agonist, and cholinergic agonist to provide labor analgesia .
Although epidural administration of the indirect cholinergic agonist neostigmine and the alpha-2 agonist clonidine show promise for relieving labor pain , these adjuvants are not commonly used for this purpose. Moreover, neostigmine is not approved by the FDA for epidural administration.
NEURAXIAL OPIOID ADMINISTRATION —
The addition of opioids to neuraxial local anesthetics appears to contravene a goal of regional analgesia, namely to provide pain relief while avoiding opioid side effects in the mother and neonate.
Early experience with epidural morphine demonstrated that significant plasma drug concentration was achieved after epidural administration . However, present-day epidural techniques for labor avoid this problem by utilizing relatively small doses of lipophilic opioids (fentanyl, sufentanil), which do not produce systemic side effects .They may, however, result in annoying, neuraxially-mediated side effects, such as pruritus and nausea.
Some authors have questioned whether the actual site of drug action is the spinal cord or the brain given the large doses of opioids administered into the epidural space and secondary systemic redistribution. Subsequent studies have shown that the site of action of lipophilic opioids administered epidurally is indeed in the neuraxis .
Intrathecal administration (compared to epidural administration) of opioids minimizes systemic side effects due to the minuscule doses of opioid used. However, opioids administered into the epidural or intrathecal space may still result in untoward effects from direct spinal effects and from cephalad spread within the CSF to the brain.
SUMMARY AND RECOMMENDATIONS
1.Catheterization of the epidural space facilitates administration of analgesics throughout labor and for instrumental or operative delivery. (See 'Continuous epidural infusion' above.)
2.Patient controlled epidural analgesia (PCEA) is a desirable approach to managing labor pain because it actively involves the parturient in her own pain management and results in less local anesthetic consumption and less motor block than standard continuous infusion techniques. (See 'Patient-controlled epidural analgesia (PCEA)' above.)
3.The onset of analgesic action is more rapid with the intrathecal route compared to the epidural route of administration. (See 'Intrathecal administration' above.)
4.The term "walking epidural" is commonly used to refer to any neuraxial technique that achieves analgesia with minimal motor block. (See 'Walking epidural' above.)
5.co-administration of different classes of analgesics has the advantage of potentiation of pain relief while minimizing the likelihood of side effects, as each analgesic is associated with distinct side effects. (see'Strategies to minimize maternal motor block' above.)
6.Small doses of opioids administered into the epidural or intrathecal space minimize systemic side effects, although annoying neuraxially-mediated side effects such as pruritus and nausea may occur. (See 'Neuraxial opioid administration' above.)
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发表于 2009-12-9 18:50:23
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发表于 2009-12-9 22:17:48
