Parameter
Change
Tidal volume
↑ 45 %
Residual volume
↓ 15 %
Respiratory rate
No change
Functional residual capacity
↓ 20 %
Minute ventilation
↑ 45 %
Progesterone acts as a direct stimulant of the respiratory center and increases the respiratory drive. Pregnancy is a state of mild respiratory alkalosis with a slight decline in PaCO2 to 30 mmHg. Metabolic compensation by the kidneys results in a fall in serum bicarbonate concentration to 20 meq/L. The increased work of breathing is perceived by many pregnant women as shortness of breath. All the changes described above are further exacerbated during labor and delivery.
Pregnant women tend to desaturate rapidly during periods of apnea as compared to their nonpregnant counterparts because of higher oxygen consumption and reduced FRC. This occurs more so in the supine position as during induction of general anesthesia. Therefore, the parturient should be adequately pre-oxygenated with 100 % oxygen before induction of general anesthesia.
The minimum alveolar concentration (MAC) for volatile anesthetic agents decreases up to 40 % during pregnancy. This, in conjunction with the rise in MV leads to rapid uptake and elimination of volatile anesthetics resulting in faster induction and emergence from anesthesia, respectively.
Cardiovascular System
Cardiac output increases early in pregnancy and by the end of second trimester, it is about 50 % higher than nonpregnant women and then remains stable in the third trimester (Table 38.2). During labor, the cardiac output increases by 40 % during the second stage above the prelabor values. It may be as high as 75 % above the predelivery values in the postpartum period. Women with limited cardiac reserve may not tolerate the increased cardiovascular demands of pregnancy. The rise in cardiac output can be attributed to an increase in both stroke volume and heart rate by 25 % each. Uterine perfusion increases from 50 ml/min to 700–900 ml/min at term. Extremities tend to be warm because of increased cutaneous blood flow and pregnant women may report nasal congestion as a result of enhanced mucosal blood flow. Mammary blood flow also increases leading to a continuous flow murmur called mammary souffle. Cardiac output falls to prelabor values about 24–72 h after delivery and returns to pre-pregnant levels 6–8 weeks postpartum.
Table 38.2
Cardiovascular physiologic changes at term pregnancy
Parameter | Change |
|---|---|
Cardiac output | ↑ 50 % |
Stroke volume | ↑ 25 % |
Heart rate | ↑ 25 % |
Ejection fraction | Increased |
Systemic vascular resistance | ↓ 20 % |
The systemic vascular resistance (SVR) begins to fall early reaching its peak around 20th week of gestation. It increases slightly during later part of pregnancy but still remaining about 20 % below the nonpregnant level at term. The fall in SVR is explained by the vasodilatation caused by progesterone, estrogen, and prostacyclins and development of the low-resistance uterine vascular bed. The systolic, diastolic, and mean arterial pressures decrease during mid-pregnancy reflecting the alterations in SVR and return to baseline by term.
The cardiac muscle undergoes eccentric hypertrophy secondary to both an increase in the blood volume and the stretch and force of contraction of heart in the gravid state. As the gravid uterus enlarges, it causes elevation of diaphragm, in turn shifting the heart anteriorly and to the left. Due to these changes, some examination findings considered abnormal in the nonpregnant population no longer remain pathological in pregnancy. They include:
Loud first heart sound and wide splitting of the second heart sound.
A grade II ejection systolic flow murmur heard along the left sternal border.
A third and a fourth heart sound during the third trimester.
Displacement of the point of maximal cardiac impulse to the left of mid-clavicular line and cephalad in the fourth intercostal space.
ECG changes such as tachycardia, axis shifts, shortening of PR and uncorrected QT intervals, depressed ST-segment, and isoelectric T waves.
Aortocaval Compression
When pregnant women assume supine position, the gravid uterus compresses the aorta and the inferior vena cava. The overall effect is a reduction in maternal systemic arterial pressure (due to reduced venous return) and uterine blood flow (because of aortic compression) leading to a fall in uteroplacental perfusion. Aortocaval compression in the supine position causing profound maternal hypotension and bradycardia is termed supine hypotension syndrome. Hence, pregnant women should be encouraged to lie on her left side after 16–20 weeks of gestation. The same effect can be achieved by placing a wedge under the right hip to maintain left uterine displacement (Fig. 38.1). This gains importance during the provision neuraxial anesthesia for labor and delivery.
Fig. 38.1
Aortocaval compression, and its relief by placing a wedge under the right hip (left uterine displacement)
Hematologic System
Both plasma and red blood cell volume increase during pregnancy. However the rise in plasma volume (55 % at term) relative to the red blood cell volume (30 % at term) is more and this leads to physiologic anemia of pregnancy. Blood volume returns to normal about 8 weeks after delivery. Physiologic advantages of this hypervolemia and hemodilution include:
Improved delivery of nutrients to the fetus.
Prevents maternal hypotension in presence of reduced vascular tone.
Compensates for hemorrhage anticipated to occur during delivery. A healthy parturient loses around 600 ml of blood during a vaginal delivery and 1,000 ml during a cesarean section.
Pregnancy is a hypercoagulable state. The concentration of most of the clotting factors increases during pregnancy except factors XI, XIII which decrease and prothrombin and factor V which remain unchanged. Gestational thrombocytopenia is seen in about 7–8 % of otherwise normal pregnancies, where the platelet count falls below 150,000/mm3 and in some this fall can be profound. It is the most common cause of thrombocytopenia in pregnancy and usually does not require treatment.
The plasma cholinesterase levels fall by about 25 % during pregnancy but are not usually associated with clinically significant prolongation of the effects of succinylcholine. The plasma albumin concentration as well as the albumin: globulin ratio fall, and the colloid oncotic pressure decreases by approximately 5 mmHg. The polymorphonuclear cell function is depressed and this is reflected by higher risk of infections and remission of the symptoms of autoimmune disease in pregnant women.
Gastrointestinal System
The stomach assumes a more horizontal position than normal and the lower esophageal sphincter tone decreases. This is attributed to progesterone as well as the rising intraabdominal pressure during the latter months of gestation. Almost 30–50 % of women experience gastroesophageal reflux, with a gastric pH under 2.5. Gastric emptying is unaltered during pregnancy but esophageal peristalsis and intestinal motility slow down under the inhibitory effects of progesterone. However, the gastric emptying is slowed in labor and more so in women who receive bolus doses of opioids for labor analgesia.
Giving importance to these considerations, pregnant women in labor are always considered “full stomach” regardless of their fasting status. In view of the potential for a difficult airway and the risk of regurgitation of stomach contents followed by pulmonary aspiration, regional anesthesia is preferred in this group of patients. If general anesthesia is required, rapid sequence induction with cricoid pressure should be carried out and the airway protected using a cuffed endotracheal tube.
Renal System
Both renal plasma flow and glomerular filtration rate increase by 75 % and 50 %, respectively, thus leading to a rise in creatinine clearance. Blood urea nitrogen and serum creatinine (decrease to 0.5–0.6 mg/dl) levels fall owing to enhanced clearance of nitrogenous metabolites from the blood. Sodium retention due to increased renin and aldosterone secretion along with elevated protein excretion promotes tissue edema. In response to alveolar hyperventilation and respiratory acidosis, the kidneys increase excretion of bicarbonates in an attempt to maintain the acid–base balance.
Endocrine System
Pregnancy induces a diabetogenic state. Human placental lactogen reduces tissue sensitivity to insulin and leads to hyperglycemia. The thyroid gland shows follicular hyperplasia and increased vascularity to support metabolism during pregnancy. However, free T3 and T4 levels remain normal. Adrenal secretion of corticosteroids is also elevated.
Musculoskeletal System
Almost 50 % parturients report back pain at term. It is proposed that the enlarging uterus increases the lumbar lordosis and the hormone relaxin (secreted by placenta) causes remodeling of the pelvic connective tissue and collagen. The lumbar lordosis also changes the center of gravity of the body. Other musculoskeletal changes occurring during pregnancy include a higher incidence of carpal tunnel syndrome, meralgia parasthetica, and increased mobility of the pelvic joints to allow passage of the fetus.
Nervous System
The MAC of volatile anesthetic agents is decreased and is likely related to elevated levels of progesterone, endorphins, and enkephalins. The local anesthetic requirement during regional anesthesia is also reduced during pregnancy due to altered nerve tissue sensitivity, compression of the dural sac, and reduction in cerebrospinal fluid volume.
Uteroplacental Blood Flow
The spiral arteries are the main source of blood supply to the uteroplacental unit. They are derived from the uterine artery (branch of the internal iliac artery). The spiral arteries lose smooth muscle in their walls during trophoblastic invasion and create a low resistance placental vascular bed. A limited ability to autoregulate in response to noxious stimuli is an important characteristic of this circulation. Uterine blood flow is directly related to the uterine perfusion pressure and inversely to the uterine vascular resistance. The following equation expresses this relationship:
Uterine blood flow is affected by hypotension (aortocaval compression, hemorrhage, sympathectomy), factors which raise uterine venous pressure (vena caval compression, uterine contractions), and those which raise uterine vascular resistance (catecholamines, stress).
Placental Function and Transfer of Drugs
The placenta produces enzymes and hormones like human chorionic gonadotropin and placental lactogen. It also acts as a permeable membrane between the mother and the developing fetus. Passive diffusion, active and facilitated transport, and pinocytosis are all involved in the transfer of substances across the placenta.
Lipid solubility, protein binding, pH, pK a, and blood flow affect drug movement across the placenta in humans. Most of the anesthetic agents like benzodiazepines, induction agents (thiopental, propofol, ketamine), inhalational agents, opioids, and local anesthetics readily cross the placenta. Among the anticholinergic drugs, atropine and scopolamine readily traverse the placental barrier, while glycopyrrolate is poorly transported. Muscle relaxants, being ionized quarternary ammonium compounds, do not readily reach the fetal circulation. Heparin does not cross the placenta; low molecular weight heparin has limited ability, whereas warfarin easily enters the fetal circulation and is associated with fetal congenital anomalies. Anti-cholinesterase agents (neostigmine, pyridostigmine) have limited potential to cross the placenta.
Fetal Monitoring
Antepartum Assessment
The goal of antepartum surveillance is to accurately determine the gestational age and evaluate fetal growth and development. Information from the history and physical examination (last menstrual period, perception of quickening, fundal height) can be used to date the pregnancy. Ultrasonography (USG) is used to calculate the expected date of delivery and identify fetal anomalies. Other parameters used to assess fetal well-being include listening to the fetal heart rate (FHR), kick count, and abdominal palpation. USG in conjunction with a triple or quadruple screen can be used to screen for trisomies in advanced maternal age. Finally, chorionic villus sampling, amniocentesis, and cordocentesis are invasive tests for fetal karyotype and definitive diagnosis of chromosomal anomalies. The non-stress test and biophysical profile are used in the later part of pregnancy to ensure continuing fetal well-being.
Intrapartum Assessment
FHR can be monitored by a simple stethoscope, Doppler ultrasound, or fetal electrocardiography. FHR tracings in conjunction with uterine contraction patterns (using tocodynamometry or intrauterine pressure catheter) provide an indirect assessment of the uteroplacental unit and fetal well-being. FHR tracing is usually described in terms of the following parameters:
1.
Baseline heart rate: The normal FHR ranges from 120 to 160 beats per minute (bpm). Bradycardia is less than 120 bpm and tachycardia is greater than 160 bpm. Changes in FHR are caused by fetal (cardiac pathology, hypoxia) as well as maternal (fever, infection, medications) factors.
2.
Variability: It is the fluctuation in the baseline FHR of two cycles or more per minute. The presence of variability indicates integrity of neural pathways. Normal variability ranges from 6 to 25 bpm. Causes of decreased variability include fetal sleep state, hypoxia, neural pathology, and maternal administration of drugs like opioids.
3.
Periodic changes: These include accelerations and decelerations. The presence of accelerations rules out fetal metabolic acidosis. However, their exact significance is unclear.
Decelerations (Fig. 38.2 ) can be
Fig. 38.2
Fetal decelerations (early, late, and variable)
(a)
Early: They coincide with uterine contractions and are not considered harmful. They reflect vagal activity due to mild hypoxia or fetal head compression.
(b)
Late: These begin 10–30s after the onset of uterine contraction and last for 10–30s after the end of contraction. They occur in response to fetal hypoxia and are considered ominous if present with decreased or absent variability.
(c)
Variable: They are variable in onset and depth in relation to the uterine contractions. They indicate umbilical cord or fetal head compression in the second stage of labor. Intervention is indicated if they are severe (less than 60 bpm) and persistent (or prolonged >30s).
Categorization of tracing patterns: To improve the utility of electronic FHR monitoring, tracing patterns have been categorized as:
Category I (normal): Strongly predictive of normal fetal acid–base status.
Category II (indeterminate): Lack of adequate evidence to be classified as normal or abnormal and does not indicate a deranged fetal acid–base profile.
Category III (abnormal): Predictive of abnormal fetal acid–base status and needs prompt evaluation.
An older invasive technique of detecting fetal acidosis is sampling fetal scalp blood to determine its pH. It is indicated in cases of a persistently abnormal FHR tracing. A less invasive form of the test is to simply stimulate the fetal scalp and watch for acceleration of FHR as a response. ST waveform analysis (STAN) of fetal electrocardiogram (ECG) is a newer technique used in combination with cardiotocography for intrapartum fetal surveillance. It is based on the rationale that fetal hypoxia causes changes in the morphology of the ST segment and T wave of the fetal ECG.
Intrapartum Fetal Resuscitation
Some of the common causes of intrapartum fetal distress include maternal hypotension, fever, uteroplacental insufficiency, uterine hypertonus, umbilical cord compression, and oligohydramnios. Initial measures taken to improve fetal oxygenation include:
Maternal repositioning to prevent aortocaval compression.
Intravenous fluids, vasopressors to treat hypotension.
Administration of supplemental oxygen via a face mask.
Discontinuation or step down of oxytocin infusion, tocolysis (terbutaline) for uterine hypertonus.
Saline amnio-infusion for oligohydramnios as a result of umbilical cord compression.
Physiology of Labor
Stages of Labor and Pain Pathways
Labor includes a series of events that are required for successful passage of the fetus through the birth canal into the external world. Mechanics of labor are described in terms of powers (force generated by uterine contractions), fetal characteristics (size, lie, presentation, station), and the bony pelvis and soft tissues of birth canal that the fetus has to traverse. Labor is divided into four stages:
Stage 1: Begins with onset of regular uterine contractions and ends with full dilatation of cervix (10 cm). It is subdivided into latent and active phases. The average duration is about 14 h in primigravidas and 7 h in parous women (sensory T10–L1).
Stage 2: This is the interval between full cervical dilatation and delivery of the baby. Cardinal events include descent of the presenting part though the maternal pelvis and requires more active participation from the parturient. The second stage prolonged if the baby is not delivered within 2 h in primiparous, and 1 h in multiparous women after complete dilatation of cervix without epidural analgesia (sensory T10–S4).
Stage 3: This lasts from delivery of the baby to expulsion of the placenta and the membranes, which takes about 30 min.
Stage 4: Some authorities describe the first 60 min after placental delivery as the fourth stage and recommend close monitoring of the parturient for signs of postpartum hemorrhage (PPH).
The discomfort associated with first stage of labor is described as “visceral pain” because of its diffuse nature and origin due to cervical dilation and stretching of the lower uterine segment. It is transmitted by C and A-delta nerve fibers to the dorsal horn of spinal cord at T10 to L1 segments.
During the second stage, the afferents that innervate the vaginal portion of cervix, vagina, and perineum are also involved in addition to those described in stage 1. These afferents are carried by the pudendal nerve to the S2–4 dorsal root ganglia. This pain can be localized to the perineum and is described as “somatic.” Successful labor analgesia using regional anesthesia techniques requires blockade of T10–L1 segments during the first stage with extension to cover the lower sacral nerve roots after complete dilatation of the cervix.
Effects of Labor Pain
Severe pain during uterine contractions causes a marked increase in MV and oxygen consumption. Hyperventilation causes respiratory alkalosis and a leftward shift of the oxygen hemoglobin dissociation curve in the mother. Compensatory hypoventilation between the contractions results in transient maternal and fetal hypoxia. The end result is diminished oxygen supply to the fetus.
The activation of the sympathetic nervous system due to pain and stress of labor leads to an increase in the levels of circulating catecholamines, cardiac output, systemic vascular resistance, and fall in uterine blood flow. Neuraxial analgesia reduces catecholamine surges. Uterine contractions cause autotransfusion of blood from uterus into the circulation. While normal parturients tolerate this increase in blood volume and cardiac work, it may be deleterious in mothers with limited cardiac reserve. As the uteroplacental unit is perfused only during uterine diastole, the decrease in uterine blood flow during contractions that occurs against a background of uteroplacental insufficiency may not be tolerated by the fetus. Therefore, effective pain relief may contribute to better outcomes in these situations.
Besides physiologic effects, a painful labor can interfere with maternal–neonatal bonding, affect future sexual relationships, and cause postpartum depression. Also, effective communication should exist between obstetricians, anesthesiologists, and the nursing personnel to identify potential high-risk patients (difficult airway, severe preeclampsia, cardiac disease). An anesthetic evaluation early in labor may be warranted in such cases so as to provide the best possible option for labor analgesia.
Labor Analgesia
Non-pharmacologic
Antenatal childbirth education, emotional support (provided by family or doula), massage, audio-therapy, and acupuncture have been used to mitigate pain and anxiety during childbirth. Transcutaneous electrical nerve stimulation (TENS) is the application of low-intensity, high-frequency electrical impulses to the lower back and is widely used in the UK and Scandinavian countries for labor analgesia. Hydrotherapy is the immersion of the parturient in warm water to cover the abdomen only during labor. Intradermal water injection consists of the injection of sterile water on the lower back and is supposed to relieve the back pain during labor. Hypnosis during childbirth is a labor intensive technique and requires prenatal training of the mother and her partner.
Systemic Labor Analgesia
This can be provided by using inhalational agents or systemic opioid administration. Systemic analgesia is used widely in institutions around the world which lack facilities for provision of safe neuraxial analgesia. It is useful in women who refuse regional anesthesia or have contraindications (coagulopathy) for provision of neuraxial blocks.
Inhalational analgesia is available in the UK as Entonox (50 % nitrous oxide + 50 % oxygen). Special scavenging equipment is necessary to prevent contamination of the environment. The mother has to be taught the technique of use so that the peak brain concentrations of nitrous oxide coincide with the peak of contraction pain. The risk of hypoxemia exists with concomitant use of parenteral opioids and faulty equipment. Recently, there has been an interest in use of volatile anesthetic agents for labor analgesia due to availability of agents with low blood–gas solubility (sevoflurane, desflurane). However, these agents can cause maternal sedation and affect uterine tone.
Parenteral opioids can be used for providing analgesia during childbirth as intermittent bolus doses. Patient controlled analgesia (PCA) is rarely used for labor analgesia in the USA. Systemic opioids should be administered in the smallest dose possible, as they cause maternal sedation and respiratory depression both in the mother and the fetus (as they cross the placenta). Opioids blunt the pain but do not provide complete analgesia, and cannot substitute analgesia provided by neuraxial techniques. Trained personnel to care for the newborn in the immediate postpartum period should be available and made aware about the risk of neonatal respiratory depression due to maternally administered opioids. Commonly used parenteral opioids used as boluses are meperidine, fentanyl, butorphanol, nalbuphine, and remifentanil (Table 38.3).
Table 38.3
Parenteral opioids for intermittent bolus use during labor
Opioid | IV Dose | IM Dose | Onset of action (min) | Duration (h) |
|---|---|---|---|---|
Meperidine | 25–50 mg | 50–100 mg | 5–10 IV; 40–45 IM | 2–3 |
Fentanyl | 25–50 mcg | 100 mcg | 2–3 IV; 10 IM | 0.5–1 |
Butorphanol | 1–2 mg | 1–2 mg | 5–10 IV; 10–30 IM | 3–4 |
Nalbuphine | 10–20 mg | 10–20 mg | 2–3 IV; 15 IM | 3–6 |
Morphine | 2–5 mg | 5–10 mg | 3–5 IV; 20–40 IM | 3–4 |
Remifentanil is an ultrashort-acting synthetic opioid with rapid onset (blood–brain equilibration time 1.2–1.4 min) and short duration of action (metabolized by plasma and tissue esterases). The analgesic half-life of remifentanil is 6 min. It is given in a dose of 0.25 mcg/kg, up to 0.5 mcg/kg with a lockout interval of 2–3 min. It has the potential to become a popular agent for use in labor PCA. As with all other opioids, careful patient monitoring is required to avoid excessive sedation and respiratory depression. General advantages of PCA are better pain relief with lower drug doses, lesser side effects, and higher patient satisfaction as the mother can self-adjust the administration of opioid as per her individual needs. Besides respiratory depression, other side effects of opioids include nausea, vomiting, delayed gastric emptying, dysphoria, and drowsiness.
Neuraxial Anesthesia
Neuraxial anesthesia for labor and delivery includes continuous epidural, combined spinal epidural (CSE), and continuous spinal and caudal blocks. Caudal blocks are infrequently used in present day obstetric anesthesia. Continuous spinal analgesia may be used in cases of an unintentional dural puncture but is not practical in most parturients. Due to the long and unpredictable nature of labor, single shot techniques are not typically useful.
Advantages of Neuraxial Labor Analgesia
Complete analgesia that prevents pain and stress induced maternal catecholamine surge and hyperventilation.
Maternal participation in the process of childbirth due to lack of sedation.
No neonatal sedation or respiratory depression.
Continuous analgesia with catheter techniques can be used to provide surgical anesthesia in eventuality of an emergency cesarean section avoiding the need for general anesthesia.
Disadvantages
Requires a skilled anesthesia provider.
May prolong the second stage of labor increasing the chances of an instrumental vaginal delivery.
Associated sympathectomy causes maternal hypotension, reduces placental circulation, and causes FHR changes.
Possibility of a patchy or failed block.
Contraindications
Absolute: Patient refusal, maternal coagulopathy, infection at puncture site, allergy to local anesthetic (LA) agents.
Relative: Maternal hypovolemia, lumbar spine pathology, untreated systemic infection. Most obstetric anesthesiologists will perform regional anesthesia in a febrile parturient with possible chorioamnionitis, provided she has received preemptive antibiotics and is not in sepsis.
Initiation of neuraxial labor anesthesia should begin with a preanesthesia evaluation along with informed consent about the benefits and complications of the procedure. The caregiver must confirm availability of the resuscitation equipment and drugs. Basic monitoring should include noninvasive blood pressure measurement (NIBP), pulse oximetry, and continuous FHR record. Non-reassuring FHR patterns are associated with neuraxial blocks due to the hypotension following sympathectomy and intrathecal opioid administration. The American Society of Anesthesiologists (ASA) Task Force on Obstetric Anesthesia recommends monitoring of FHR before and after initiation of regional analgesia for labor pain management. Intravenous access should be established and maternal hydration started with a non-dextrose containing balanced salt solution (lactated ringer’s). While some providers give a fluid bolus (1,000 ml) during initiation of regional block, the ASA Task Force does not recommend a fixed volume to be infused. Aseptic precautions must be maintained during block placement.
Lumbar Epidural Block
A lumbar epidural block is usually placed by the anesthesiologist when the parturient is having active labor contractions, with cervical dilation of 4–6 cm, and absence of fetal distress. The lumbar epidural space (usually L3–4/L4–5) is identified using a 17 or 18G Tuohy needle with the mother in the sitting (commonly) or lateral position (Table 38.4). A 19 or 20G flexible catheter is passed into the epidural space (2–4 cm) to provide continuous labor analgesia. An epidural test dose is given to recognize unintentional intravascular or subarachnoid placement. A typical test dose consists of epidural injection of lidocaine 1.5 % with epinephrine 5 mcg/ml (1:200,000) to a total volume of 3 ml.
Table 38.4
Conduct of epidural analgesia for labor
Monitors | On |
Position of patient | Sitting (usually)/lateral |
Back skin preparation | Betadine × 3 times |
Lumbar space | L3–4/L2–3 |
Local skin infiltration | 1–2 ml of 1 % lidocaine |
Needle | 17G Tuohy |
Technique | Loss of resistance to air/saline |
(If wet tap) | Remove needle and go one space above/insert catheter into the subarachnoid space |
Epidural catheter insertion | 2–4 cm into epidural space |
Aspiration of catheter | Negative for heme and CSF |
Test dose | 3 ml of 1.5 % lidocaine with 1:200,000 epinephrine |
Agent | 0.25 % bupivacaine/ropivacaine 5–10 ml |
Desirable level of anesthesia | T10 |
PCEA | Start |
One should avoid test dose administration during an active maternal contraction. An increase in the maternal heart rate by 20 bpm within 1 min and motor blockade in 3–5 min may indicate intravascular or intrathecal placement. After ruling out a malpositioned catheter, epidural analgesia is initiated using a bolus injection of anesthetic agents (5–10 ml of 0.25 % bupivacaine or 0.25 % ropivacaine ± an opioid) and maintained with a continuous infusion (for example, 0.125 % bupivacaine plus 2 mcg/ml fentanyl, 10 ml/h, demand bolus 3–5 ml every 6–10 min, 4 h limit of 80 ml). The desired segmental anesthetic level is T10. The epidural catheter is removed (tip intact) after delivery when the parturient is stable to be sent to the postpartum unit.
Combined Spinal Epidural Block
This is usually performed as a needle-through-needle technique. After the lumbar epidural space is identified as described above, a long 25 or 27G spinal needle is introduced through the Tuohy needle. An intrathecal agent is injected after dural puncture (CSF flow) and the spinal needle is withdrawn. A catheter is then threaded into the epidural space, fixed to skin, and used for continuous analgesia.
Advantages
Faster onset of analgesia as compared to epidural block alone.
Intrathecal injection of an opioid alone without local LA agent in early labor allows good pain relief without motor blockade. A combination of opioid with LA in advanced, rapidly progressing labor provides good sacral analgesia within minutes.
Lower dose of opioid is required as compared to systemic or epidural dose.
Disadvantages
Higher incidence of maternal pruritis and FHR changes noted after intrathecal administration of opioids.
Dural puncture is required though the incidence of postdural puncture headache (PDPH) is not any higher as compared to epidural analgesia.
After initiation of CSE, it is difficult to evaluate functioning of the epidural catheter for 1–2 h until the effect of intrathecally administered drugs wears off. It may not be a practical option when an adequately functioning epidural catheter has to be ensured (difficult airway, FHR changes with high possibility of an urgent cesarean section).
Choice of Drugs
A combination of long-acting amide LA and lipid soluble opioid is commonly used for labor epidural analgesia. Advantages of using a combination are:
Lower doses of both agents act synergistically to provide superior analgesia.
Lesser incidence of unwanted effects (motor blockade by LA or pruritis due to opioids).
Faster onset and duration of analgesia.
Reduced shivering.
Local Anesthetic Agents
Traditionally, bupivacaine has been used in varying concentrations to provide epidural labor analgesia. Peak effect is seen at 20 min and analgesia lasts up to 90 min. It is highly protein bound limiting placental transfer. Ropivacaine and levobupivacaine (not available in the USA) are newer LAs similar to bupivacaine as far as the pharmacokinetic and pharmacodynamic properties are concerned. However, they are associated with less motor blockade and cardiotoxicity as compared to bupivacaine. All three provide adequate labor analgesia without affecting mode of delivery, labor duration, and neonatal outcome. Lidocaine is not commonly used for initiation of labor epidural analgesia because of its short duration of action. Chloroprocaine is used to provide surgical anesthesia for cesarean section or instrumental vaginal delivery due to its short onset of action. An initial epidural volume of 5–20 ml is usually required at initiation followed by 8–15 ml/h continuous infusion to maintain analgesia (Table 38.5).
Table 38.5
Drugs for initiation and maintenance of neuraxial labor analgesia
Drugs | Initiation of epidural analgesia | Initiation of spinal analgesia | Maintenance of epidural analgesia (continuous infusion/PCEA) |
|---|---|---|---|
Local anesthetics | |||
Bupivacaine | 0.0625–0.125 % | 1.25–2.5 mg | 0.0625 %–0.125 % |
Ropivacaine | 0.1–0.2 % | 2.5–4.5 mg | 0.1 %–0.2 % |
Levobupivacaine | 0.0625–0.125 % | 2.5–4.5 mg | – |
Opioid | |||
Fentanyl | 50–100 mcg | 15–25 mcg | 1.5–3 mcg/ml |
Sufentanil | 5–10 mcg | 1.5–5 mcg | 0.2–0.33 mcg/ml |
Opioids
Lipid soluble opioids fentanyl and sufentanil are used in combination with low concentration of LAs for neuraxial labor analgesia. Morphine is not very popular for this purpose because of its slower onset and long duration of action with undesirable side effects (pruritus, nausea, vomiting). For maintenance of analgesia, a low concentration solution of a LA with an opioid is administered either as a continuous infusion or patient controlled technique. For CSE, intrathecal lipid soluble opioid along with a low dose of LA or opioid alone is used to initiate analgesia when a CSE is performed and epidural infusion is then started for maintenance.
Adjuvants
Additives like epinephrine, clonidine, and neostigmine can be added to epidural or intrathecal solutions to prolong the duration of analgesia. However, they may cause severe hypotension and other side effects and, therefore, must be used with caution. Currently, clonidine is not recommended or approved for intrathecal use in obstetric patients.
Patient Controlled Epidural Analgesia
Patient Controlled Epidural Analgesia (PCEA) uses a programmable pump to deliver anesthetic agents into the epidural space for maintaining analgesia. PCEA parameters that can be adjusted include rate of background infusion, patient controlled bolus doses, lock-out interval, and maximum dose per hour. Typical PCEA settings are a background infusion rate of 6–12 ml/h, a patient controlled bolus dose of 3–5 ml with a lockout interval of 6–15 min using a combination of dilute LA solution and opioid. When a background infusion is not used, bolus dose is adjusted at 8–12 ml with lockout interval of 10–20 min.
Advantages of PCEA
Reduced incidence of unscheduled clinician intervention for breakthrough pain.
Reduced total anesthetic consumption and lower extremity motor blockade.
Maternal satisfaction is equal or better than the continuous infusion techniques.
Newer Advances
Computer-integrated PCEA is a delivery system that automatically adjusts the background infusion rate based on the number of PCEA demands. A disposable PCEA device has been compared with a standard electronic PCEA device. Disposable devices are less bulky, which may facilitate ambulation during labor. However, they lack programmability and are associated with increased costs.
Ambulatory Analgesia
Popularly known as “walking epidural,” it refers to the ability of a parturient to ambulate safely after initiation of neuraxial analgesia. It typically consists of low dose of anesthetic agents (usually an opioid) that provides pain relief without causing motor blockade. Regular epidural analgesia is initiated once active labor starts. Although the ability to ambulate may not affect labor outcome, excessive motor blockade does prolong the second stage of labor and increases the chances of having an operative vaginal delivery.
Side Effects of Neuraxial Analgesia
1.
Hypotension : Sympathetic blockade following neuraxial analgesia causes peripheral vasodilation and hypotension in 10 % of parturients. Prolonged severe hypotension affects uteroplacental perfusion and causes fetal acidosis. Preventive strategies employed include avoiding aortocaval compression and preloading/co-loading with intravenous fluids. Hypotension is treated with additional intravenous fluids, oxygen, and vasopressors like ephedrine (5–10 mg iv) or phenylephrine (40–100 mcg iv) as bolus doses.
2.
Pruritis: This is an opioid related side effect. Nalbuphine (2.5–5 mg iv) is popularly used to treat opioid induced pruritis.
3.
Nausea, vomiting: This may be related to the labor itself, as a side effect of neuraxial opioids or due to hypotension following institution of neuraxial block. Hypotension should be treated as above, and antiemetics (ondansetron) administered as needed.
4.
Urinary retention: Occurs due to blockade of sacral nerve roots that supply the urinary bladder and opioid induced suppression of detrusor contractility. A Foley’s catheter is usually inserted after initiation of neuraxial analgesia.
5.
Delayed gastric emptying: Labor as well as bolus opioid administration prolongs gastric emptying time. However, low dose epidural infusion with fentanyl and bupivacaine does not affect gastric emptying.
6.
Shivering: A common occurrence in labor with loss of heat due to sympathetcomy. Warming blankets should readily be provided.
Complications of Neuraxial Analgesia
1.
Failed analgesia: This refers to no neuroblockade, inadequate density, and unilateral block or missed segments. This is usually managed by additional doses of LA (testing to see whether an epidural is working), or repeating the epidural procedure.
2.
Accidental dural puncture and postdural puncture headache (PDPH): PDPH can occur after an intentional dural puncture during spinal anesthesia or an unintentional dural puncture with an epidural needle. The risk of developing a headache after accidental dural puncture with an epidural needle is about 52 %. The headache is described as fronto-occipital, radiating to the neck, worsening in upright position, and relieved on lying down in bed. It may be accompanied with nausea, photophobia, neck stiffness, and tinnitus. The headache usually appears within 48 h after dural puncture and disappears within a week in 95 % of cases without intervention. Diagnosis is clinical but brain imaging may be indicated in the presence of atypical symptoms to rule out other causes of postpartum headache (pseudotumor cerebri, pneumocephalus, posterior reversible encephalopathy syndrome, subdural hematoma).
Management strategies described include maintaining adequate hydration, caffeine, sumatriptan, epidural blood patch (prophylactic/therapeutic), epidural morphine, and intrathecal catheters. An epidural catheter placed intrathecally during an accidental dural puncture with an epidural needle and left in situ for 24 h reduces the incidence and severity of PDPH. The catheter can be used to provide analgesia during labor and surgical anesthesia for abdominal delivery if needed. For an epidural blood patch, up to 20 ml of the patient’s blood is collected aseptically after the epidural needle is in place, and then injected slowly via the epidural needle (Table 38.6). Patients may feel significant pressure in the back during the injection. Relief of headache is almost immediate, and patients are discharged home.
Table 38.6
Conduct of an epidural blood patch
Monitors | On |
Patient position | Usually sitting/lateral |
Lumbar space | Same or near the first dural puncture site |
Back skin preparation | Betadine × 3 times, local 1–2 ml of 1 % lidocaine |
Needle | 17G Tuohy |
Loss of resistance | Air/saline
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