CHAPTER 60 Obstetric Analgesia and Anesthesia
1 What are the most commonly used parenteral opioids for labor analgesia? Which side effects are of special concern to the parturient?
Table 60-1 summarizes commonly used parenteral opioids and their side effects. In general, intravenous medications help the parturient tolerate labor pain but do not provide complete analgesia. The incidence of side effects and efficacy of analgesia are dose dependent. Maternal sedation and nausea are common. Opioids easily cross the placenta and may cause a decrease in fetal heart rate variability. In addition, intravenous opioids may cause neonatal respiratory depression and neurobehavioral changes.
Patient-controlled analgesia (PCA) has been associated with greater patient satisfaction, less risk of maternal respiratory depression, less need for antiemetic use, and better pain relief despite lower drug doses. PCA is especially useful if epidural anesthesia is contraindicated or not available. The most experience has been gained with meperidine and fentanyl using the regimens noted in Table 60-1.
In most laboring women epidural analgesia provides the most effective pain relief and reduces maternal catecholamine levels, potentially improving uteroplacental perfusion. Epinephrine and norepinephrine secretion increase as a result of painful contractions and may prolong labor by decreasing uterine contractility through their β-agonist activity. Painful contractions may also lead to maternal hyperventilation and respiratory alkalosis, which in turn shifts the oxyhemoglobin dissociation curve to the left, decreasing delivery of oxygen to the fetus. Most important, the benefits of enhanced maternal well-being with adequate analgesia and the risk of posttraumatic stress symptoms caused by inadequate analgesia should not be underestimated.
Relief of labor-induced pain is sufficient reason for placing an epidural catheter. Analgesia can be readily converted to anesthesia by increasing the local anesthetic concentration, thus facilitating forceps or cesarean delivery. Labor analgesia benefits patients with hypertension and some types of cardiac disease (e.g., mitral stenosis) because it blunts the hemodynamic effects that accompany uterine contractions (increased preload, tachycardia, increased systemic vascular resistance, hypertension, and hyperventilation). The contraindications to epidural anesthesia include patient refusal, coagulopathy, uncontrolled hemorrhage, and infection at the site of needle introduction. Relative contraindications include systemic maternal infection, elevated intracranial pressure, prior spinal instrumentation with hardware, and certain neurologic diseases.
5 Discuss the importance of a test dose and suggest an epidural test dose regimen. When and why is this regimen used?
The test dose is used to diagnose subarachnoid or intravenous placement of the epidural catheter, thereby preventing total spinal anesthesia or systemic toxicity from local anesthetics. A common test dose is 3 ml of 1.5% lidocaine (45 mg) with 1:200,000 epinephrine (15 mcg). If the test dose of local anesthetic is administered, subarachnoid, motor, and sensory block will appear within 3 to 5 minutes. If the test dose is injected intravenously (IV), tachycardia results within 45 seconds (increase in heart rate of 30 beats/min) because of the epinephrine additive. Every time an epidural catheter is injected, it is a test dose of sorts; always monitor for signs of possible intravenous or subarachnoid local anesthetic injection. Aspiration of the catheter may not be adequate to identify all intrathecal or intravascular catheters. If there is any doubt in the practitioner’s mind about the exact location of the epidural catheter, the catheter should be removed and replaced.
6 What are the characteristics of the ideal local anesthetic for use in labor? Discuss the three most common local anesthetics used in obstetric anesthesia. How does epinephrine affect the action of local anesthetics?
The ideal local anesthetic for labor would have rapid onset of action, minimal risk of toxicity, minimal motor blockade with effective sensory blockade, and a minor effect on uterine activity and placental perfusion. Bupivacaine and ropivacaine are most commonly used for obstetric epidural analgesia. Lidocaine and chloroprocaine are most commonly used for obstetric surgical anesthesia.
Bupivacaine, an amide, is the most commonly used local anesthetic for obstetric analgesia. Pain relief after epidural injection is first noted by the patient after 10 minutes; 20 minutes is required to achieve peak effect. Analgesia usually lasts approximately 2 hours. Dilute solutions provide excellent sensory analgesia with minimal motor blockade. During early labor 0.125% bupivacaine or even lower concentrations is often adequate, whereas a 0.25% concentration may be required during the active phase of labor. Because bupivacaine is highly protein bound, its transplacental transfer is limited. Addition of epinephrine (1:200,000) to bupivacaine speeds its onset and lengthens its duration of action but also increases the intensity of motor blockade (which is not desirable in laboring patients).
Lidocaine is also an amide local anesthetic used in concentrations of 0.75% to 1.5% for sensory analgesia, but it crosses the placenta more readily than bupivacaine and produces more motor block than bupivacaine; thus it is rarely used outside the operating room. Analgesia lasts approximately 45 to 90 minutes and is usually apparent within 10 minutes.
2-Chloroprocaine is an ester local anesthetic. Onset of analgesia is rapid and lasts approximately 40 minutes; this short duration limits its usefulness in labor. Chloroprocaine 3% is frequently used to increase the anesthetic level quickly for cesarean section or instrumental vaginal delivery. Chloroprocaine has a very short half-life in maternal and fetal blood because it is metabolized by plasma esterase; thus it may be the safest of the commonly used local anesthetics.
Ropivacaine is slightly less potent than bupivacaine; therefore concentrations of 0.1% to 0.2% are used during labor. Onset, duration, and sensory block are similar to equipotent doses of bupivacaine. Motor blockade is slightly less than bupivacaine and may be an advantage. It has less cardiotoxicity (if inadvertently injected intravenously) because it binds less avidly to sodium channels of cardiac conduction tissue. Ropivacaine costs significantly more than bupivacaine.