Summary
The pregnant patient undergoes various physiologic changes which allow them to adapt to the stress of pregnancy, labor, and delivery. The physiologic changes of pregnancy are summarized in Table 24.1.
Physiologic Changes of Pregnancy
The pregnant patient undergoes various physiologic changes which allow them to adapt to the stress of pregnancy, labor, and delivery. The physiologic changes of pregnancy are summarized in Table 24.1.
Table 24.1 Summary of physiologic changes of pregnancy
MAC, minimum alveolar concentration; SV, stroke volume; HR, heart rate; CVP, central venous pressure; PCWP, pulmonary capillary wedge pressure; LVH, left ventricular hypertrophy; TV, tidal volume; RR, respiratory rate; ABG, arterial blood gas; FRC, functional residual capacity; RBC, red blood cell; GERD, gastroesophageal reflux disease; GFR, glomerular filtration rate; BUN, blood urea nitrogen.
Neurologic System
Elevated progesterone levels in pregnancy cause a decrease in minimum alveolar concentration (MAC) by up to 40%. Pregnant patients are also more sensitive to neuraxial local anesthetics and require a lower dose compared to nonpregnant patients.
Cardiovascular System
Cardiac output increases in pregnancy secondary to an increase in stroke volume and heart rate. Cardiac output is highest immediately after delivery of the fetus due to autotransfusion of uteroplacental blood as the evacuated uterus contracts. A mild decrease in blood pressure typically accompanies pregnancy due to a decrease in systemic vascular resistance related to the vasodilatory effects of progesterone. Central venous pressure (CVP) and capillary wedge pressure do not change during pregnancy. After about 20 weeks of gestation, the parturient may also experience supine hypotension syndrome when lying flat due to the gravid uterus compressing the inferior vena cava (IVC) and decreasing venous return. Management involves placing the patient in left uterine displacement, which displaces the uterus off the IVC and increases venous return.
Airway and Respiratory System
Pregnancy is associated with an increased incidence of difficult airway and mask ventilation, compared to the general population. As pregnancy progresses, there is an increase in airway edema and capillary engorgement, leading to higher Mallampati scores. Increased friability of the oral mucosa predisposes these patients to an increased risk of bleeding from airway manipulation. Pregnant patients also have decreased functional residual capacity and increased oxygen consumption, making them more prone to rapid desaturation after preoxygenation. Minute ventilation is increased primarily due to an increase in tidal volume.
Hematologic System
In pregnancy, plasma volume increases disproportionately compared to red blood cell mass, resulting in physiologic anemia of pregnancy. Normal hemoglobin values during pregnancy are between 10.5 and 12 g dL−1. Pregnancy is a hypercoagulable state, with increased production of all clotting factors, except factors XI and XIII. The anticoagulant factor protein S is decreased, and increased resistance to protein C develops. A significant increase in fibrinogen is also seen, with normal values in pregnancy of >400 g dL−1. Thrombocytopenia also develops due to dilution, as well as increased consumption.
Gastrointestinal System
Increased progesterone during pregnancy reduces lower esophageal sphincter tone and slows gastric emptying and gastrointestinal (GI) motility, predisposing pregnant patients to increased reflux symptoms and higher aspiration risk. Pregnant patients also have an increase in gastric volume and acidity, with a volume of >25 mL and pH <2.5, respectively. All pregnant patients beyond the first trimester are considered to have a full stomach and be at increased risk of regurgitation and aspiration; thus, they should all receive a nonparticulate antacid to reduce the risk of pneumonitis, should aspiration occur.
Stages of Labor and Anatomy of Labor Pain
Labor is divided into three stages:
The first stage of labor commences with maternal perception of regular uterine contractions and ends with complete cervical dilation of approximately 10 cm, through which the fetus can be expelled. Pain during this stage is primarily visceral and is transmitted via afferent C fibers accompanying sympathetic nerve fibers to enter the spinal cord at T10–L1 spinal segments. Pain during this stage is caused by uterine contractions, accompanied by dilation of the cervix and stretching of the lower uterine segment.
The second stage of labor commences with complete cervical dilation and ends with delivery of the fetus. Pain during this stage is primarily somatic, caused by distension of the vaginal vault and perineum, and is transmitted via the pudendal nerve to the S2–S4 spinal segments.
The third stage of labor involves delivery of the placenta. Pain during this stage is also transmitted via sacral somatic fibers via the pudendal nerve (S2–S4).
Methods of Labor Analgesia
Pharmacologic
Systemic opioids can be used for labor analgesia. However, the risk of maternal sedation, respiratory depression, and loss of protective airway reflexes, and the proximity to the time of delivery warrant their judicious use. Maternal side effects of opioids include nausea/vomiting, pruritus, and decreased gastric motility. All opioids readily cross the placenta and can have effects on the neonate, including decreased fetal heart rate variability and dose-related neonatal respiratory depression after birth.
Fentanyl is a highly lipid-soluble synthetic opioid, commonly used for labor analgesia due to its short half-life and no active metabolites. Doses of 50–100 μg hr−1 can be used with no significant effect on neonatal Apgar scores and respiratory effort compared to mothers not receiving fentanyl.
Morphine is seldomly used for labor analgesia due to significant maternal sedation and accumulation of its active metabolite (morphine-6-glucuronide). Maternal side effects include histamine release causing pruritus and rash, and an increased risk of respiratory depression.
Remifentanil patient-controlled analgesia (PCA), although less efficacious than epidural analgesia, offers superior pain relief, with a decreased risk of neonatal side effects compared to other intravenous opioid analgesics. Metabolism depends on plasma and tissue esterases, and it is rapidly metabolized in both mother and fetus. The primary risk associated with use of remifentanil is respiratory depression, and thus careful oxygenation and ventilation monitoring are required throughout treatment.
Nitrous oxide can be used for labor analgesia and is typically administered via self-inhalation just before and during contractions in a 50:50 blended mixture with oxygen. Although inferior to epidural analgesia, it is a safe and effective method to provide labor analgesia without resultant hypoxia, unconsciousness, or loss of protective airway reflexes in the parturient who desires a less invasive approach or has contraindications to neuraxial techniques. Its use requires appropriate scavenging equipment in labor and delivery rooms to prevent occupational exposure.
Regional Techniques
Regional techniques provide excellent analgesia, with minimal depressant effects on the mother and fetus. Commonly employed regional techniques in obstetric anesthesia include central neuraxial blocks – spinal, epidural, combined spinal/epidural (CSE). Paracervical, pudendal, and lumbar sympathetic blocks are less commonly employed due to unfamiliarity with the technique and an increased risk of both maternal and fetal side effects. During the first stage of labor, visceral pain impulses from T10 to L1 must be blocked. In the second stage of labor, analgesia must be extended to also include somatic pain impulses from S2 to S4.
Neuraxial analgesia is the most reliable and effective method for reducing pain during labor, and is the only form of analgesia that provides complete analgesia for both stages of labor. Contraindications to neuraxial block include patient refusal or inability to cooperate, increased intracranial pressure secondary to mass lesion, uncorrected maternal hypovolemia or shock, infection at the site of needle insertion, and coagulopathy.
Lumbar Epidural Analgesia
Lumbar epidural analgesia is a safe, effective, and versatile technique that can be used to provide pain relief during labor and vaginal delivery, while also allowing flexibility to make the block denser, prolong its duration, and convert to epidural anesthesia if operative delivery is required. Epidural analgesia is typically initiated after an epidural catheter is inserted into the epidural space between the L2 and L5 intervertebral space. Analgesia is typically maintained with a continuous infusion of local anesthetic and opioid through the catheter. Addition of opioids, such as fentanyl or sufentanil, to the local anesthetic solution allows the administration of a more dilute local anesthetic to minimize side effects and complications from each drug. Long-acting amide local anesthetics, such as bupivacaine 0.25% or ropivacaine 0.1%, are typically administered as they provide excellent sensory analgesia while also minimizing motor blockade.
Spinal Analgesia
In spinal analgesia, a single injection in the subarachnoid or intrathecal space for labor analgesia is quick to perform and offers the advantage of a fast and reliable onset of neural blockade. However, because of its limited duration of action and unpredictable duration of labor, this technique may not be useful for most laboring women. A single-shot spinal injection is typically utilized for labor analgesia in a parturient who is unable to hold still, to facilitate placement of an epidural, and is usually reserved for when the duration of labor can be reasonably estimated, such as in multiparous women with advanced dilation or in the second stage of labor.
Continuous Spinal Analgesia
Continuous spinal analgesia with a spinal catheter can be utilized in the case of accidental dural puncture. However, the high incidence of postdural puncture headache (PDPH) precludes the elective placement of spinal catheters through epidural needles in most patients. When utilized, continuous spinal analgesia provides excellent analgesia and the ability to convert to surgical anesthesia in the event cesarean delivery is required.
Combined Spinal/Epidural
CSE has become an increasingly popular technique in obstetric anesthesia for labor analgesia. It combines the rapid, reliable onset of profound labor analgesia with minimal motor blockade from the spinal injection, with the flexibility and duration associated with a continuous epidural technique. The needle-through-needle technique is most commonly utilized, which involves identification of the epidural space, followed by insertion of a 25–27 G spinal needle into the intrathecal space. Upon confirmation of free-flowing cerebrospinal fluid (CSF), an opioid or a local anesthetic, or a combination of both, is administered. The spinal needle is removed, and a catheter is threaded into the epidural space for continuous analgesia.
Other Regional Techniques
Paracervical, Lumbar Sympathetic, and Pudendal Nerve Blocks
Lumbar sympathetic, paracervical, and pudendal nerve blocks can be used to block specific nerve plexuses for pain relief during labor. Unfavorable risk–benefit profiles of these blocks have limited their use when other options are available for labor analgesia.
Paracervical Block
Paracervical block can be used to provide analgesia for the first stage of labor. Bilateral paracervical blocks interrupt transmission of pain impulses from the uterus and cervix. Although effective for pain relief during the first stage of labor, its use has fallen out of favor due to its association with fetal bradycardia and maternal local anesthetic toxicity.
Lumbar Sympathetic Block
Lumbar sympathetic block can also be used to provide analgesia during the first stage of labor, as it interrupts painful transmission of cervical and uterine impulses. Its use has fallen out of favor due to difficulty in the block technique and the risk of intravascular injection.
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