Regional anesthesia is achieved by injection of a local anesthetic (Table 24–2) around the nerves that pass from spinal segments to the peripheral nerves responsible for sensory innervation of a portion of the body. More recently, narcotics have been added to local anesthetics to improve analgesia and reduce some side effects of local anesthetics. Regional nerve blocks used in obstetrics include the following: (1) lumbar epidural and caudal epidural block, (2) subarachnoid (spinal) block, (3) combined spinal epidural block, and (4) pudendal block.
Infiltration of a local anesthetic drug and pudendal block analgesia carry minimal risks. The hazards increase with the amount of drug used. The safety and suitability of regional anesthesia depend on proper selection of the drug and the patient and the obstetrician–gynecologist's knowledge, experience, and expertise in the diagnosis and treatment of possible complications. Major conductive anesthesia and general anesthesia in obstetrics require specialized knowledge and expertise in conjunction with close maternal and fetal monitoring. This field of expertise has developed as a subspecialty within anesthesia, reflecting the need for specialized understanding of the obstetric patient and her response and of the fetal responses to anesthesia.
Regional anesthesia is appropriate for labor analgesia, caesarean delivery, and other obstetric operative procedures (eg, postpartum tubal ligation, cervical cerclage). Most patients prefer to remain awake; however, occasionally a choice is made to provide general anesthesia.
The anesthesiologist will assess the patient to determine the relative risks of general versus regional anesthesia. For example, some forms of valvular heart disease may contraindicate regional block, and general anesthesia may be considered more appropriate. Other contraindications to regional anesthesia include infection, coagulopathy, hypovolemia, progressive neurologic disease, and patient refusal.
The woman who is well informed and has a good rapport with her physician generally is a calm and cooperative candidate for regional or general anesthesia. The patient and her partner should be well informed early in her pregnancy of the options for labor anesthesia as well as for caesarean section if that circumstance arises. The anesthesiologist can be involved early in pregnancy if the patient has special concerns about anesthesia (family history of anesthetic risk, previous back surgery, coagulation problems). Some hospitals have obstetric anesthesia preassessment clinics that deal with these patient concerns.
A local anesthetic drug blocks the action potential of nerves when their axons are exposed to the medication. Local anesthetic agents act by modifying the ionic permeability of the cell membrane to stabilize its resting potential. The smaller the nerve fiber, the more sensitive it is to local anesthetics because the susceptibility of individual nerve fibers is inversely proportional to the cross-sectional diameter of the fibers. Hence, with regional anesthesia, the patient's perception of light touch, pain, and temperature and her capacity for vasomotor control are obtunded sooner and with a smaller concentration of the drug than is the perception of pressure or the function of motor nerves to striated muscles. The exception to this rule is the sensitivity of autonomic nerve fibers that are blocked by the lowest concentration of local anesthetic despite their being larger than some sensory nerves.
Only anesthetic drugs that are completely reversible and nonirritating and cause minimal toxicity are clinically acceptable. Other desirable properties of regional anesthetic agents include rapidity of onset, predictability of duration, and ease of sterilization. Table 24–2 summarizes the local anesthetics commonly used in obstetrics and gynecology together with their uses and doses.
All local anesthetics have certain undesirable dose-related side effects when absorbed systemically. All these drugs are capable of stimulating the central nervous system and may cause bradycardia, hypertension, or respiratory stimulation at the medullary level. Moreover, they may produce anxiety, excitement, or convulsions at the cortical or subcortical level. This response stimulates grand mal seizures because it is followed by depression, loss of vasomotor control, hypotension, respiratory depression, and coma. Such an episode of indirect cardiovascular depression often is accentuated by a direct vasodilatory and myocardial depressant effect. The latter is comparable to the action of quinidine. This effect explains why lidocaine is useful for treatment of certain cardiac arrhythmias.
Chloroprocaine is an ester derivative that was popular in 1970s primarily because of its rapid onset and short duration of action and its low toxicity to the fetus. It is metabolized by plasma cholinesterase and therefore does not demand liver enzyme degradation, as do the more complex and longer acting amide derivatives. Chloroprocaine has a half-life of 21 seconds in adult blood and 43 seconds in neonatal blood. Direct toxic effects on the fetus are minimized because fewer drugs are available for transfer in the maternal compartment.
The potency of chloroprocaine is comparable to that of lidocaine and mepivacaine, and the drug is 3 times more potent than procaine. Its average onset of action ranges from 6–12 minutes and persists for 30–60 minutes, depending on the amount used. Chloroprocaine is often used for urgent caesarean delivery when epidural catheter is already placed to avoid general anesthesia.
Bupivacaine, the amide local anesthetic, is related to lidocaine and mepivacaine but has some very different physicochemical properties. It has a much higher lipid solubility, a higher degree of binding to maternal plasma protein, and a much longer duration of action. More than with other local anesthetics, the concentration of bupivacaine can be reduced to produce sensory block with minimal motor block. Because injection of bupivacaine for labor pain relief now is mostly in the form of continuous small-volume and minimal concentration administration via a pump mechanism, the complications previously of concern, such as hypotension and convulsions, are now rare.
A word of caution is needed regarding the administration of bupivacaine for caesarean delivery. This drug has been implicated in certain cardiovascular catastrophes associated with initial drug injection, such as cardiac arrests that were refractory to full and appropriate resuscitative attempts. Although these catastrophes are rare, the practitioner is well advised to inject no more than 5 mL of the drug at any one time, to wait 4–5 minutes, then to repeat the procedure until the desired volume has been delivered. The maximum concentration of bupivacaine now allowed by the Food and Drug Administration (FDA) for obstetric epidural anesthesia is 0.5%. The dose of more than 3 mg/kg is considered a toxic dose now. The safety of bupivacaine can be enhanced by giving it in fractional doses (eg, 5 mL every 5 minutes).
Ropivacaine is a newer amide local anesthetic introduced into the United States in the mid-1990s. It is less lipid-soluble than bupivacaine, and initial studies suggested that it produced less motor blockade and was less cardiotoxic than its homologue bupivacaine. Later studies have been less convincing in documenting improved efficacy and safety, but ropivacaine has replaced bupivacaine in some institutions. There is ongoing study of the safety and efficacy of levobupivacaine, the levorotatory isomer of bupivacaine, which may also prove less cardiotoxic than its racemic parent molecule. Both of these newer amide local anesthetics are used in doses and concentrations similar to those of bupivacaine.
Local Infiltration Analgesia
Local tissue infiltration of dilute solutions of anesthetic drugs generally yields satisfactory results because the target is the fine nerve fibers. Nevertheless, one must keep in mind the dangers of systemic toxicity when large areas are anesthetized or when reinjection is required. It is good practice, therefore, to calculate in advance the milligrams of drug and volume of solution that may be required to keep the total dosage below the accepted toxic dose.
Infiltration in or near an area of inflammation is contraindicated. Injections into these zones may be followed by rapid systemic absorption of the drug as a result of increased vascularity of the inflamed tissues. Moreover, the injection may introduce or aggravate infection.
Regional Analgesia Techniques
This analgesic technique is well suited to obstetric anesthesia. Either bolus injections or continuous infusion of local anesthetics is used for labor, vaginal delivery, or caesarean surgery. Narcotics are added to supplement the quality of the block.
After the patient is evaluated, an epidural block can be placed once labor is established. Drug dosages can be adjusted as circumstances change. The catheter can be used for surgery and postoperative analgesia if necessary. The second stage of labor is prolonged by epidural anesthesia; however, the duration of the first stage is unaffected. The use of outlet forceps is increased, but fetal outcome is not adversely affected by epidural block.
The epidural block technique must be exact, and inadvertent massive (high) spinal anesthesia occasionally occurs. Other undesirable reactions include the rapid absorption syndrome (hypotension, bradycardia, hallucinations, and convulsions), postpartum backache, and paresthesias. Epidural block should eradicate pain between T10 and L1 for the first stage of labor and between T10 and S5 for the second stage of labor.
The procedure is as follows. Inject 3 mL of a 1.5% aqueous solution of lidocaine or similar agent into the catheter as a test dose. If spinal anesthesia does not result after 5–10 minutes, inject an additional 5 mL. Inject 10 mL of the anesthetic solution in total to slowly accomplish an adequate degree and suitable level of anesthesia. Once the block is established, a continuous infusion of 10–12 mL/h will maintain the block for labor. Bupivacaine 0.125–0.25% is most often used for an epidural block, with fentanyl 2–5 μg/mL in the epidural mixture.
The mother is nursed in a wedged or lateral position to prevent aortocaval compression. The sympathectomy produced by the block predisposes the patient to venous pooling and reduced venous return. Maternal blood pressure must be measured frequently when the epidural is in effect.
Caudal anesthesia (Fig. 24–4) is an epidural block approached through the caudal space. It can provide selective sacral block for the second stage of labor; however, it is rarely used now because of complications specific to the obstetric patient. The descent of the fetal head against the perineum, in addition to the sacral edema at term, obscures the landmarks of the sacral hiatus. This makes the caudal procedure technically challenging, and reports of transfixing the rectum and fetal skull puncture with the epidural needle have led many anesthesiologists to avoid this technique. Lumbar epidural anesthesia is considered a safer alternative.
Caudal catheter in place for continuous caudal anesthesia.
Spinal anesthesia currently is the anesthetic of choice for caesarean delivery. Spinal anesthesia can be performed more quickly than epidural anesthesia and provides ideal operating conditions, including dense sensory and motor block. The onset of sympathectomy is more abrupt than with epidural block, so care must be taken to ensure that the patient is adequately preloaded with 1.5–2 L of saline solution before performing the technique. Spinal anesthesia is used less commonly these days to alleviate the pain of delivery and the third stage of labor. The advantages of spinal anesthesia are that the mother remains conscious to witness delivery, no inhalation anesthetics or analgesic drugs are required, the technique is not difficult, and good relaxation of the pelvic floor and lower birth canal is achieved. Prompt anesthesia is achieved within 5–10 minutes. The dosage of spinal anesthetic is small. Complications are rare and easy to treat. However, spinal headache occurs in 1–2% of patients.
Combined Spinal–Epidural Analgesia
The use of combined spinal–epidural anesthesia (CSE) became popular in the mid 1990s as an alternative to epidural anesthesia for labor. A small dose of local anesthetic and narcotic (2.5 mg bupivacaine and 25 μg fentanyl) is injected through a spinal needle, which is introduced through the epidural needle and advanced into the intrathecal space. The spinal needle is withdrawn and the epidural catheter placed for later use. The spinal medication produces immediate pain relief and minimal motor block and may allow ambulation. Later in labor, the epidural catheter is used for continuous infusion of epidural solution, similar to that described for standard epidural anesthesia in labor.
Detractors of CSE argue that the technique may increase the incidence of post–lumbar puncture headache and that ambulation even after low-dose spinal injection is unsafe for both mother and baby. Finally, because the technique is technically cumbersome, it may be associated with higher complication rates, although the studies did not support this statement.
The most serious consequence of spinal or epidural anesthesia is maternal mortality. Maternal deaths associated with use of 0.75% bupivacaine for caesarean delivery and labor were reported in the late 1980s, prompting the FDA to outlaw the use of this drug in obstetrics. These deaths were attributed to venous uptake of the drug and immediate and lasting myocardial depression from the local anesthetic, which did not respond to appropriate cardiac resuscitative efforts. Today maternal mortality associated with regional anesthesia is lower, primarily because bolus dosing of high concentrations of local anesthesia is no longer performed.
Most side effects of spinal or epidural anesthesia are secondary to block of the sympathetic nerve fibers that accompany the anterior roots of the spinal thoracic and upper lumbar nerves (thoracolumbar outflow). Thus many physiologic regulating mechanisms are disturbed. The blood pressure falls as a result of loss of arterial resistance and venous pooling—assuming no compensation is made by change of the patient's position (eg, Trendelenburg position). If high thoracic dermatomes (T1–T5) are blocked, alteration of the cardiac sympathetic innervation slows the heart rate and reduces cardiac contractility. Epinephrine secretion by the adrenal medulla is depressed. Concomitantly, the unopposed parasympathetic effect of cardiac slowing alters vagal stimulations. As a result of these and related changes, shock follows promptly, especially in hypotensive or hypovolemic patients. Moreover, a precipitous fall in the blood pressure of the arteriosclerotic hypertensive patient is inevitable.
Fluids, oxygen therapy for adequate tissue perfusion, shock position to encourage venous return, and pressor drugs given intravenously are recommended.
In the past, postdural puncture headache (PDPH) due to leakage of cerebrospinal fluid through the needle hole in the dura was an early postoperative complication in up to 15% of patients. Small-caliber needles (25F) decrease the incidence of headache to 8–10%. With the introduction of pencil-point Whitaker and Sprotte spinal needles, the incidence of PDPH has been reduced to 1–2%. Therapy for PDPH includes recumbent position, hydration, sedation, and, in severe cases, epidural injection of 10–20 mL of the patient's fresh blood to “seal” the defect.
Rarely, spinal or epidural anesthesia caused nerve injury and transient or permanent hypesthesia or paresthesia. Excessive drug concentration, sensitivity, or infection may have been responsible for some of these complications. The incidence of serious complications of spinal or epidural anesthesia is considerably lower than that of cardiac arrest during general anesthesia.
Paracervical block is no longer considered a safe technique for the obstetric patient. In the past, paracervical anesthesia was used to relieve the pain of the first stage of labor. Pudendal block was required for pain during the second stage of labor. Sensory nerve fibers from the uterus fuse bilaterally at the 4–6 o'clock and 6–8 o'clock positions around the cervix in the region of the cervical–vaginal junction. Ordinarily, when 5–10 mL of 1% lidocaine or its equivalent is injected into these areas, interruption of the sensory input from the cervix and uterus promptly follows.
Many now consider paracervical block to be contraindicated in obstetrics because of the potential adverse fetal effects. Many reports in the literature place the incidence of fetal bradycardia at 8–18%. However, recent work with accurate fetal heart rate monitoring associated with continuous uterine contraction patterns suggests that the incidence is closer to 20–25%. Some researchers have attempted to investigate the significance of the bradycardia. One explanation is that an acid–base disturbance in the fetus does not occur unless the bradycardia lasts longer than 10 minutes and that neonatal depression is rare unless associated with delivery during the period of bradycardia. There seems to be little difference in the incidence and severity of fetal bradycardia by paracervical block between complicated and uncomplicated patients. Other disadvantages of paracervical block include maternal trauma and bleeding, fetal trauma and direct injection, inadvertent intravascular injection with convulsions, and short duration of the block.
Pudendal block has been one of the most popular of all nerve block techniques in obstetrics. The infant is not depressed, and blood loss is minimal. The technique is simplified by the fact that the pudendal nerve approaches the spine of the ischium on its course to innervate the perineum. Injection of 10 mL of 1% lidocaine on each side will achieve analgesia for 30–45 minutes approximately 50% of the time.
Both the transvaginal and transcutaneous methods are useful for administering a pudendal block. The transvaginal technique has important practical advantages over the transcutaneous technique. The “Iowa trumpet” needle guide can be used, and the operator's finger should be placed at the end of the needle guide to palpate the sacrospinous ligament, which runs in the same direction and is just anterior to the pudendal nerve and artery. Appreciating the sensation of the needle puncturing the ligament usually is difficult. This facet of the technique (no definite end point) may make it difficult for the inexperienced clinician to perform. Aspiration of the syringe for possible inadvertent entry into the pudendal artery should be accomplished, and, if no blood is returned, 10 mL of local anesthetic solution should be injected in a fanlike fashion on the right and left sides. The successful performance of the pudendal block requires injection of the drug at least 10–12 minutes before episiotomy. Often in clinical practice, pudendal block is performed within 4–5 minutes of episiotomy, so the local anesthetic may not have adequate time to take effect.
Advantages and Disadvantages
Advantages of pudendal nerve block are its safety, ease of administration, and rapidity of onset of effect. Disadvantages include maternal trauma, bleeding, and infection; rare maternal convulsions due to drug sensitivity; occasional complete or partial failure; and regional discomfort during administration.
The pudendal perineal block, like any other nerve block, demands some technical experience and knowledge of the innervation of the lower birth canal. Nevertheless, in spite of a well-placed bilateral block, skip areas of perineal analgesia may be noted. The possible reason is that although the pudendal nerve of S2–S4 derivation does contribute to the majority of fibers for sensory innervation to the perineum, other sensory fibers also are involved. For example, the inferior hemorrhoidal nerve may have an origin independent from that of the sacral nerve and therefore will not be a component branch of the pudendal nerve. In this case, it must be infiltrated separately. In addition, the posterior femoral cutaneous nerve (S1–S3) origin may contribute an important perineal branch to the anterior fourchette bilaterally. In instances in which this nerve plays a major role in innervation, it must be blocked separately by local skin infiltration.
Two other nerves contribute to the sensory innervation of the perineum: the ilioinguinal nerve, of L1 origin, and the genital branch of the genitofemoral nerve, of L1 and L2 origin. Both of these nerves sweep superficially over the mons pubis to innervate the skin over the symphysis of the mons pubis and the labium majus. Occasionally, these nerves must also be separately infiltrated to provide optimal perineal analgesic effect. Thus it should be apparent that a simple bilateral pudendal nerve block may not be effective in many cases. For maximum analgesic effectiveness, in addition to a bilateral pudendal block, superficial infiltration of the skin from the symphysis medially to a point halfway between the ischial spines may be necessary. Thus a true perineal block may be regarded as a regional technique.
Either lumbar epidural or caudal epidural block should eradicate pain between the T10 and S5 levels for the second stage. All of these nerves are denervated because they all are derived from L1–S5 segments.
Palpate the ischial spines vaginally. Slowly advance the needle guide toward each spine. After placement is achieved, the needle is advanced through the guide to penetrate approximately 0.5 cm. Aspirate, and if the needle is not in a vessel, deposit 5 mL below each spine. This blocks the right and left pudendal nerves. Refill the syringe when necessary, and proceed in a similar manner to anesthetize the other areas specified. Keep the needle moving while injecting and avoid the sensitive vaginal mucosa and periosteum.
Withdraw the needle and guide approximately 2 cm and redirect toward an ischial tuberosity. Inject 3 mL near the center of each tuberosity to anesthetize the inferior hemorrhoidal and lateral femoral cutaneous nerves.
Withdraw the needle and guide almost entirely and then slowly advance toward the symphysis pubica almost to the clitoris, keeping approximately 2 cm lateral to the labial fold and approximately 1–2 cm beneath the skin. Injection of 5 mL of lidocaine on each side beneath the symphysis will block the ilioinguinal and genitocrural nerves.
Use of needle guide (“Iowa trumpet”) in pudendal anesthetic block. (Reproduced, with permission, from Benson RC. Handbook of Obstetrics & Gynecology. 8th ed. Los Altos, CA: Lange; 1983.)
If the procedure explained is carefully and skillfully done, only slight discomfort will be felt during the injections. Prompt flaccid relaxation and good anesthesia for 30–60 minutes can be expected. A summary of anesthetic approaches in labor is shown in Figure 24–6.
First-stage management in a primipara can be divided into 3 phases. Phase I (early labor) should be managed by simple reassurance and verbal commentary if the patient has received adequate antepartum education. An epidural may be performed once labor is well established. Phase II can be handled by a segmental epidural block, continued reassurance, a sedative–hypnotic drug, a narcotic, or a tranquilizer. Phase III, the accentuated phase of labor, can be handled by segmental epidural block, a combination tranquilizer and analgesic, or a caudal epidural block. However, use of reassurance and verbal commentary in conjunction with prepared childbirth methods may be adequate for some patients to tolerate the discomfort of phase III labor.
Prevention & Treatment of Local Anesthetic Overdosage
The correct dose of any local anesthetic is the smallest quantity of drug in the greatest dilution that will provide adequate analgesia. The pregnant patient is more likely to have an intravascular drug injection because of venous distention in the epidural space and may be more susceptible to the toxic effects of local anesthetics (Table 24–3). Injection of the drug into a highly vascularized area will result in more rapid systemic absorption than, for example, injection into the skin. To prevent too-rapid absorption, the operator can add epinephrine to produce local vasoconstriction and prolong the anesthetic. A final concentration of 1:200,000 is desirable, especially when a toxic amount is approached. Epinephrine is contraindicated in patients with increased cardiac irritability of medical or drug origin.
Table 24–3. Toxic Doses of Local Anesthetics Commonly Used in Obstetrics.
Treatment of local anesthetic overdosage manifested by central nervous system toxicity (a convulsion) is generally achieved effectively and without incident. However, the clinician must be aware of certain basic principles. These include the recognition of prodromal signs of a central nervous system toxic reaction and immediate treatment as required. A toxic central nervous system reaction to local anesthetics consists of ringing in the ears, diplopia, perioral numbness, and deep, slurred speech. An adequate airway must be maintained, and the patient should receive 100% oxygen, with respiratory assistance if necessary. Protection of the patient's airway and immediate injection of thiopental 50 mg or midazolam 1–2 mg usually stop the convulsion immediately. Succinylcholine was recommended in the past, but it is a potent neuromuscular relaxant that requires placement of an endotracheal tube with positive-pressure ventilation. Studies have indicated that cellular metabolism is greatly increased during convulsive episodes so that a definite increase in cellular oxygenation occurs—hence the use of a depressant selective for the hypothalamus and thalamus because these sites are the foci of irritation.
Local anesthetics-induced cardiotoxicity, especially by bupivacaine, is serious consequence when local anesthetic overdosage happens. The treatment of this complication is usually difficult, and the patient may suffer from arrhythmias (ventricular tachycardia) to even cardiac arrest. Intralipid intravenous infusion is recommended for bupivacaine-induced cardiotoxicity. Current guidelines suggest that 20% lipid emulsion initially be administered as a bolus of 1.5 mL/kg over 1 minute. After completion of the bolus, a continuous infusion of 0.25 mL/kg/min should be started. If the patient does not respond to the initial bolus, 1 to 2 additional boluses may be administered. The rate of the infusion may be increased to 0.5 mL/kg if there is persistent hypotension. The infusion should be continued until 10 min after the patient regains hemodynamic stability. An upper limit of 10 mL/kg is the recommended upper limit for administration in 30 minutes. Any patient that has had local anesthetic toxicity should be monitored for 12 hours after the event, as recurrence of cardiovascular instability has been shown to occur even after lipid administration.