Anesthesia in the Normal and High-Risk Patient



Anesthesia in the Normal and High-Risk Patient


Bhavani Shankar Kodali

Andrew M. Malinow



Introduction

Women in labor require special anesthetic considerations because of the physiologic changes of pregnancy1 including



  • Airway anatomy that could pose a difficult intubation;


  • Metabolism and the respiratory system resulting in hastened hypoxemia during apnea;


  • The gastrointestinal system predisposing to regurgitation and aspiration;


  • Abdominal pressure of the growing uterus on both the aorta and inferior vena cava resulting in supine hypotensive syndrome of pregnancy; and


  • Mechanical, hormonal, and biochemical factors that result in increased spread of intrathecal and epidural local anesthetic agents.

An understanding of obstetric pain pathways is critical in providing adequate analgesia for labor and delivery and anesthesia for cesarean delivery. Uterine pain impulses are transmitted by A-delta and C visceral afferent nerve fibers that travel with sympathetic nerves to the uterine and cervical plexuses, the pelvic (inferior hypogastric) plexus, the middle and superior hypogastric, and the aortic plexuses. Together these fibers travel into the lumbar and lower thoracic sympathetic chain, terminating in the dorsal horn of the spinal cord at thoracic segmental level 10 to lumbar segment 1 (Figure 26.1).1 Nociceptive stimulation continues from uterine contractions and distension of the lower uterine segment throughout the late first stage and second stage of labor, accompanied by a progressive increase in somatic pain resulting from distention of the pelvic floor, vagina, and perineum by the descending fetal presenting part. Pain is transmitted by somatic nerve fibers, primarily branches of the pudendal nerve (derived from the anterior primary divisions of sacral nerves S2 [sacral segmental level], S3, and S4), including the inferior hemorrhoidal, perineal, and dorsal nerve[s] of the clitoris.

During cesarean delivery, additional nociceptive pathways are involved in the transmission of pain. Most cesarean deliveries are performed with a horizontal infraumbilical skin incision, involving the T11 to T12 dermatomes. During surgery, retraction of the skin may involve two to four more rostral sensory dermatomes and intraperitoneal manipulation and dissection involve poorly localized visceral pain pathways. Therefore, adequate labor neuraxial analgesia involves a “block” up to T10; for cesarean delivery, adequate anesthesia requires a “block” up to T4 to prevent discomfort associated with peritoneal traction.1


Anesthesia for Labor and Delivery in a Patient With a Routine Pregnancy

The most effective labor analgesia is provided by neuraxial spinal, continuous labor epidural (CLE), combined spinal epidural (CSE), or continuous spinal anesthesia (CSA) injection. For labor, neuraxial analgesia is superior to nonpharmacological methods (ie, Lamaze, water birthing, transcutaneous electrical nerve stimulation), nitrous oxide labor analgesia, and parenteral opioids (ie, fentanyl, remifentanil, and dexmedetomidine)1 (Figure 26.2A-C).

With all but single-injection spinal analgesia, an epidural catheter is routinely sited at the vertebral L2 to L4 level.







CLE infusions contain low-concentration local anesthetics, such as bupivacaine (0.0625%-0.125%) or ropivacaine (0.1%),2,3 routinely augmented by the addition of fentanyl (2 µg/mL). Infusion of such dilute solutions provide effective labor analgesia with minimal or no weakness of abdominal and lower limb muscles, allowing the patient the ability to move in her bed while sensing the urge to push during the second stage of labor and vaginal delivery.

In CSE, a 25- to 27-gauge spinal needle is introduced through an epidural needle into the intrathecal space (Figure 26.2C). Bupivacaine (1.5-2 mg) with fentanyl (25 µg) is injected to provide quicker onset of pain relief, and then it is immediately followed by the insertion of epidural catheter through the epidural needle.

CSA involves puncture of the dura to facilitate placement of a catheter to allow titration of small injections of local anesthetic or opioid, a technique employed mainly (and rarely) in the management of high-risk delivery (vide infra).

Analgesia is provided via a patient-controlled electromechanical pump in either continuous, patient-controlled epidural analgesia (PCEA)4 or programmed intermittent epidural bolus (PIEB) analgesia. Now routinely employed, PCEA involves a continuous infusion of local anesthetic supplemented with patient-controlled boluses to manage “breakthrough” pain; PIEB mode substitutes preprogrammed pump bolus delivery (preset intervals of 30 or 45 minutes) for the continuous infusion.2

There is no increase in the incidence of cesarean delivery with the current practice of neuraxial labor analgesia; there is an increase in assisted vaginal delivery compared to patients receiving nonneuraxial labor analgesia or no analgesia.5

The choice of anesthetic technique for cesarean delivery depends on several factors, including: experience of anesthesiologist and obstetrician; indication for and urgency of cesarean delivery; presence of an in situ functioning epidural catheter placed for labor analgesia; and patient factors (such as patient cooperation, maternal preference,
obesity, and presence of coagulation disorders, spine deformities, and anticipated difficult airway). Neuraxial anesthesia is almost always preferred to general anesthesia as it is safer for the parturient. The case-fatality rate for neuraxial anesthesia is low but not insignificant; therefore, neuraxial anesthesia must be cautiously administered.6 The complications of neuraxial anesthesia include post-dural puncture headache (1%-1.5%), back pain (no correlation with neuraxial techniques), spinal epidural hematoma (0.6 per 100,000), and rarely epidural abscess. Although the case-fatality rate for
general anesthesia in obstetrics has decreased by over two-thirds since 1991, it is still 1.7 times that for neuraxial anesthesia (6.5 vs 3.8 per million).7 However, the need for general anesthesia cannot be eliminated entirely; it is required for emergent cesarean delivery and in the case when neuraxial anesthesia has failed or is contraindicated. It is imperative that, when required, the anesthesiologist be adept at administering general anesthesia safely. Airway changes occur in pregnancy as well as during labor; therefore, examination of the maternal airway is critical for safe administration of general anesthesia.8






Spinal anesthesia for cesarean delivery is achieved by subarachnoid injection of bupivacaine (10-15 mg) through a 25- to 27-gauge needle inserted at vertebral level L3 to L4. This dose yields surgical anesthesia for up to 120 minutes. Both Whitacre and Sprotte spinal needles are designed with a noncutting leading edge and are associated with a decreased incidence of post-dural puncture headache. Fentanyl (10-20 µg) is routinely added to the local anesthetic to enhance the quality of sensory blockade. Further addition of preservative-free morphine (150-200 µg), especially when combined with postoperative administration of nonopioid adjuvants (eg, acetaminophen or nonsteroidal anti-inflammatory drugs [NSAIDs]), provides excellent postoperative pain relief for about 18 to 24 hours.1 If prolonged duration of surgery is anticipated, CSE allows additional epidural catheter injections to supplement the required level of anesthesia. The epidural catheter can also be injected with preservative-containing morphine (2-3 mg) to provide postoperative pain relief as an alternative to intrathecal morphine.1 If a parturient in labor with an in situ epidural catheter requires cesarean delivery, the T4 rostral level of surgical anesthesia can be achieved in approximately 10 to 15 minutes after injection of 2% lidocaine with 1:200,000 epinephrine (usually 18-24 mL injected in intermittent 3- to 5-mL bolus doses). If immediate cesarean delivery is required, injection of 3% chloroprocaine is often used to expeditiously achieve surgical anesthesia (approximately 7-10 minutes) and thus avoid general anesthesia.1 Prophylactic administration of intravenous metoclopramide and ondansetron minimizes the incidence of intraoperative and immediate postoperative nausea and vomiting.


Anesthesia for Labor and Delivery in a Patient With a High-Risk Pregnancy


General Principles

Management of the high-risk pregnancy involves meticulous antenatal care given by the obstetrician and consultant subspecialists to achieve maternal physiologic stability in the face of maternal comorbidity. In labor, the goal is to maintain fetal well-being (in general, meaning maintaining uteroplacental blood flow) while providing maternal cardiorespiratory stability. It is widely accepted that vaginal delivery is safer than cesarean delivery for almost any pregnancy. Yet, the onset of labor heralds physiologic stress, both gradual and sudden, which can disrupt any equilibrium already achieved. The requested induction of labor analgesia for expected vaginal delivery or the required induction of surgical anesthesia (neuraxial or general) for cesarean delivery may also disrupt maternal physiologic stability. Any anesthetic plan for an expected vaginal delivery, often beginning with induction of labor from an unfavorable cervix, must take into account of a prolonged time period (eg, 1-3 days), from induction to delivery leaving open a window for physiologic perturbation not seen during the relatively short window associated with cesarean delivery. At this time, the parturient is more physiologically unstable than would be encountered at a previous, planned, nonemergent cesarean delivery. In a parturient with stenotic valvular disease, even maternal tachycardia associated with hours of repetitive contraction-associated “autotransfusion,” maternal discomfort, or fever from chorioamnionitis might destabilize cardiac function. In addition, the fetal effects of a prolonged induction (eg, chorioamnionitis) might indicate an induction of surgical anesthesia for unplanned emergent (or even immediate) cesarean delivery.

The choice of surgical anesthesia may also be dictated by the expected maternal physiologic response to the induction of anesthesia. An example would be a patient with pulmonary disease, who, in a semiseated position, well tolerates appropriate upper thoracic sensory-dermatomal level of neuraxial anesthesia but, later, might decompensate with the rostral diaphragmatic displacement that occurs when the surgical table is placed near-level for abdominal delivery. At that point, a previously eschewed induction of general anesthesia now becomes required for mechanical ventilation.


Less frequently, fetal condition influences the choice of surgical anesthesia for high-risk conditions. Seating a patient with super-morbid obesity for an expected difficult insertion of a neuraxial needle is contraindicated by a prolapsed fetal cord or small part. Just a delay in induction of surgical anesthesia for an indicated immediate abdominal delivery (eg, prolonged fetal bradycardia) might further complicate the degree of fetal/neonatal (patho-) physiologic stress.

Therefore, any well-communicated delivery plan should include not only an analgesia/anesthesia plan for labor/vaginal delivery (ie, “Plan A”) but also for induction of surgical anesthesia for an unplanned nonemergent (“Plan B”) or immediate, emergent abdominal delivery (“Plan C”).

A few high-risk conditions are addressed next, using the principles stated above while organizing a plan for safe conduct of analgesia/anesthesia with recommendations and special considerations.


Obesity

Obesity (body mass index [BMI] ≥ 30 kg/m2)9,10 is associated with profound physiologic changes, such as an increase in oxygen consumption, carbon dioxide production, and alveolar ventilation. An increase in abdominal mass forces the diaphragm cephalad, accentuating the decrease in functional residual capacity (FRC) and expiratory reserve volume (ERV). Excess adipose tissue over the chest wall together with poor lung compliance resembles a restrictive lung disease. In obesity, the work of breathing is increased. Placing the patient supine, in lithotomy, or on the bed in Trendelenburg position all accentuate the reduction in lung volume. Together with the decrease in FRC and ERV, all exacerbate ventilation/perfusion mismatch, maternal arterial hypoxemia, and atelectasis. Left uterine displacement (LUD) and supplemental oxygen by mask are standard.

Pickwickian syndrome (obesity-hypoventilation syndrome) occurs in pregnancy and is characterized by somnolence, hypercarbia, right-sided heart failure, and chronic hypoxemia.11 It may be associated with obstructive sleep apnea. Difficult airway management should be anticipated. Obesity is also associated with an increase in gastric volume, poor gastric emptying, and hyperacidity as well as an increased risk of aspiration.12

Due to the risk of respiratory compromise, caution should be used if prescribing parenteral narcotics or self-administered inhaled nitrous oxide in the laboring patient with obesity. Analgesia after single injection of subarachnoid local anesthetic/opioid solution lasts less than 120 minutes.

More importantly, it does not provide a catheter for injection of successive doses of local anesthetic, later necessary for prolonged labor or for abdominal delivery. Siting a neuraxial catheter (as part of epidural, CSE, or CSA analgesia) and injecting dilute solutions of local anesthetic/opioid is the preferred technique for labor analgesia. In the patient with morbid obesity, successful siting of a neuraxial catheter is often technically difficult and requires a prolonged duration period, necessitating ultrasound imaging and/or retrieval of long-length (eg, 5-, 6- or 7-inch) epidural needles required to pass through superficial adipose layers to reach the epidural/subarachnoid space. As for parturients without obesity, a T8 to T10 sensory dermatomal level is adequate for labor analgesia. In parturients with super-morbid obesity (BMI > 40), we have induced CSA for labor analgesia. Unlike an epidural catheter, the withdrawal of cerebrospinal fluid (CSF) before injection of local anesthetic is reassuring that the catheter tip is correctly placed, predictive that injected drug will be effective, and a surgical level of anesthesia can easily be obtained.

For cesarean delivery, neuraxial anesthesia at the rostral T4 sensory dermatomal level is preferred to general. In patients with obesity, due to the (often) prolonged duration of surgery, many anesthesiologists prefer siting a neuraxial catheter (eg, CLE, CSE, CSA) to single-injection spinal anesthesia. If general anesthesia is required, then there should immediate execution of a well-thought out and previously communicated plan. Obesity is also associated with an increase in gastric volume, poor gastric emptying, and hyperacidity as well as an increased risk of aspiration.12 Before induction, patients should routinely receive immediate-acting, acid aspiration chemoprophylaxis (some combination of nonparticulate antacid and parenteral metoclopramide); some anesthesiologists will also inject a parenteral dose of an H2 antagonist to be effective at the time of tracheal extubation.

Difficult airway management should be anticipated. Every preanesthetic evaluation must include a thorough evaluation of the airway. There is an increased likelihood of difficult tracheal intubation in patients with obesity, perhaps due to increased
neck circumference (soft tissue) making proper prepositioning of the head impossible before laryngoscopy. Although rapid sequence induction of general anesthesia with tracheal intubation is routine, some anesthesiologists prefer awake laryngoscopy/intubation. Practiced protocols for the difficult airway should be in place. A cache of laryngoscopic equipment (including different types and sizes of metal blades, video, and fiber-optic laryngoscopes and topical airway mucosal anesthetics) should be readily available. At the end of the surgical procedure, tracheal extubation should occur only when the patient is awake with airway reflexes intact.


Cardiac Disease

Before delivery, the perinatal team will medically optimize the patient suffering cardiovascular disease, helping maintain pregnancy until a planned (near-) term delivery. On occasion, maternal or fetal deterioration mandates unplanned delivery. To maintain cardiovascular stability, the anesthesiologist will focus on the cardiovascular stress of labor and expected vaginal or unplanned abdominal delivery.13 Central to any plan is the maintenance of forward flow, based on providing/maintain homeostasis of four basic cardiac parameters—preload, afterload, contractility, and heart rate—throughout delivery until immediate bedside responsibilities are transferred back to the cardiologist.

The induction of neuraxial anesthesia is associated with sympathetic-mediated vasodilation, and this coupled with unanticipated obstetric hemorrhage can decrease circulating volume returning to the heart (preload). Right ventricular preload is maintained with a combination of metered bolus infusions of intravenous fluid and/or low doses of vasopressor drugs, mitigating maternal tachycardia. Maternal tachycardia can result in decreased ventricular filling time and decreased forward flow and increased myocardial oxygen demand. Autotransfusion associated with regular uterine contractions as well as with postdelivery uterine can increase cardiac output, leading to acute cardiac failure in a patient who cannot compensate increases in cardiac output.

Although pregnancy is associated with a decrease in afterload, an unexpected increase in afterload in response to labor pain or even to an administered drug (eg, methylergonovine) may impede forward flow, destabilizing a patient with cardiac disease suffering a regurgitant lesion. The associated increase in myocardial oxygen demand may destabilize a patient with myocardial ischemia.

Most drugs currently used to induce or maintain general anesthesia have little or no effect on cardiac contractility. However, the induction of anesthesia in a patient with cardiac disease is much different in dose and time sequence of administration as compared to routine induction of general anesthesia for immediate abdominal delivery. The anesthetic plan for immediate abdominal delivery under general anesthesia should be well-communicated between obstetrician and anesthesiologist, especially with reference to any expected delay (prolonged time sequence) and any expected neonatal effects of the chosen induction “drugs.” For example, bolus intravenous injection of opioids (eg, fentanyl [3-5 µg/kg] or equivalent doses of alfentanil or remifentanil) can blunt the endogenous catecholamine-induced pressor response to maternal intubation but may delay surgical anesthesia by 60 to 90 seconds, or (rarely) may affect immediate neonatal condition.

Routine labor and vaginal/abdominal delivery are marked by episodes of maternal tachycardia in response to reflex vasodilation, use of vasopressors, tracheal intubation, injection of terbutaline, hyper- or hypothermia, hypoxemia, and anemia. The anesthesiologist routinely avoids pharmacologically induced increases or decreases in maternal heart rate in a patient already medically optimized. The anesthesiologist attempts to minimize such stimuli and, if unavoidable, minimize the elevation in heart rate. Again, a prolonged induction of labor is associated with the cardiovascular effects of uterotonic prostaglandins, early labor pain, tocolytic terbutaline, chorioamnionitis, and even maternal anxiety seen with obstetric emergencies; all may destabilize maternal cardiovascular status.

In addition to pulse oximetry (measuring both maternal peripheral arterial saturation and heart rate), routine monitoring of a patient with cardiac disease is often expanded to include continuous electrocardiography; siting an intra-arterial catheter to measure maternal systemic arterial pressure; siting a central venous catheter in anticipation of infusion of vasoactive drugs; and intermittent “point-of-care” (bedside) ultrasound to intermittently image inferior caval diameter, biventricular and septal motion, and interstitial lung water.

A plan to discontinue scheduled anticoagulation before induction of neuraxial analgesia/anesthesia should be communicated to the anesthesiologist.
Neuraxial analgesia/anesthesia for labor can be modified for selected patients undergoing assisted vaginal delivery indicated to shorten the expulsive phase of delivery. For expected vaginal delivery, the patient is usually placed in a low lithotomy (with tilt or manual uterine displacement) or modified Sims position.

Neuraxial anesthesia for cesarean delivery can be induced using epidural anesthesia or spinal anesthesia knowing that the relative (“slow” or “fast,” respectively) onset of upper thoracic sympathectomy will increase peripheral capacitance, decreasing central circulating volume, and often increase maternal heart rate. The anesthesiologist may induce a two-step CSE anesthetic injecting subarachnoid opioid and then may inject a local anesthetic via the catheter to achieve surgical anesthesia while titrating intravenous crystalloid boluses to maintain normovolemia.

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Jun 19, 2022 | Posted by in OBSTETRICS | Comments Off on Anesthesia in the Normal and High-Risk Patient
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