Airway Management in Pregnancy
Lucienne Lutfy-Clayton
Margaret Goodrich
Kamil Skotnicki
OVERVIEW
Airway management is challenging in pregnancy due to the anatomic and physiologic changes that occur, impacting oxygenation, ventilation, and securing of the airway. The overall airway failure rate during obstetric anesthesia is 1 in 390 cases.1 This rate is 10-fold greater than that of general anesthesia. General anesthesia use in pregnant patients is declining and may be contributing to failed airways due to decreased provider experience.2 The rate of failed obstetric airways in the emergency department setting is unknown. Pregnant patients are considered to have difficult airways with potential for rapid desaturation because of changes in oxygen reserve, physiologic hyperventilation, aortocaval compression by the gravid uterus, upper airway edema and bleeding, and risk of aspiration.1,3
Managing an emergent obstetric airway is stressful for health care providers but can be mitigated by using a systematic planned approach while considering the unique alterations that occur throughout pregnancy. Success is enhanced with practice as well as knowledge of algorithms adapted for the obstetric airway. The high stakes of these airways create an increased cognitive load for the health care team; however, the use of checklists can offload cognitive burden, improve safety, and standardize airway preparation.4 Although the emergent obstetric airway is a rare event, a systematic approach to all airways and practice with difficult airways can obviate the stress inherent in these situations.
ANATOMY
Anatomic changes in pregnancy include upper airway edema, increased capillary engorgement, and mucosal friability due to increases in blood volume and estrogen levels.5,6 These anatomic alterations contribute to an increased risk of bleeding and swelling that reduces glottic visualization. Mallampati scores gradually increase throughout pregnancy2 and correlate with the glottic view during intubation (Figure 4.1). Weight gain and obesity in pregnancy increase the risk for both difficult intubation and failed intubation. These anatomic changes continue, and should be considered, during the postpartum period.
Figure 4.1: Mallampati score. Designed for an awake patient, for emergent cases open the mouth and look at the amount of visualized posterior pharynx. Grade I: soft palate, uvula, anterior/posterior tonsillar pillars. Grade II: soft palate and uvula. Grade III: soft palate and base of uvula. Grade IV: hard palate. The likelihood of difficult laryngoscopy increases with Grade III and Grade IV views. This scoring system correlates with glottic view and is a surrogate for difficulty. Mallampati scores are difficult to perform in the emergency department; therefore, looking in the mouth can be effective in detecting masses, edema, trauma, bleeding, vomit, and physical barriers such as jaw wiring. (Left image from Barash PG, Cullen BF, Stoelting RK, et al. Clinical Anesthesia. 8th ed. Philadelphia, PA: Wolters Kluwer; 2017. Right image from Johnson J. Bailey’s Head and Neck Surgery. 5th ed. Philadelphia, PA: Wolters Kluwer; 2013.) |
Upper airway edema progresses throughout pregnancy. Mucosal edema also increases in preeclampsia and with intravenous fluid administration. Labor causes dynamic changes in intraoral edema, resulting in a worsening Mallampati classification as well as decreased oral volume and surface area.1 Increased vascularity and friability of the intraoral tissues raise the risk of bleeding and obscuring visualization of the glottic opening as well as video laryngoscope (VL) optics. Each additional attempt may worsen bleeding, making successful intubation less likely.
Increased breast size, obesity, and the gravid uterus can act as physical barriers to intubation. Obesity has been found to increase the risk of difficult intubation, particularly when the weight is more than 90 kg.7 The diaphragm is elevated by the gravid uterus, reducing the functional residual capacity (FRC) and expiratory reserve volume. This reduction can be 20% or more in the recumbent, lithotomy, and Trendelenburg positions.5,6 The work of breathing in parturients is increased due to reduced chest compliance and increased chest height, leading to shallow hyperventilation.5 Pregnant patients are also predisposed to obstructive sleep apnea, with higher rates of snoring, up to 25% in the obese population, indicating upper airway obstruction during sleep.5 Obstructive sleep apnea is associated with increased rates of failed airways and difficult intubation in nonpregnant operating room cases.8
PHYSIOLOGY
The physiologic changes during pregnancy resulting in increased oxygen demand are present during the first and second trimester and progress to term. Tidal volume, respiratory rate, ventilation perfusion mismatch, and carbon dioxide (CO2) diffusing capacity all increase in pregnancy. Concomitantly, expiratory reserve volume, residual volume, FRC, total lung capacity, airway resistance, arterial oxygen partial pressure, and arterial CO2 partial pressure are all reduced.5 Additionally, oxygen consumption and CO2 production increase linearly with weight gain.5 CO2 and progesterone increase the sensitivity of the medullary respiratory centers, resulting in increased minute ventilation.5 These cumulative changes decrease the time to desaturation for the gravid female and induce a respiratory alkalosis.1,5 The lower esophageal sphincter tone is reduced secondary to progesterone,1 resulting in increased gastric reflux, delayed gastric emptying, and increased risk for aspiration.
Cardiac output is increased up to 50% during pregnancy, which accelerates the onset of paralysis with nondepolarizing paralytics and may affect the duration of action of other medications used for induction. Aortocaval compression by the gravid uterus can be compounded by “obese supine hypotension syndrome”. By placing the pregnant patient, especially the obese pregnant patient, at increased risk of hypotension, syncope, and reduced uterine blood flow, especially in the supine position.5 Avoid the supine position and consider lateral uterine displacement in these patients.3 Monitor for signs of hypovolemia in pregnant patients as hypotension may not be evident until a loss of 25% to 30% of the blood volume has occurred.9,11 The fetus is at greater risk of complications during maternal hypotension.
MANAGEMENT
Noninvasive Ventilation
Noninvasive ventilation (NIV) for short-term support (<48 hours) is safe in respiratory conditions that can improve rapidly. Case reports describe NIV use in pregnant patients with pulmonary edema, pneumonia, and in the perioperative period. NIV mitigates the risks associated with intubation, rapid sequence intubation (RSI) medications, and postintubation sedation. The potential concern for aspiration secondary to reduced lower esophageal tone and delayed gastric emptying in pregnant patients is not substantiated in the literature.9 NIV should only be considered for pregnant patients who can protect their airway and have a respiratory illness that is short lived. The emergency provider should consider the risks and benefits of NIV on a case-by-case basis.
The Systematic Approach to Intubation
A systematic approach is recommended on all emergent airways. This involves careful preparation in four distinct stages (Figure 4.2).5 First pass success (FPS) is the goal, resulting in decreased risk of adverse events, morbidity, and mortality.10,11 This approach is fast, efficient, and addresses the most commonly missed or overlooked steps that can lead to failed airways. A checklist with key steps can help the team accomplish all four stages effectively (Table 4.1).
Figure 4.2: Sequence of steps of preparation for emergent obstetric airway. These preparation steps can improve your first pass success in the emergent obstetric airway, as well as mentally preparing yourself and your team. (Modified from Cook TM, Woodall N, Harper J, et al.; Fourth National Audit Project. Major complications of airway management in the UK: results of the Fourth National Audit Project of the Royal College of Anaesthetists and the Difficult Airway Society. Part 2: intensive care and emergency departments. Br J Anaesth. 2011;106(5):632-642.) |
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TABLE 4.1 Checklist of Steps to Review Prior to an Emergent Obstetric Airway | ||||||||||||||||||||||||||||||||||||||||||||||||||||
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Prepare the Patient
Preoxygenation
Patients benefit from preoxygenation prior to intubation by increasing the safe apneic period. This increased time to hypoxemia is critical for pregnant patients given their reduced FRC and increased metabolic demand. A minimum of 3 minutes of preoxygenation with a non-rebreather mask at a flow rate greater than 15 L/min (as high as the regulator will allow) prior to induction is recommended, but a longer time period is desirable.12,13 This allows for “denitrogenation” as well as oxygenation. If tolerated, continuous positive airway pressure (CPAP) with a positive end-expiratory pressure (PEEP) of 10 cm H2O has the potential to deliver a greater amount of preoxygenation in the pregnant patient.14 Patients should be in a ramped, heads up, or reverse Trendelenburg position to optimize preoxygenation.12 An upright position of 20° to 30° increases the FRC and safe apneic time in obese and nonobese pregnant patients.3
Apneic oxygenation is the delivery of oxygen through an open conduit to the glottis during apnea. In elective operating room cases, this can increase the safe apneic time by more than 2 minutes.12 The FPS is increased with apneic oxygenation in emergency intubations when there is no hypoxia.12 A patent airway is essential for apneic oxygenation and is best achieved by proper positioning of the patient as well as the use of adjuncts such as an oropharyngeal airway (OPA) or nasopharyngeal airway (NPA). With sedative and paralytic use, the posterior structures of the oropharynx are at risk of collapsing and closing off the airway. Maneuvers that maintain patency of the posterior pharyngeal structures include the jaw thrust, external auditory meatus to sternal notch positioning, and head elevation. Use caution with NPA insertion in pregnant patients due to the risk of epistaxis secondary to increased vascularity of the nasal mucosa.13
Positioning
Patient outcomes are improved when placed in a ramped, heads up, or upright position; therefore, consider using blankets under the shoulders, a commercial ramp, or place the patient in reverse Trendelenburg. This positioning helps mitigate the anatomic and physiologic changes that occur in pregnant patients.8 Patients benefit from the ear to sternal notch position, where the external auditory meatus is in-line with the sternal notch, creating the most direct pathway to the trachea and improving preoxygenation as well as apneic oxygenation.12 Although left lateral decubitus positioning improves blood return by moving the gravid uterus off the inferior vena cava (IVC), this position is technically challenging during intubation1,3,5,15; therefore, lateral displacement of the uterus is recommended instead.
Medications
Request medications while preoxygenating and positioning the patient. This will facilitate the procedure and allow flexibility in approach. Table 4.2 summarizes the medication recommendations for the emergent obstetric airway. Premedications are avoided given the increased risk of errors, delays incurred, and lack of evidence to support their use. Hypotension in pregnancy may not be evident until loss of 25% to 30% of the blood volume7,11; therefore, push dose pressors can be given prior to induction in patients with suspected hypovolemia; phenylephrine in 50 to 200 mcg IV pushes are considered safe for use in pregnancy. Asthmatic patients are at increased risk of bronchospasm due to mucosal edema; thus, ketamine can be used as an induction agent to improve respiratory insufficiency and bronchospasm in these patients.
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