Local Anesthetic/Local Anesthetic–Opioid Mixtures
Epidural and spinal (intrathecal) analgesia more commonly utilizes local anesthetics either alone or with opioids for labor and delivery. Analgesia during the first stage of labor requires neural blockade at the T10–L1 sensory level, whereas pain relief during the second stage of labor requires neural blockade at T10–S4. Continuous lumbar epidural analgesia is the most versatile andmost commonly-employed technique, because it can be used for pain relief for the first stage of labor as well as analgesia/anesthesia for subsequent vagi-nal delivery or cesarean section, if necessary. “Single-shot” epidural, spinal, or combined spinal epidural analgesia may be appropriate when pain relief is ini-tiated just prior to vaginal delivery (the second stage). Obstetric caudal injections have largely been abandoned because of less versatility; although effective for perineal analgesia/anesthesia they require large volumes of local anesthetic to anesthe-tize upper lumbar and lower thoracic dermatomes. They have also been associated with early paralysis of the pelvic muscles that may interfere with normal rotation of the fetal head, and with a small risk of accidental puncture of the fetus.
Absolute contraindications to regional anesthe-sia include patient refusal, infection over the injection site, coagulopathy, marked hypovolemia, and true allergies to local anesthetics. The patient’s inability to cooperate may prevent successful regional anes-thesia. Neuraxial anesthesia and full anticoagulation is a dangerous combination. Regional anesthesia should generally not be performed within 6–8 h of a subcutaneous minidose of unfractionated heparin or within 12–24 h of administration of low-molecu-lar-weight heparin (LMWH). Thrombocytopenia or concomitant administration of an antiplatelet agent increases the risk of spinal hematoma. A vaginal birth after cesarean (VBAC) delivery is not consid-ered a contraindication to regional anesthesia dur-ing labor. Concern that the anesthesia may mask pain associated with uterine rupture during VBAC may not be justified, because dehiscence of a lower segment scar frequently does not cause pain even without epidural anesthesia; moreover, changes in uterine tone and contraction pattern may be more reliable signs.
Before performing any regional block, appro-priate equipment and supplies for resuscitation should be checked and made immediately available. Minimum supplies include oxygen, suction, a mask with a positive-pressure device for ventilation, a functioning laryngoscope and blades, endotracheal tubes (6 or 6.5 mm), oral and nasal airways, intrave-nous fluids, ephedrine, atropine, propofol, and suc-cinylcholine. The ability to frequently monitor blood pressure and heart rate is mandatory. A pulse oxim-eter and capnograph should be readily available.
Epidural analgesia for labor may be administered in early labor after the patient has been evaluated by her obstetrician. When dilute mixtures of a local anesthetic and an opioid are used, epidural analgesia has little if any effect on the progress of labor. Concerns that regional analgesia will increase the likelihood of oxytocin augmentation, operative (eg, forceps) delivery, or cesarean section, are unjustified. It is often advantageous to place an epidural catheter early, when the patient is less uncomfortable and can be positioned more easily. Moreover, should an urgent or emergent cesarean section become necessary, the presence of a well-functioning epidural catheter makes it possible to avoid general anesthesia.
Parturients may be positioned on their sides or in the sitting position for the procedure. The sitting position often makes it easier to identify the midline and spine in obese patients. When epidural anesthe-sia is being given for vaginal delivery (second stage), the sitting position helps ensure good sacral spread.
Because the lumbar epidural space pressure may be positive in some parturients, correct iden-tification of the epidural space may be difficult. Unintentional dural puncture will occur even in experienced hands; the incidence of “wet taps” in obstetric patients is 0.25–9%, depending on clinician
experience. Many practitioners add a compress-ible air bubble to the saline syringe and bounce the plunger to ensure that it moves freely and does not stick to the syringe wall ( Figure 41–1A and C). Most clinicians advocate the midline approach, whereas a minority favors the paramedian approach. For the placement of a lumbar epidural catheter in the obstetric patient, most anesthesiologists advance the epidural needle with the left hand, which is braced against the patient’s back, while applying continuous pressure to a glass syringe filled with sterile saline (Figure 41–1A and C). Alternatively, some make use of the “wings” of the Weiss epidural needle by advancing it with both hands few millimeters at a time ( Figure 41–1B). A change of tissue resistance is then tested continuously using tactile feedback when advancing the needle and by intermittently applying pressure to the air-filled loss-of resistance syringe. The later technique allows for precise con-trol of needle advancement and may allow a better distinction of various tissue densities. If air is used for detecting loss of resistance, the amount injected should be limited; injection of larger volumes of air (>2–3 mL) in the epidural space has been associated with patchy or unilateral analgesia and headache. The average depth of the lumbar epidural space in obstetric patients is reported to be 5 cm from the skin. Placement of the epidural catheter at the L3–4 or L4–5 interspace is generally optimal for achiev-ing a T10–S5 neural blockade. Ultrasound guidance has recently been offered as tool in assisting with the placement of an epidural catheter. This technique allows the practitioner to judge the depth of the epi-dural space and estimate the best angle of needle insertion. The potential benefit of this technique is most obvious in obese patients with poor anatomic landmarks. However, the technique is highly user-dependent, and few practitioners have adopted it.
If unintentional dural puncture occurs, the anesthetist has two choices: (1) place the epidural catheter in the subarachnoid space for continu-ous spinal (intrathecal) analgesia and anesthesia , or (2) remove the needle and attempt placement at a higher spinal level. The intrathecally-placed epidural catheter may be used as continuous spinal anesthetic, possibly reducing the incidence of post–dural puncture headache. If used in this fash-ion, an infusion of 0.0625–0.125% bupivacaine with fentanyl, 2–3 mcg/mL starting at 1–3 mL/h, is a rea-sonable choice.
Many clinicians advocate use of a multiholed cath-eter instead of a single-holed catheter for obstetric anesthesia. Use of a multiholed catheter may be associated with fewer unilateral blocks and greatly reduces the incidence of false-negative aspiration when assessing for intravascular or intrathecal cath-eter placement. Advancing a multiholed catheter 4–6 cm into the epidural space appears to be optimal for obtaining adequate sensory levels. A single-hole catheter need only be advanced 3–5 cm into the epi-dural space. Shorter insertion depths (<5 cm), how-ever, may favor dislodgment of the catheter out of the epidural space in obese patients following flexion/ extension movements of the spine. Spiral wire-reinforced catheters are very resistant to kinking. A spiral or spring tip, particularly when used without a stylet, is associated with fewer, less intense pares-thesias and may also be associated with a lower inci-dence of accidental intravascular insertion.
The addition of opioids to local anesthetic solutions for epidural anesthesia has dramatically changed the practice of obstetric anesthesia. The synergy between epidural opioids and local anesthetic solu-tions reflects separate sites of action, namely, opiate receptors and neuronal axons, respectively. When the two are combined, very low concentrations of both local anesthetic and opioid can be used. More importantly, the incidence of adverse side effects, such as hypotension and drug toxicity, is likely reduced. Although local anesthetics can be used alone, there is rarely a reason to do so. Moreover, when an opioid is omitted, the higher concentration of local anesthetic required (eg, bupivacaine, 0.25%, and ropivacaine, 0.2%) for adequate analgesia can impair the parturient’s ability to push effectively as labor progresses. Bupivacaine or ropivacaine in con-centrations of 0.0625–0.125% with either fentanyl, 2–3 mcg/mL, or sufentanil, 0.3–0.5 mcg/mL, is most often used. In general, the lower the concentration of the local anesthetic the greater the concentration of opioid that is required. Very dilute local anes-thetic mixtures (0.0625%) generally do not produce motor blockade and may allow some patients to ambulate (“walking” or “mobile” epidural). The long duration of action of bupivacaine makes it a popu-lar agent for labor. Ropivacaine may be preferable because of its reduced potential for cardiotoxicity . At equi-analgesic doses, ropiva-caine and bupivacaine appear to produce the same degree of motor block.
The effect of epinephrine-containing solutions on the course of labor is somewhat controversial. Many clinicians use epinephrine-containing solu-tions only for intravascular test doses because of concern that the solutions may slow the progres-sion of labor or adversely affect the fetus; others use only very dilute concentrations of epinephrine such as 1:800,000 or 1:400,000. Studies comparing these various agents have failed to find any differences in neonatal Apgar scores, acid–base status, or neurobe-havioral evaluations.
Initial epidural injections may be done either before or after the catheter is placed. Administration through the needle can facilitate catheter place-ment, whereas administration through the catheter ensures proper function of the catheter. The follow-ing sequence is suggested for epidural activation:
· Test for unintentional subarachnoid or intravascular placement of the needle or catheter with a 3-mL test dose of a local anesthetic with 1:200,000 epinephrine (controversial; see the section on Prevention of Unintentional Intravascular and Intrathecal Injections). Many clinicians test with lidocaine 1.5% because of less toxicity following unintentional intravascular injection and a more rapid onset of spinal anesthesia than with bupivacaine and ropivacaine. The test dose should be injected between contractions to help reduce false positive signs of an intravascular injection (ie, tachycardia due to a painful contraction).
· If after 5 min signs of intravascular or intrathecal injection are absent, with the patient supine and left uterine displacement, administer 10 mL of the local anesthetic– opioid mixture in 5-mL increments, waiting 1–2 min between doses, to achieve a T10–L1 sensory level. The initial bolus is usually composed of 0.1–0.2% ropivacaine or 0.0625–0.125% bupivacaine combined with either 50–100 mcg of fentanyl or 10–20 mcg of sufentanil.
· Monitor with frequent blood pressure measurements for 20–30 min or until the patient is stable. Pulse oximetry should also be used. Oxygen is administered via face mask if there are any significant decreases in blood pressure or oxygen saturation readings.
· Repeat steps 2 and 3 when pain recurs until the first stage of labor is completed. Alternatively, a continuous epidural infusion technique may be employed using bupivacaine or ropivacaine in concentrations of 0.0625–0.125% with either fentanyl, 1–5 mcg/mL, or sufentanil, 0.2–0.5 mcg/mL at a rate of 10 mL/h, which subsequently is adjusted to the patient’s analgesic requirements (range: 5–15 mL/h). A third choice would be to use patient-controlled epidural analgesia (PCEA). Some studies suggest that total drug requirements may be less and patient satisfaction is greater with PCEA compared with other epidural techniques. PCEA settings are typically a 5-mL bolus dose with a 5–10 min lockout and 0–12 mL/h basal rate; a 1-h limit of 15–25 mL may used. Migration of the epidural catheter into a blood
vessel during a continuous infusion technique may be heralded by loss of effective analgesia; a high index of suspicion is required because overt signs of systemic toxicity may be absent. Erosion of the catheter through the dura results in a slowly progressive motor blockade of the lower extremities and a rising sensory level.
Administration for the second stage of labor extends the block to include the S2–4 dermatomes. Whether a catheter is already in place or epidural anesthesia is just being initiated, the following steps should be undertaken:
· If the patient does not already have a catheter in place, identify the epidural space while the patient is in a sitting position. A patient who already has an epidural catheter in place should be placed in a semiupright or sitting position prior to injection.
· Give a 3-mL test dose of local anesthetic (eg, lidocaine 1.5%) with 1:200,000 epinephrine. Again, the injection should be completed between contractions.
· If aft er 5 min signs of an intravascular or intrathecal injection are absent, give 10–15 mL of additional local anesthetic–opioid mixture at a rate not faster than 5 mL every 1–2 min.
· Administer oxygen by face mask, lay the patient supine with left uterine displacement, and monitor blood pressure every 1–2 min for the first 15 min, then every 5 min thereafter.
Safe administration of epidural anesthesia is criti-cally dependent on avoiding unintentional intrathecal or intravascular injection. Unintentional intravascular or intrathecal placement of an epidural needle or catheter is possible even when aspiration fails to yield blood or cerebrospinal fluid (CSF). The incidence of unintentional intravascular or intrathecal placement of an epidural catheter is 5–15% and 0.5–2.5%, respectively. Even a properly placed catheter can subsequently erode into an epidural vein or an intrathecal position. This possi-bility should be considered each time local anes-thetic is injected through an epidural catheter.
Test doses of lidocaine, 45–60 mg, bupivacaine, 7.5–10 mg, ropivacaine, 6–8 mg, or chloroprocaine, 100 mg, can be given to exclude unintentional intra-thecal placement. Signs of sensory and motor block-ade usually become apparent within 2–3 min and 3–5 min, respectively, if the injection is intrathecal.
In patients not receiving β-adrenergic antago-nists, the intravascular injection of a local anesthetic solution with 15–20 mcg of epinephrine consistently increases the heart rate by 20–30 beats/min within 30–60 s if the catheter (or epidural needle) is intra-vascular. This technique is not always reliable in parturients because they often have marked spon-taneous baseline variations in heart rate with con-tractions. In fact, bradycardia has been reported in a parturient following intravenous injection of 15 mcg of epinephrine. Moreover, in animal studies, 15 mcg of epinephrine intravenously reduces uterine blood flow. Alternative methods of detecting unintentional intravascular catheter placement include eliciting tinnitus or perioral numbness following a 100-mg test dose of lidocaine or eliciting a chronotropic effect following injection of 5 mcg of isoproterenol. The use of dilute local anesthetic solutions and slow injection rates of no more than 5 mL at a time may also enhance detection of unintentional intravascular injections before catastrophic complications develop.
Use of the head-down (Trendelenburg) position is controversial because of its potentially detrimental effects on pulmonary gas exchange.
Techniques using CSE analgesia and anes-thesia may particularly benefit patients withsevere pain early in labor and those who receive anal-gesia/anesthesia just prior to delivery. Intrathecal opioid and local anesthetic are injected after which an epidural catheter is left in place. The intrathecal drugs provide nearly immediate pain control and have minimal effects on the early progress of labor, whereas the epidural catheter provides a route for subsequent analgesia for labor and delivery or anes-thesia for cesarean section. Addition of small doses of local anesthetic agents to intrathecal opioid injec-tion greatly potentiates their analgesia and can sig-nificantly reduce opioid requirements. Thus, many clinicians will inject 2.5 mg of preservative-free bupivacaine or 3–4 mg of ropivacaine with intrathe-cal opioids for analgesia in the first stage of labor. Intrathecal doses for CSE are fentanyl, 5–10 mcg, or sufentanil, 5 mcg. Some studies suggest that CSE techniques may be associated with greater patient satisfaction and lower incidence of PDPH than epi-dural analgesia alone. A 24- to 27-gauge pencil-point spinal needle (Whitacre, Sprotte, or Gertie Marx) is used to minimize the incidence of PDPH.
The spinal and epidural needles may be placed at separate interspaces, but most clinicians use a needle-through-needle technique at the same inter-space. Use of saline for identification of the epidural space may potentially cause confusion of saline for CSF. With the needle-through-needle technique, the epidural needle is placed in the epidural space and a long spinal needle is then introduced through it and advanced farther into the subarachnoid space. A distinct pop is felt as the needle penetrates the dura. The needle-beside-needle technique typically employs a specially designed epidural needle that has a channel for the spinal needle. After the intrathecal injection and withdrawal of the spinal needle, the epidural catheter is threaded into position and the epidural needle is withdrawn. The risk of advancing the epidural catheter through the dural hole created by the spinal needle appears to be negligible when25-gauge or smaller needle is used. The epidural catheter, however, should be aspirated carefully and local anesthetic should always be given slowly and in small increments to avoid unintentional intrathe-cal injections. Moreover, epidural drugs should be titrated carefully because the dural hole may facili-tate entry of epidural drugs into CSF and enhance their effects.
Spinal anesthesia given just prior to delivery—also known as saddle block—provides profound anesthe-sia for operative vaginal delivery. Use of a 22-gauge or smaller, pencil-point spinal needle (Whitacre, Sprotte, or Gertie Marx) decreases the likelihood of PDPH. Hyperbaric tetracaine (3–4 mg), bupiva-caine (2.5–5 mg), or lidocaine (20–40 mg) usually provides excellent perineal anesthesia. Addition of fentanyl (12.5–25 mcg) or sufentanil (5–7.5 mcg) significantly potentiates the block. A T10 sensory level can be obtained with slightly larger amounts of local anesthetic. Three minutes after injection, the patient is placed in the lithotomy position with left uterine displacement.
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