Holly Ann Muir
Richard D. Zane
The Clinical Challenge
Along with many other physiological 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. Despite advances in difficult airway management, the incidence of airway management failure in the parturient patient at term remains ten times higher than in the age-matched, nonpregnant population. Pregnancy causes an approximately 20% reduction in expiratory reserve volume, residual volume, and functional residual capacity (FRC), which, along with an increased maternal basal metabolic rate and oxygen demand by the fetal unit, leads to rapid desaturation of pregnant women when rendered apneic. Maternal minute ventilation increases early in pregnancy largely due to an increase in tidal volume. This results in alteration of “normal” blood gas parameters, which must be taken into account 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/L in order to maintain a normal maternal pH. Mechanical ventilation must provide some degree of hyperventilation in order to maintain maternal pH, and it is reasonable to increase minute ventilation by approximately 20% for the pregnant woman in first trimester, progressing to 40% by term. During 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, thus impairing maternal and fetal perfusion. This can be mitigated to a certain degree by placing the patient in the left lateral decubitus position.
Pregnancy also can affect laryngoscopy and bag-mask ventilation (BMV). Weight gain and increased breast size may make direct laryngoscopy difficult, while mucosal venous engorgement of the nasal passages and pharynx cause airway tissues to become friable and prone to bleeding. This mucosal edema can also lead to distortion of the airway structures, leading to difficulty both in identifying structures and in passing the endotracheal tube through the larynx and trachea. This upper airway distortion can be worsened by pre-eclampsia, 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 (6.5–7.0 on average). BMV can be difficult because of redundant and engorged upper airway tissues and limitations of chest wall and diaphragmatic excursion by the breasts and abdominal contents, respectively.
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 as an increase in resting gastric volume. Gastroesophageal sphincter tone is also reduced in pregnancy. With enlargement of the uterus, increasing pressure is exerted on the stomach, combined with a reduction in gastroesophageal sphincter tone, so the risk of reflux is especially high. 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.
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, while the pharmacodynamics and pharmacokinetics of atracurium, a bis-quaternary ammonium benzylisoquinoline compound, are not changed.
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 can be anticipated, regardless of the absence of difficult airway markers. 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 previously, which may create airway difficulty beyond the sixth month of pregnancy.
1. Use the difficult airway algorithm for patients in the third trimester of pregnancy. If careful assessment indicates that RSI is reasonable, have backup devices readily at hand, and anticipate more rapid oxyhemoglobin desaturation than for the nonpregnant patient. Avoid nasotracheal intubation, if possible. The mucosa may be engorged, edematous, and friable, and nasotracheal intubation is more likely to lead to mucosal damage and bleeding.
2. Preoxygenate carefully, 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.
3. 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.
4. Although there is not hard evidence in support of Sellick's maneuver, it is widely used and recommended in pregnant patients, in particular, because of the gastrointestinal changes described previously. An assistant trained in the application of cricoid pressure may be important in this situation.
5. Although rescue airway devices, such as the LMA, intubating LMA, and the Combitube, can be used in the event of failed intubation as for the nonpregnant patient, the enhanced risk of aspiration in pregnancy creates additional urgency for definitive airway control. The successful placement of one of these extraglottic rescue devices may achieve adequate gas exchange, giving the provider additional time to secure a definitive airway and avoid a surgical airway. Nonetheless, as the term pregnant patient may rapidly desaturate, cricothyroidotomy should not be delayed when intubation fails and BMV is not successful.
Recommended Intubation Sequence
· Preparation: A detailed difficult airway examination, including LEMON, MOANS, RODS, and SHORT, should always be performed before making a decision regarding the appropriateness of RSI (see Chapter 7). When the routine difficult airway assessment does not predict difficulty, obesity, enlarged breasts, and physiological airway edema can nevertheless complicate the ability to successfully secure the airway of the pregnant patient. Even if predictors of a difficult airway are not present, the difficult airway algorithm should be employed for late term pregnancy. If, during application of the difficult airway algorithm, the intubator does not have confidence that BMV and intubation will be successful, it may be advisable to consider an awake, sedated, topically anesthetized technique, such as awake flexible or rigid fiberoptic or video laryngoscopy, or awake direct laryngoscopy.
Assemble your airway equipment both for immediate management and potential rescue of a failed airway. Be sure to include a selection of smaller-size endotracheal tubes with stylets loaded; a bougie, short-handle laryngoscope if direct laryngoscopy will be attempted; and, if available, a rescue device with which you are familiar and equipment for a surgical airway.
· Preoxygenation: Eight vital capacity breaths or 3 minutes with 100% oxygen. Use left lateral decubitus positioning when in the supine position to avoid aortocaval compression; tilt the abdomen slightly to the left with a wedge or pillow under the right hip to displace the gravid uterus from the inferior vena cava.
· Pretreatment: Avoid pretreatment with defasciculating agents. Lidocaine and fentanyl are indicated as for the nonpregnant patient if time allows. Fentanyl should be used in the eclamptic or pre-eclamptic patient to mitigate hypertensive responses. Note the previously mentioned caveats regarding opioids.
· Paralysis with Sedation: As for the nonpregnant patient. Unless contraindicated, succinylcholine remains the gold standard for achieving rapid paralysis in a dose of 1 to 1.5 mg/kg. If succinylcholine is contraindicated, a nondepolarizing agent should be administered at the full dose recommended for RSI, despite the risk of prolonged effect after administration. Rocuronium currently offers the most favorable onset time when given in a dose of 1 mg/kg. The choice of induction agent is dictated by maternal hemodynamic condition as in the nonpregnant patient. There is no evidence to support the use of one particular induction agent in pregnancy.
· Positioning: Intubation success can be significantly enhanced with proper positioning prior to administration of induction agents. For the obese parturient patient or one with excessive breast tissue placing a roll, a pillow, or a liter bag of IV fluid vertically between the shoulder blades moves the glottic structures forward and assists in displacing the breasts away from the neck. Positioning of the occiput is equally important because too much extension of the neck can move the glottic structures anteriorly and impede visualization. Placing a pad or folded sheet under the patient's head to bring it into a neutral position may eliminate this. A head up position can help with ventilation both in the spontaneously breathing patient and when positive-pressure ventilation is required.
Cricoid pressure may be more important in the pregnant patient, but evidence supporting its use is scant. If glottic visualization is difficult, cricoid pressure may be released in order to improve the view.
· Placement with proof: It is critical to confirm placement of the endotracheal tube using both auscultation and confirmation of end-tidal carbon dioxide before releasing cricoid pressure.
Specific Clinical Considerations
Management of the Failed Intubation
As with any patient where emergent intubation is required expeditiously, unanticipated difficulty can be encountered despite a careful difficult airway assessment. In the pregnant patient, the approach needs to be modified slightly to accommodate the anticipated physiological impairments imposed by pregnancy. Primarily, this is driven by the rapidity with which the mother desaturates and the commonly encountered airway edema and friability. The recommendation for number of attempts at laryngoscopy prior to moving to a rescue device is reduced to two from three in the pregnant patient, unless success on the third attempt is believed to be highly likely. Even though maintaining oxygenation and ventilation is important for all patients, it is paramount in the pregnant patient, who has reduced physiological reserve. In this circumstance, one should always choose a rescue device with which he or she is most facile and has the most experience. Properly performed, two-handed, two-person BMV may buy time to allow an alternative to cricothyrotomy. Nevertheless, one needs to be prepared to move on to a surgical airway if the rescue device is not able to provide adequate ventilation. Keep in mind that upper airway edema is a common cause of inability to both visualize the glottic structures and ventilate with BMV or supraglottic devices. Resistance to diaphragmatic excursion by the uterus and the weight of the gravid breast on the chest will further impede successful ventilation, so the surgical approach may be the only option.
Postintubation Management
Pregnancy is associated with an increased metabolic rate, which requires progressively increased minute ventilation as the pregnancy progresses. At term, this translates into a 30% to 50% increase in minute ventilation. Arterial blood gases or pulse oximetry and end-tidal carbon dioxide monitoring will aid in adjusting the ventilation parameters. Modest adjustments of both rate (start at 12–14 per minute) and tidal volume (start at 12 mL/kg) will meet the ventilatory need. If ventilation pressures are high, placing the patient in reverse Trendelenburg and left lateral decubitus position to move the abdominal contents down off the diaphragm may bring some improvement.
Tips and Pearls
Taking the time to position the patient slightly head up with a roll between the shoulders and good support under the occiput before induction and attempting intubation may improve success.
Supraglottic airway edema is a common cause for failure in the pregnant patient; therefore, a smaller (6.5–7.0 mm ID) endotracheal tube may be required. In the event of a failed airway when placing an LMA for rescue, briefly decreasing the degree of cricoid pressure may improve success at placement.
When choosing pharmacological agents to facilitate intubation, the general rule of thumb is: “if it benefits the mother in the acute setting, it will ultimately benefit the fetus.”
Evidence
1. Which extraglottic device is best for the pregnant patient? There are no randomized studies comparing the various rescue airway devices in pregnant patients. There are, however, a number of case reports detailing the use of the LMA Classic, LMA ProSeal, LMA Fastrach, laryngeal tube, and Combitube. Naturally, these case reports largely focus on the positive outcomes when these devices are used so the risks and benefits of each are difficult to discern. None of these devices provide complete protection against aspiration, which remains a significant concern in the airway management of the pregnant patient. The laryngeal tube modification with the second lumen for passing an oral gastric tube and the LMA ProSeal with a similar design may offer at least a conduit for stomach contents to exit if regurgitation occurs. Both the Combitube and the laryngeal tube have a lumen/balloon that can provide some barrier to regurgitation; however, experience with both devices in obstetrics is limited.
Although the LMA ProSeal, laryngeal tube, and Combitube may offer some advantage against the risk of aspiration, they cannot be used to secure definitive endotracheal intubation as can the intubating LMA. Currently, the intubating LMA is probably the best choice as a rescue device in the pregnant patient because it can be used as both a rescue device and an intubation device. Nonetheless, the choice of rescue device should be influenced by operator experience (1,2,3,4,5,6,7).
References
1. Backus Chang A. Physiologic changes of pregnancy. In: DH Chestnut, ed. Obstetric Anesthesia: Principles and Practice, 3rd ed. Philadelphia: Mosby; 2004:15–36.
2. Crosby ET. The difficult airway in obstetric anaesthesia. In: Benumof JL, ed. Airway Management: Principles and Practice. St. Louis, MO: Mosby; 1996:638–665.
3. Dennehy KC, Pian-Smith MC. Airway management of the parturient. Int Anesthesiol Clin 2000;38:147–159.
4. Lewin SB, Cheek TG, Deutschman CS. Airway management in the obstetric patient. Crit Care Clin 2000;16:505–513.
5. Minville V, N'guyen L, Coustet B, et al. Difficult airway in obstetrics using ILMA Fastrach. Anesth Analg 2004;99:1873.
6. Munnur U, de Boisblanc B, Suresh M. Airway problems in pregnancy. Crit Care Med 2005;33:S259–S268.
7. Zand F, Amini A. Use of the laryngeal tube-S for airway management and prevention of aspiration after a failed tracheal intubation in a parturient. Anesthesiology 2005;102:481–483.