THE CLINICAL CHALLENGE
The physiologic and anatomic changes associated with pregnancy may pose challenges to all facets of airway management including oxygenation, ventilation, and securing the airway. Along with many physiologic changes, late-term pregnancy also presents unique difficulties related to the airway. In fact, complications related to airway management in the parturient patient are the most significant cause of anesthetic-related maternal mortality. Pregnancy changes the patient’s anatomy and physiology in a variety of distinct ways:
• Oxygen reserve and depletion: There is an approximately 20% reduction in expiratory reserve volume, residual volume, and functional residual capacity (FRC) and an increased maternal basal metabolic rate and oxygen demand by the fetal unit. These changes lead to more rapid desaturation during apnea (approximately 3 minutes compared to 6-8 minutes for the normal nonpregnant adult) and are further exacerbated in the obese pregnant patient.
• Physiologic hyperventilation: Progesterone increases the ventilatory drive and leads to hyperventilation. Maternal minute ventilation increases early in pregnancy largely because of an increase in tidal volume. This results in alteration of “normal” blood gas parameters, which must be considered when managing mechanical ventilation. Maternal PaCO2 falls to approximately 32 mm Hg, which is associated with a compensatory decrease in bicarbonate from 26 to 22 mEq per L in order to maintain a normal maternal pH. Mechanical ventilation must provide some degree of hyperventilation in order to maintain maternal pH. A reasonable approach is to increase the minute ventilation by approximately 20% for the pregnant woman in the first trimester, increasing to 40% by term.
• Cardiopulmonary compromise in late pregnancy: In the late stages of pregnancy when the patient is placed supine, the effects of the gravid uterus on the diaphragm and, occasionally, increased breast size on the chest wall further decrease the FRC. In addition to decreasing FRC, supine positioning in the late second and third trimester of pregnancy can result in aortocaval compression by the gravid uterus. This significantly reduces blood return to the heart, impairing maternal and fetal perfusion. This can be mitigated to a certain degree by placing the patient in the left lateral decubitus position.
• Effects on laryngoscopy and bag-mask ventilation (BMV): Pregnancy also can affect laryngoscopy and BMV. Weight gain, greater resistance to chest expansion by abdominal contents, and increased breast size may make BMV difficult in a manner analogous to that seen with an obese patient. Enlarged breast size can also make insertion of a laryngoscope more difficult. The effects of estrogen and increased blood volume contribute to mucosal edema of the nasal passages and pharynx, causing airway tissues to become redundant, friable, and more prone to bleeding, especially with airway manipulation. This mucosal edema can also lead to dynamic distortion of the airway structures and difficulty both in identifying structures and in passing the endotracheal tube through the upper airway to the glottis. This upper airway distortion can be worsened by preeclampsia, active labor with pushing, and the infusion of large volumes of crystalloid fluids. Vascular engorgement also leads to a decrease in luminal size in the trachea, requiring a smaller than expected endotracheal tube (6.5 to 7 on average). The engorged upper airway tissues also can make BMV more difficult.
• Increased propensity for aspiration: As pregnancy progresses, gastric acid secretion increases, causing a decrease in maternal gastric pH as well as an increase in gastrin levels, a reduction in gastric activity, and an increase in gastric emptying time that can result in an increase in resting gastric volume. Gastroesophageal sphincter tone is also reduced in pregnancy. Enlargement of the uterus increases the pressure exerted on the stomach, which combined with a reduction in gastroesophageal sphincter tone, increases the risk of reflux. A “full stomach” should always be a concern in these patients. Administration of neuromuscular blockade will exacerbate this further by causing a loss of supporting abdominal muscle tone. These normal changes in gastrointestinal physiology start early in the second trimester, but become most problematic in the mid- to late second and third trimesters.
• Effects on neuromuscular blocking agents: Maternal plasma cholinesterase activity is reduced by 25%; however, this does not result in any significant effects on elimination, half-life, or duration of effect of succinylcholine. Pregnancy, however, does result in enhanced sensitivity to the aminosteroid muscle relaxants such as vecuronium and rocuronium, which may prolong their effect.
APPROACH TO THE AIRWAY
In early pregnancy, fluid and FRC changes predominate, but the airway itself is unchanged. As pregnancy progresses, difficulty in both intubation and BMV should be anticipated. Nevertheless, the approach to airway management in the pregnant patient is no different from that of any other emergent intubation, except for consideration of the unique features of pregnancy described in The Clinical Challenge section, which may create airway difficulty beyond the sixth month of pregnancy.
Key issues to consider for airway management in these patients are as follows:
1. Anatomically, think of the third trimester pregnant patient as analogous to an obese patient, and use the difficult airway algorithm. If careful assessment using the LEMON, ROMAN, RODS, and SMART mnemonics (see Chapter 2) indicates that rapid sequence intubation (RSI) is reasonable, have backup devices readily at hand, and anticipate more rapid oxyhemoglobin desaturation than for the nonpregnant patient.
2. If flexible endoscopic intubation is the chosen method, avoid the nasal route in favor of the oral. The mucosa may be engorged, edematous, and friable, and nasotracheal intubation is more likely to lead to mucosal damage and bleeding.
3. Preoxygenate fully, using at least eight vital capacity breaths or 3 minutes of breathing 100% oxygen; as FRC is reduced, oxygen consumption is increased, and apnea leads to desaturation more rapidly. This is best provided with the patient in an upright position while passively breathing through either a bag and mask apparatus or a non-rebreather (NRB) facemask with oxygen delivered at the “flush flow rate” (see Chapter 5) of 40 to 70 L per minute. Passive oxygenation during the apneic phase, using nasal cannula at 5 to 15 L per minute flow, should be used routinely if possible. Although neither have been studied in term pregnant women, they significantly delay desaturation in obese patients who have similar physiologic aberrations.
4. All opioids and induction agents may reduce maternal blood flow to the placenta and, therefore, blood flow to the fetus. These agents also cross the placental barrier. Because muscle relaxants are quaternary ammonium salts and are fully ionized, they do not readily cross the placenta. Antihypertensive agents such as metoprolol, labetalol, and esmolol cross the placenta and carry a risk of inducing fetal bradycardia. In the context of emergent airway management, however, maternal well-being supersedes the potential for fetal exposure. When these agents are administered and delivery of the fetus is imminent, the caregiver charged with the management of the neonate immediately after delivery should be fully briefed regarding the agents administered to the mother.
5. Although there is no hard evidence in support the use of the “Sellick maneuver,” it is widely used and recommended in pregnant patients, however, because of the gastrointestinal changes described previously. We consider the maneuver optional, but if it is to be used, the person tasked with applying it should be trained and skilled in the application of cricoid pressure.
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