Anatomy of Labor Pain
In the first stage of labor, pain travels via sympathetic nerve fibers (going through the inferior hypogastric plexus on the way to the sympathetic chain) that originate from the T10-L1 segments of the spinal cord (referred to the back as well as abdominal wall).
Pain for the second stage is transmitted via the pudendal nerve (S2-4)
Risks include maternal sedation and respiratory depression, loss of protective airway reflexes, and the risk of neonatal depression. Opiates cross the placenta, thus they should be used conservatively. In many patients they do not supply adequate analgesia, thus their use is declining [Stoelting R and Miller R. Basics of Anesthesia, 5th ed. p. 483, 2007]. Benzodiazepines also cross the placenta – diazepam is particularly dangerous as neonates have difficulty excreting it, thus it can last for up to a week [Stoelting R and Miller R. Basics of Anesthesia, 5th ed. p. 483, 2007]. Ketamine, up to 1 mg/kg in divided doses, can provide analgesia without loss of consciousness or airway reflexes, but is often used in combination with other analgesics.
Meperidine is probably the most commonly used opioid (25-50 mg IV), but it rapidly crosses the placenta and can remain in the neonate for up the three days [Kuhnert BR et al. Am J Obstet Gynecol 133: 909, 1979]. Fentanyl also crosses the placenta, although single doses of < 1 ucg/kg or appear to have no effects on fetal APGAR scores [Eisele JH et al. Anesth Analg 61: 179, 1982; Frolich et al. Can J Anaesth 53: 79, 2006]. Remifentanil has recently gained traction in the OB anesthesia setting, as it is ultra short-acting and is rapidly metabolized by the neonate. Beware the use of butorphanol as it is associated with a 75% incidence of fetal sinusoidal HR. [Hatjis CG and Meis PJ. Obstet Gynecol 67: 377, 1986]
Hydroxyzine and promethazine, both of which are phenothiazines, have a combination of anxiolytic and antiemetic effects but are notable in that they can decrease fetal beat-to-beat heart rate variability.
Midazolam has no fetal effects at doses < 0.02 mg/kg [Frolich et al. Can J Anaesth 53: 79, 2006], and diazepam at 2.5-10 mg PO has minimal effects on Apgar scores.
Ketamine has a unique combination of properties which make it useful in certain settings (ex. hemodynamic instability), but at doses > 1 mg/kg has been shown to increase the intensity of uterine contractions [Marx GF et al. Anesthesiology 50: 163, 1979] and has also been shown to cause neonatal depression.
Both nitrous oxide (ex. Etonox in U.K.) and volatile anesthetics can be used for labor analgesia, as well as for uterine relaxation.
Medications to Avoid
Morphine (higher incidence of neonatal depression);
The advantages of epidural analgesia include avoidance of hyperventilation, reduced maternal catecholamines, and the ability to alter the level of analgesia (including a T4 level if necessary for Cesarean section). Prior to initiating epidural analgesia, it is critical that resuscitation equipment be available. Extra-thecal placement is confirmed with 45 mg lidocaine or 7.5 mg bupivacaine (neither of which should produce a spinal if injected extra-thecally). For intravascular testing, negative aspiration was shown to be relatively reliable in testing a multiorifice epidural catheter, detecting 47 of 48 intravascular catheters in one study [Norris MC et al. Anesth Analg 88: 1076, 1999; FREE Full-text at Anesthesia & Analgesia]. Given the success of negative aspiration, the use of epinephrine for this purpose is discouraged in the pregnant patient, as false positives sometimes occur [Mulroy M and Glosten B. Anesth Analg 86: 923, 1998] and there is a theoretical possibility of reducing uteroplacental perfusion secondary to the alpha-agonist effects, as has been seen in pregnant ewes. [Hood DD et al. Anesthesiology 64: 610, 1986]
After placement of an epidural, both the fetus and the uterus should be monitored, as intra-thecal (but not epidural?) administration of opiates has been shown to produce uterine hyperactivity and subsequent fetal bradycardia in at least one case report [Friedlander JD et al. Reg Anesth 22: 378, 1997]. Nitroglycerin is an effective uterine relaxant in obstetric emergencies, as has been proven clinically as well as in animal models [Segal S et al. Anesth Analg 86: 304, 1998]
In the past, there was concern that premature placement of an epidural would prolong labor or increase the rate of Cesarean section. Most early randomized, controlled trials comparing the early to late epidurals were small and contained significant crossover [Eltzschig HK et al. N Engl J Med 348: 319, 2003]. Sharma’s 2004 metaanalysis, which included > 2000 patients, found no difference [Sharma SK et al. Anesthesiology 100: 142, 2004 metaanalysis], as did Segal et al’s study of sentinel events (institutions where epidural use quickly increased found no increase in Cesarean section rates) [Segal S et al. Am J Obstet Gynecol 183: 974, 2000]. The debate about the utility of early epidurals was put to rest by Wong’s group, who conducted a RCT of 750 nulliparous women, randomized to thecal fentanyl at first instance of pain vs. IV pain medicine at first instance (early group received epidural analgesia at the second request for analgesia but in the late group not until cervical dilatation achieved 4.0 cm or a third request for analgesia was made), the early epidural group offered the following advantages – “The median time from the initiation of analgesia to complete dilatation was significantly shorter after intrathecal analgesia than after systemic analgesia (295 minutes vs. 385 minutes, P<0.001), as was the time to vaginal delivery (398 minutes vs. 479 minutes, P<0.001). Pain scores after the first intervention were significantly lower after intrathecal analgesia than after systemic analgesia (2 vs. 6 on a 0-to-10 scale, P<0.001). The incidence of one-minute Apgar scores below 7 was significantly higher after systemic analgesia (24.0 percent vs. 16.7 percent, P=0.01)” [Wong CA et al. N Engl J Med 352: 655, 2005]. Placement of the epidural should NEVER be delayed simply in order to wait for an arbitrary level of cervical dilation. [Camann W. NEJM 352: 718, 2005]
Most anesthesiologists strive for a T10-L1 band of analgesia early in labor, which should cover the pain of uterine contractions and cervical dilation. As delivery approaches, increasing volumes are infused in order to cover the perineum. Local anesthetics are usually delivered as incremental boluses followed by a continuous infusion, which minimizes the risk of subarachnoid or intravascular delivery. Patient controlled epidurals have recently come into fashion, with a metaanalysis showing a reduction in anesthetic interventions, lower doses of local anaesthetic, and less motor block as compared to continuous dosing [van der Vyver M et al. Br J Anaesth 89: 459, 2002]. Ambulation can sometimes be preserved if the concentration of local anesthetics is lowered and opiates (ex. fentanyl 2 ucg/kg) are added.
25 or 26 ga. pencil point needle is used to reduce the risk of headache. Lidocaine, tetracaine, bupivacaine, and ropivacaine are commonly used. Women are often maintained in a seated position for up to 2 minutes in an effort to disproportionately anesthetize the perineal region. Note that a true “saddle block” would not completely relieve pain because the uterus itself would not be blocked, thus to obtain complete analgesia from a spinal increased doses are used and the sitting position is maintained for no more than 30 seconds (this often allows for a T10 level). In the late stages of labor, low-dose local anesthesia (ex 2.5-5 mg bupivacaine) combined with opiates (10-25 ucg fentanyl) can provide 90-120 minutes of analgesia without producing excessive muscle weakness.
Combined Spinal-Epidural Analgesia
Accomplished by placing a spinal needle through the epidural needle and injecting either local anesthetic, opioid, or both. If an opiate is injected alone, one can achieve ~ 2 hours of analgesia without motor or sympathetic blockade prior to starting the epidural (i.e., the patient can ambulate). The major advantage of the combined spinal-epidural technique is its rapid onset and increased reliability as compared to the epidural.
Pudendal blocks are often used for delivery. Paracervical blocks are now rarely used.
Esters (procaine, chloroprocaine, tetracaine) are metabolized by plasma cholinesterase, which reduces the risk of maternal toxicity as well as fetal transfer. Amides (lidocaine, bupivacaine, ropivacaine), by contrast, are metabolized by the liver, making them longer-acting as well as opening the possibility of fetal transfer (and “ion trapping”). Local anesthetic toxicity manifests in the neonate as muscular weakness.