30 OBSTETRICS Flashcards
1
Q
- How do the maternal intravascular fluid, plasma, and erythrocyte volumes change
during pregnancy?
A
- During pregnancy the maternal intravascular fluid volume increases from its
prepregnancy volume. The increase in intravascular volume begins in the first
trimester of pregnancy. By term, the intravascular fluid volume has increased by
about 35% above the prepregnancy state. The plasma volume increases by
approximately 45% at term. The erythrocyte volumein the pregnant patientincreases
by approximately 20%. Because the plasma volume increases by over twice as
much as the erythrocyte volume, the woman has a relative physiologic anemia.
That is, the hematocrit of the pregnant patient is relatively less than her prepregnancy
state. This is termed the physiologic anemia of pregnancy. (515, Table 33-1)
2
Q
- How does the coagulation status change during pregnancy?
A
- The pregnant woman at term is in a hypercoagulable state secondary to increases in factors I, VII, VIII, IX, X, and XII, and decreases in factors XI, XIII, and Antithrombin III.
- This results in an approximately 20% decrease in prothrombin time (PT) and partial thromboplastin time (PTT). Platelet count may remain normal or decrease 10% by term.
3
Q
- What is the average maternal blood loss during the vaginal delivery of a newborn?
What is the average maternal blood loss during cesarean delivery?
A
- The average maternal blood loss during vaginal delivery of a newborn is 300 to 500mL.
- The average maternal blood loss during the delivery of a newborn by cesarean delivery is 800 to 1000 mL, but blood loss during a cesarean delivery is greatly variable.
- The increase in intravascular fluid volume and the hypercoagulable state of the mother help to counter the blood losses incurred during this time.
- The contracted uterus after either type of delivery creates an autotransfusion of approximately 500 mL of blood, which decreases the overall effect of the blood loss on the mother.
4
Q
- How does the maternal cardiac output change from nonpregnant levels?
A
4.
- Maternal cardiac output increases 10% by the tenth week of gestation, and at term pregnancy increases by approximately 40% to 50% of its prepregnancy value.
• Cardiac output is equal to the product of stroke volume and heart rate. The increase in cardiac output is primarily due to an increase in stroke volume. The increase in heart rate during pregnancy is less and is therefore only a minimal contributor to the increase in cardiac output.
• Labor is associated with further increases in cardiac output with output above prelabor values by 10% to 25% during the first stage and 40% in the second stage. The greatest increase in cardiac output occurs just after delivery, when it increases by as much as 80% above prelabor values. This is the maximal change in cardiac output in the woman.
• Cardiac output decreases substantially toward prepregnant values by 2 weeks postpartum.
5
Q
- In an uncomplicated pregnancy, what changes occur in blood pressure, systemic vascular resistance, and central venous pressure?
A
- The systolic blood pressure of the woman having an uncomplicated pregnancy does not exceed her prepregnancy blood pressure and typically decreases secondary to a 20% reduction in systemic vascular resistance at term.
- Systolic, mean, and diastolic blood pressure may all decrease 5% to 15% by 20 weeks
gestational age and gradually increase toward prepregnant values as the pregnancy progresses towards term. - Central venous pressure does not change during pregnancy despite the increased plasma volume because venous capacitance increases.
6
Q
- What is the supine hypotension syndrome?
What symptoms accompany the syndrome?
A
- Supine hypotension syndrome, as the name implies, is the decrease in blood pressure seen when the pregnant patient lies in the supine position after midgestation.
- The supine hypotension syndrome occurs because of a decrease in cardiac output by approximately 10% to 20%.
- When the pregnant woman is in the supine position, the gravid uterus compresses the inferior vena cava, resulting in decreased venous return and decreased preload for the heart.
- Symptoms that accompany the hypotension include diaphoresis, nausea, vomiting, and possible changes in cerebration. Symptoms must be present for the patient to be considered susceptible to supine hypotension syndrome.
7
Q
- What compensatory mechanisms do most women have that prevents them from experiencing supine hypotension syndrome and how can maternal hypotension be minimized?
A
- Most pregnant women, when lying in the supine position, are able to compensate for the possible decrease in blood pressure that results from the compression of the inferior vena cava by the gravid uterus.
- One compensatory mechanism includes maintaining venous return by diverting blood flow from the inferior vena cava to the paravertebral venous plexus. The blood then goes to the azygos vein and returns to the heart via the superior vena cava. Dilation of the epidural veins may make unintentional intravascular placement of an epidural catheter more likely. A “test dose” is given before dosing an epidural catheter to decrease the likelihood of an unrecognized intravascular placement before initiating
neuraxial blockade.
• Another compensatory mechanism is an increase in peripheral sympathetic nervous system activity. This increases peripheral vascular tone and helps to maintain venous return to the heart. Regional anesthesia, however, can interfere with these compensatory mechanisms by causing sympathetic nervous system blockade, rendering the pregnant woman at term more susceptible to decreases in blood pressure. The gravid uterus can also compress the lower abdominal aorta and lead to arterial hypotension in the lower extremities, but maternal symptoms or decreases in systemic blood pressure as measured in the arms are often not reflective of this decrease. The major clinical significance of the aortocaval compression is the decrease in placental and uterine blood flow that results.
• The decrease in blood flow through the uteroplacental unit leads to a decrease
in blood flow to the fetus. The aortocaval compression can be minimized by having the woman lie in the lateral position. Uterine displacement can also be used, typically with displacement being to the left because the inferior vena cava sits just to the right of and anterior to the spine. Left uterine displacement is easily accomplished by table tilt or the placement of a wedge or folded blanket under the right hip, elevating the hip by 10 to 15 cm. (516-517,
Figures 33-1 and 33-2)
8
Q
- What are some aspects of the upper airway that undergo physiologic change in pregnancy?
What are the clinical implications of these changes?
A
- There is significant capillary engorgement of the mucosal layer of the upper airways and increased tissue friability during pregnancy.
- There is increased risk of obstruction from tissue edema and bleeding with instrumentation of the upper airway.
- Additional care is needed during suctioning, placement of airways (avoid nasal instrumentation if possible), direct laryngoscopy, and intubation.
- In addition, because the vocal cords and arytenoids are often edematous, smaller-sized cuffed endotracheal tubes (6.0 to 6.5 mm internal diameter) may be a better selection for intubation of the trachea for these patients.
- The presence of preeclampsia, upper respiratory tract infections, and active pushing with associated increased venous pressure further exacerbate airway tissue edema, making both intubation and ventilation more challenging.
9
Q
- How is minute ventilation changed during pregnancy from nonpregnant levels?
How does the resting maternal PaCO2 change as a result of the change in minute
ventilation?
A
9.
- During pregnancy, the minute ventilation increases to about 50% above prepregnancy levels.
- This change occurs in the first trimester of pregnancy and remains elevated for the duration of the pregnancy.
- An increase in tidal volume is the main contributor to the increase in minute ventilation seen, with only small increases in respiratory rate from prepregnancy.
- During the first trimester, as a result of the increase in minute ventilation, the resting maternal PaCO2 decreases from 40 mm Hg to about 30 or 32 mm Hg.
- Arterial pH, however, remains only slightly alkalotic (7.42 to 7.44) secondary to increased renal excretion of bicarbonate ions.
10
Q
- How do the binding characteristics of hemoglobin change during
pregnancy?
A
- Maternal hemoglobin has less of an affinity for binding oxygen during pregnancy, which facilitates downloading oxygen to the tissues and the fetus.
- The hemoglobin dissociation curve is thus shifted to the right with the P-50 increasing from 27 to approximately 30 mm Hg.
11
Q
- What are the changes in maternal lung volumes that occur with pregnancy? What
are the anesthetic implications of these changes?
A
- Maternal lung volumes start to change in the second trimester. This is a result of
mechanical compression by the gravid uterus as it enlarges and forces the
diaphragm cephalad. This leads to a decrease in the woman’s functional residual
capacity by approximately 20% at term. This decrease is a result of approximately
equal decreases in both the expiratory reserve volume and residual lung volume.
This can result in a functional residual capacity less than closing capacity and
increased atelectasis in the supine position. There is no significant change in
vital capacity seen during pregnancy. The rates of change in the alveolar
concentration of inhaled anesthetics during induction and emergence from
anesthesia are both increased secondary to the increase in minute ventilation and
decrease in functional residual capacity. Clinically this, along with the decrease in MAC that accompanies pregnancy, leads to a more rapid achievement of an anesthetized state than when the patient is not pregnant. Apnea in the woman rapidly leads to arterial hypoxemia. There are at least two explanations for this.
- First, a decreased functional residual capacity and subsequent decreased oxygen
reserve are contributors.
- Second, aortocaval compression and decreased venous return leading to decreases in cardiac output may also contribute. The decrease in cardiac output would lead to an increase in overall oxygen extraction and therefore decrease the level of oxygenation of blood returning to the heart.
- Third, maternal oxygen consumption is increased by 20% at term, with further ncreases noted during labor. Because of the rapid decrease in maternal PaO2 with apnea or hypoventilation, preoxygenation with 100% O2 for 3 minutes or four maximal breaths over the 30 seconds just prior to the induction of emergent general anesthesia is recommended.
12
Q
- How does maternal PaO2 change during pregnancy?
A
- Maternal PaO2 changes during the progression from early gestation to term.
- Early in gestation, the PaO2 in the mother is slightly increased over prepregnancy values to over 100 mm Hg breathing room air.
- This is secondary to maternal hyperventilation and subsequent decreased PaCO2 during this time.
- As the pregnancy progresses, the PaO2 is normal or even slightly decreased. The decrease in PaO2 during the course of pregnancy likely results from airway closure and associated intrapulmonary shunt.
13
Q
- What are the gastrointestinal changes in pregnancy that render the woman vulnerable to regurgitation of gastric contents? What clinical implication does this have?
A
13.
- There are at least four gastrointestinal changes in pregnancy that render the woman significantly vulnerable to the regurgitation of gastric contents beyond midgestation.
- The enlarged uterus acts to displace the stomach and pylorus cephalad from its usual position.
- This repositions the intraabdominal portion of the esophagus into the thorax and leads to relative incompetence of the physiologic gastroesophageal sphincter.
- The tone of the gastroesophageal sphincter is further reduced by the higher progesterone and estrogen levels of pregnancy. Gastric pressure is increased by the gravid uterus.
- Gastrin secreted bythe placenta stimulates gastric hydrogen ion secretion. The pH of the woman’s gastric fluid is predictably low as a result. Reflux and subsequent esophagitis are common during pregnancy.
- During labor, gastric emptying is delayed and intragastric fluid volume tends to be increased as a result. (Epidural analgesia alone does not alter gastric emptying.) Anxiety, pain, and the administration of opioids can further decrease gastric emptying. Clinically, this means that the pregnant patient must always be treated as if she has a full stomach. - Regardless of what amount of time has elapsed since her last ingestion of solids, she is at increased risk of regurgitation and aspiration of gastric contents. This includes the routine use of nonparticulate antacids, rapid sequence induction, cricoid pressure, and cuffed endotracheal intubation as part of general anesthesia induction sequence in a pregnant woman after approximately 20 weeks gestational age.
- Pharmacologic interventions that are recommended in the woman to help minimize the risks of pulmonary aspiration are aimed at decreasing the severity of acid pneumonitis should aspiration occur
- The administration of antacids to pregnant women before the induction of anesthesia is common practice. This is as an attempt to increase the pH of gastric contents. Sodium citrate is the antacid commonly used. Of note, the antacid must be nonparticulate, because aspiration of particulate matter contained in some antacids is in itself a hazard.
- Metoclopramide can be useful for decreasing the gastric fluid volume of pregnant women in active labor who require general anesthesia. It can significantly decrease gastric volume in as little as 15 minutes, although gastric hypomotility associated with prior opioid administration reduces the effectiveness of metoclopramide.
- H2 receptor antagonists increase gastric fluid pH in pregnant women approximately one hour after administration without producing adverse effects, and are additionally recommended by some.
14
Q
- How do the epidural and subarachnoid spaces change in pregnancy? How is the sensitivity to local anesthetics different in the pregnant versus nonpregnant patient? How are the dosing requirements for neuraxial anesthesia affected by these changes?
A
- During pregnancy, both the epidural and intrathecal spaces are decreased in volume from their prepregnancy state. This occurs because of the engorgement of epidural veins and the increased intraabdominal pressure resulting from the progressive enlargement of the uterus. However, CSF pressure does not increase with pregnancy.
- The decrease in the epidural space decreases the required volume of local anesthetic necessary to achieve a particular level of anesthesia by facilitating its spread in the epidural space. The decreased intrathecal space also facilitates the spread of spinal anesthetic and decreases the dose required from prepregnancy values.
- There appears to be an increased sensitivity to local anesthetics by women who are pregnant. The decreased local anesthetic requirement in pregnant women appears to have a biochemical component to it as well as a mechanical one.
- This is based on the observation of decreased neuraxial local anesthetic doses as early as the first trimester, before significant uterine enlargement
15
Q
- How do renal blood flow and glomerular filtration rate change in pregnancy?
At what gestational month of pregnancy is this change at a maximum? How does this affect the normal upper limits of creatinine and blood urea nitrogen in pregnant patients?
A
- Renal blood flow and glomerular filtration rate in the woman are both increased.
- By the third month of pregnancy the increase is about 50% to 60%.
- This results in a decrease in what is considered the normal upper limit of both the blood urea nitrogen and serum creatinine concentrations during
16
Q
- Does hepatic blood flow change during pregnancy?
How are plasma protein concentrations and plasma cholinesterase activity altered by pregnancy?
A
- Liver blood flow does not change significantly with pregnancy.
- Plasma protein concentrations are reduced in pregnancy secondary to dilution. The decreased albumin levels can create increased blood levels of highly protein bound drugs.
- Plasma cholinesterase, or pseudocholinesterase, decreases in activity by about 25% during pregnancy. This decrease in activity is first noted by about the tenth week of gestation and persists for as long as 6 weeks postpartum.
- There is no clinical manifestation of this change in plasma cholinesterase activity, and n
17
Q
- How are maternal and fetal blood delivered to the placenta?
A
- The function of the placenta is to unite maternal and fetal circulations. The union allows for the physiologic exchange of nutrients and waste.
- Maternal blood is delivered to the placenta by the uterine arteries. Fetal blood is delivered to the placenta by the two umbilical arteries. Nutrient rich blood is returned from the placenta to the fetus via a single umbilical vein.
- The two most important determinants of placental function are uterine blood flow and the characteristics of the substances to be exchanged across the placenta.
18
Q
- What is uterine blood flow (UBF) at term?
A
18.
- Uterine blood flow increases during gestation from approximately 100 mL/min before pregnancy to 700 mL/min at term.
- Adequate uterine blood flow must be maintained to ensure placental circulation is adequate and therefore guarantee fetal well-being.
- About 80% of the uterine blood flow perfuses the placenta and 20% supports the myometrium.
19
Q
- What are the determinants of UBF?
A
- During pregnancy uterine blood flow has limited autoregulation, and the uterine vasculature is essentially maximally dilated under normal pregnancy conditions.
- Uterine blood flow is proportional to the mean blood perfusion pressure to the uterus and inversely proportional to the resistance of the uterine vasculature.
- Decreased perfusion pressure can result from systemic hypotension secondary to hypovolemia, aortocaval compression, or decreased systemic resistance from either general or neuraxial anesthesia.
- Uterine blood flow also decreases with increased uterine venous pressure. This can result from vena caval compression (supine position), uterine contractions (particularly uterine tachysystole as may occur with oxytocin administration), or significant abdominal musculature contraction (Valsalva during pushing).
- Additionally, extreme hypocapnia (PaCO2 <20 mm Hg) associated with hyperventilation secondary to labor pain can reduce UBF to the point of fetal hypoxemia and acidosis.
- Epidural or spinal anesthesia does not alter UBF as long as maternal hypotension is avoided.
- Endogenous catecholamines induced by stress or pain and exogenous vasopressors have the capability of increasing uterine arterial resistance and decreasing UBF, although both ephedrine or phenylephrine are used clinically in moderate amounts to maintain uterine perfusion pressure when the pregnant patient is hypotensive.
20
Q
- What factors affect the transfer of oxygen between the mother and fetus?
A
- Transfer of oxygen to the fetus is dependent on a variety of factors including the ratio of maternal to fetal umbilical blood flow, the oxygen partial pressure gradient, the respective hemoglobin concentrations and affinities, the placental diffusing capacity, and the acid-base status of the fetal and maternal blood (Bohr effect)
21
Q
- What factors affect placental exchange of drugs and other substances?
What is the most reliable way to minimize fetal transfer of a drug?
A
- Transfer of drugs and other substances less than 1000 Da from the maternal circulation to the fetal circulation and vice versa is primarily by diffusion.
- Some factors that affect the exchange of substances from the maternal circulation to the fetus include the concentration gradient of the substance across the placenta, maternal protein binding, molecular weight, lipid solubility, and degree of ionization of the substance.
- The most reliable way to minimize the amount of drug that reaches the fetus is by minimizing the concentration of the drug in the maternal blood.
22
Q
- What common drugs used in anesthesia have limited ability to cross the placenta?
Which readily cross the placenta?
FETAL UPTAKE AND DISTRIBUTION OF DRUGS
A
- Nondepolarizing neuromuscular blocking drugs have a high molecular weight and
low lipid solubility. These two characteristics together limit the ability of nondepolarizing neuromuscular blocking drugs to cross the placenta. - Succinylcholine is highly ionized, preventing it from diffusing across the placenta despite its low molecular weight.
- Additionally, both heparin and glycopyrolate have significantly limited placental transfer.
- Placental transfer of barbiturates, local anesthetics, and opioids is facilitated by the relatively low molecular weights of these substances.
- Nondepolarizing neuromuscular blocking drugs have a high molecular weight and
23
Q
- How does the pH of fetal blood affect the transfer of drugs? What is ion trapping?
A
- Fetal blood is slightly more acidic than maternal blood, with a pH about 0.1 unit less than maternal blood pH.
- The lower pH of fetal blood facilitates the fetal uptake of drugs that are basic.
- Weakly basic drugs, such as local anesthetics and opioids that cross the placenta in the nonionized state, become ionized in the fetal circulation.
- This results in an accumulated concentration of drug in the fetus for two reasons.
- First, once the drug becomes ionized it cannot readily diffuse back across the placenta. This is known as ion trapping.
- Second, a concentration gradient of nonionized drug is maintained between the mother and the fetus.
- In the case of lidocaine administration, this may mean that if the fetus was distressed and acidotic and lidocaine was given in sufficient doses to the woman, lidocaine may accumulate in the fetus.
24
Q
- What characteristics of the fetal circulation are protective against the distribution of large doses of drugs to vital organs?
A
- First, about 75% of the blood that is coming to the fetus via the umbilical vein passes through the liver. This allows for a significant amount of metabolism of the drug to take place before going to the fetal arterial circulation and delivery to the heart and brain.
- Second, drug contained in the umbilical vein blood enters the inferior vena cava via the ductus venosus. This blood is diluted by drug-free blood returning from the lower extremities and pelvic viscera of the fetus, resulting in a decrease in the concentration of the drug that is in the inferior vena cava.
- In addition, despite decreased liver enzyme activity in comparison to adults, fetal/neonatal enzyme systems are adequately developed to metabolize most drugs.
25
Q
- Name the stages of labor and what events define each stage.
A
- Labor is a continuous process divided into three stages.
1. The first stage is the onset of labor until the cervix is fully dilated. - This first stage is further divided into the latent and active stage. The active phase begins at the point when the rate of cervical dilation increases (often between 3 and 5 cm.).
2. The second stage of labor begins when the cervix is fully dilated and ends when the neonate is born. - This stage is referred to as the “pushing and expulsion” stage.
3. The third and final stage begins once the neonate is delivered and is completed when the placenta is delivered.
26
Q
- What is an “active phase arrest”? What is an “arrest of descent”?
A
- If a woman fails to dilate adequately in the active phase despite pharmacologic interventions, it is considered active phase arrest and will result in cesarean delivery.
- During the second stage of labor, the patient may not be able to “push” the neonate out of the pelvis. This is termed arrest of descent. If the neonate is low enough in the pelvis, the obstetrician can perform an instrumented vaginal delivery via vacuum or forceps, otherwise a cesarean delivery is required.
27
Q
- In the first stage of labor, describe the associated sensory levels and where the end organ afferent nerve impulses are initiated.
A
- During the first stage of labor (cervical dilation), the majority of painful stimuli result in afferent nerve impulses from the lower uterine segment and cervix. This pain is typically visceral in nature (dull, aching, poorly localized).
- The nerve cell bodies are located at the dorsal root ganglia of T10 to L1 level.
28
Q
- In the second stage of labor, describe the associated sensory levels and where the and organ afferent nerve impulses are initiated.
A
- In the second stage of labor, afferents innervating the vagina, perineum, and pelvic floor travel primarily via the pudendal nerve to the dorsal root ganglia of the S2 to S4 levels.
- This pain is typically somatic in nature (sharp and well localized).
29
Q
- For each stage of labor, describe which analgesic techniques benefit the pregnant
woman and why.
A
- The first and second stages of labor can employ neuraxial techniques such as an epidural or combined spinal epidural (CSE).
- Although used less frequently, a paracervical block can also be used during the first stage of labor.
- A single shot spinal or pudendal block can be used for the second stage of labor.
- Typically, the obstetrician performs both the paracervical and pudendal block.
30
Q
- Describe the different nonpharmacologic techniques used for labor and the efficacy of each.
A
- A variety of nonpharmacologic techniques for labor analgesia exist.
- These include hypnosis, the breathing techniques described by Lamaze, acupuncture,
acupressure, the LeBoyer technique, transcutaneous nerve stimulation, massage, hydrotherapy, vertical positioning, presence of a support person, intradermal water injections, biofeedback, and many others. - A meta-analysis reviewing the effectiveness of a support individual (e.g., doula, family member) noted that women with a support individual used less pharmacologic analgesia methods, had a decreased length of labor, and a lower incidence of cesarean delivery.
- In a 2006 retrospective survey, nonpharmacologic methods of tub immersion and massage were rated more or equally effective in relieving pain compared to use of opioids in labor.
31
Q
- List the different systemic medications used for labor analgesia and their active metabolites, if any.
A
31.
- Fentanyl is commonly used for labor analgesia because it is short acting with no active metabolite.
- Morphine was used more frequently in the past, but currently is rarely used. Its active metabolite (morphine-6-glucuronide) has a prolonged duration of analgesia, the half-life is longer in neonates compared to adults, and it produces significant maternal sedation. - Meperidine is still one of the most frequently used opioids worldwide. Maternal half-life of meperidine is 2 to 3 hours with half-life in the fetus and newborn significantly greater (13 to 23 hours) and more variable.
- In addition, meperidine is metabolized to an active metabolite (normeperidine) that can significantly accumulate after repeated doses. With increased dosing and shortened time between doses, there are increased neonatal risks.
32
Q
- What is “morphine sleep”?
A
- In latent labor, obstetrical providers may use intramuscular morphine combined with phenergan for analgesia, sedation, and rest termed “morphine sleep.”
- This produces analgesia for approximately 2.5 to 4 hours with an onset of 10 to
20 minutes.
33
Q
- How is remifentanil used as a labor analgesic and what are the indications for its use?
A
- Remifentanil patient controlled analgesia (PCA) has been considered for women who have contraindications to neuraxial blockade.
- Although pain was improved with remifentanil compared to those without pharmacologic intervention, a randomized control trial comparing epidural analgesia to remifentanil PCA had overall pain scores that were lower in the epidural group.
- More sedation and hemoglobin desaturation were noted during remifentanil analgesia, but there was no difference between groups in fetal and neonatal outcome measures.
34
Q
- Are benzodiazepines used in pregnancy and if so, when?
A
- Diazepam is used in obstetrics; however, it will readily cross the placenta and yield roughly equal maternal and fetal blood levels.
- Since neonates have a limited ability to excrete the active metabolites, use of diazepam has been associated with neonatal respiratory depression.
- Midazolam is a shorter acting anxiolytic, but also rapidly crosses the placenta, and large induction doses have been associated with profound neonatal hypotonia.
- Their use has been controversial given their amnestic properties. In specific obstetrical situations, use of midazolam in small doses may be beneficial.
35
Q
- When is ketamine used in labor and delivery and what additional benefits does it provide for pain control?
A
- During labor ketamine can be administered for urgent situations in divided IV doses (10 to 20 mg) totaling less than 1 mg/kg.
- Ketamine in these doses willprovide rapid analgesia, and is useful for vaginal delivery and episiotomy. It has a rapid onset (30 seconds) and minimal duration of action (<5 minutes).
- When given with opioids, ketamine can act synergistically to reduce the amount of opioid necessary to produce adequate analgesia.
36
Q
- List the different types of neuraxial analgesia?
A
- Neuraxial analgesia typically involves the administration of local anesthetics, often with the coadministration of opioid analgesics or other adjunctive analgesics.
- Bupivacaine and ropivacaine are the most commonly used local anesthetics.
- Epidural, spinal, and combined spinal and epidural (CSE) are the techniques typically used for labor analgesia.
37
Q
- When would you use each type of neuraxial analgesia for labor pain?
A
- Anytime a woman in labor without contraindications requests neuraxial analgesia,
regardless of the stage of her labor, a neuraxial blockade may be placed. - The timing of placement does not depend on an arbitrary cervical dilation.
- A single shot spinal analgesic has a finite analgesic duration depending on the local
anesthetic and this should be taken into account when using this technique. - For example, a single shot spinal analgesic is ideal if an obstetrician is performing an instrumented vaginal delivery in a woman without previous neuraxial block.
- The other neuraxial techniques employ catheter delivery techniques and can be extended throughout the length of the labor.
- Anytime a woman in labor without contraindications requests neuraxial analgesia,
38
Q
- Should laboring women remain “nothing per oral (NPO)” after placement of an epidural or combined spinal and epidural (CSE)?
A
- Otherwise healthy women in labor may have modest amounts of clear liquids.
- However, in a complicated labor (e.g., by morbid obesity, difficult airway, concerning
fetal status), the decision to restrict oral intake should be individualized.
39
Q
- What is a walking epidural and what are the associated risks?
A
- Placing opioids alone in the epidural or intrathecal space (if placing a CSE) is considered a walking epidural.
- The local anesthetic solution, while providing analgesia, has minimal effects on sympathetic or motor nerves. This allows the woman to ambulate after tests for motor blockade indicate that she is not at risk of falling. Even so, the woman should be closely monitored and ideally should only ambulate when accompanied because proprioception and balance may be impaired.
- Use of neuraxial opioids is associated with dose-related maternal side effects including pruritus, sedation, and nausea. In addition, the administration of intrathecal opioids can result in fetal bradycardia independent of hypotension.
- The mechanism for fetal bradycardia is unclear, but may result from uterine hyperactivity following the rapid onset of analgesia.
40
Q
- What drugs are used or being evaluated as adjuvant neuraxial drugs for labor analgesia?
A
- Clonidine and neostigmine are epidural adjuncts to local anesthetics and have been evaluated for use in neuraxial blockade.
- They prolong the blockade and limit the dose of local anesthetic infusion.
- Neostigmine is still undergoing evaluation for its use in obstetric anesthesia and currently is not recommended for standard practice. It increases acetylcholine binding within the spinal cord stimulating nitric oxide to produce analgesia. Neostigmine also produces refractory nausea and vomiting if given intrathecally, but minimal nausea and vomiting if given epidurally.
- Clonidine inhibits the release of substance P in the dorsal horn and produces analgesia. - It also increases the level of acetylcholine in the cerebral spinal fluid. Intrathecal clonidine can produce excellent analgesia, but sedation and hypotension are common.
41
Q
- Name the tissue layers and ligaments encountered when placing an epidural and in what order the anesthesiologist encounters each.
A
- When placing an epidural, the needle traverses the skin and subcutaneous tissues,
supraspinous ligament, interspinous ligament, the ligamentum flavum, and is advanced into the epidural space.
42
Q
- The American Society of Anesthesiologists (ASA) recommendations regarding aseptic technique for placement of neuraxial block include what specific precautions?
A
- Aseptic technique should be used during neuraxial procedures, including
(1) jewelry removal (e.g., rings and watches), hand washing, and use of caps, masks, and sterile gloves;
(2) use of individually packaged chlorhexidine (preferred) or povidone-iodine (preferably with alcohol) for skin preparation, allowing for adequate drying time;
(3) sterile draping; and
(4) use of sterile occlusive dressings at the catheter insertion site
- Aseptic technique should be used during neuraxial procedures, including
43
Q
- What are the interspaces where the neuraxial block for labor analgesia is placed and what are the risks of placing the neuraxial block higher or lower than this range of interspaces?
A
- For neuraxial analgesia the needle is normally inserted between L1 and L4.
- If the needle is placed too high, there is a risk of puncturing the conus medullaris if the needle inadvertently punctures the dura. In addition, coverage of sacral roots needed during the second stage may be inadequate.
- If the catheter is placed lower than L4, the neuraxial block may not adequately cover the nerves that innervate the uterus and may not provide the necessary labor analgesia for uterine contractions and cervical dilation.
44
Q
- What is a “test dose” and what does it assess?
A
- Prior to the infusion of local anesthetic through an epidural catheter a test dose typically composed of 3 cc of 1.5% lidocaine with 1:200,000 epinephrine is administered. - The anesthesia provider waits 3 minutes to confirm that the needle is not intravascular with no increase in heart rate or blood pressure, and no systemic symptoms of lidocaine infusion have resulted such as tinnitus or perioral tingling.
- Additionally, the patient is asked to move her lower extremities to confirm that the bolus was not placed intrathecally, which would result in motor blockade.
45
Q
- Can a test dose be used with a CSE?
A
- A test dose can and should be used with the CSE.
- The test dose will both confirm whether the catheter is intravascular by changes in heart rate and blood pressure, and unintended intrathecal catheter placement can still be assessed as the patient should still be able to move her lower extremities after
typical spinal local anesthetic doses in the spinal portion of the CSE.
46
Q
- What type of needle is used in placement of spinal analgesia and why?
A
- A 24 to 26-gauge “pencil point” spinal needle is commonly selected for CSE to reduce the risk of a postdural puncture headache.
47
Q
- What is a “saddle block” and when is it used during labor and delivery?
A
47.
- If primarily perineal anesthesia is needed (i.e., forceps delivery, perineal laceration
repair), the patient may be left in the sitting position for 2 to 3 minutes after intrathecal injection with hyperbaric local anesthetic to concentrate the sensory block in the perineal region (“saddle block”).
- A true saddle block anestheti (requiring more time in the sitting position) does not produce complete uterine pain relief because the afferent fibers (extending to T10) from the uterus are not blocked.
48
Q
- What are the contraindications to neuraxial procedures?
A
- Certain conditions contraindicate neuraxial procedures.
- These include: (
1) patient refusal,
(2) infection at the needle insertion site,
(3) significant coagulopathy,
(4) hypovolemic shock,
(5) increased intracranial pressure from mass lesion, and
(6) inadequate provider expertise. - Other conditions such as systemic infection, neurologic disease, and mild coagulopathies are relative contraindications and should be evaluated on a case-by-case basis.
