49.3.1 Why Do Parturients Have More Airway Complications Compared to the General Population?
The parturient is at significantly greater risk for airway complications and difficult intubations than her nonpregnant counterpart.16 A wide range of both anatomical and physiological changes occur during pregnancy and many of these may impact the airway directly, or indirectly (Table 49-1). Many of the changes are hormonally driven and the gravid uterus has a significant impact on the respiratory, cardiovascular, and gastrointestinal systems. Finally, there are a number of abnormal pregnancy-related processes that impact heavily on the parturient airway.
Table 49-1 Factors Affecting Management of the Parturient Airway ||Download (.pdf)
Table 49-1 Factors Affecting Management of the Parturient Airway
|Weight gain (12-20 kg)|
- Enlarging gravid uterus
- Increasing total body water and interstitial fluid
- Increasing blood volume
- Deposition of new fat
- Enlargement of the breasts
- Decrease in respirator reserve volume
- Decrease in functional residual capacity (20%-30%)
- Increased oxygen consumption
- More rapid desaturation
- Increased oral, nasal, pharyngeal, and tracheal mucosal edema
- Vascular engorgement of oral, pharyngeal, and nasal capillaries
- Edema of face and neck
- Advancement of Mallampati classification with pregnancy
- Advancement of Mallampati classification with bearing down during labor
- Inferior caval syndrome (supine hypotensive syndrome) requiring left uterine tilt
- Steadily increasing intragastric pressure as pregnancy progresses
- Decreased lower esophageal sphincter tone due to increasing progesterone
- Symptomatic gastroesophageal reflux
- Distortion of gastric anatomy
- Increased gastric acidity
49.3.2 How Do the Physiological Changes Associated with Pregnancy Impact the Airway of Parturients?
The difficulties in airway management for obstetrical patients may be related to a number of factors as discussed in the following sections.
During pregnancy, average weight gain can be 12 to 20 kg over the parturient prepregnant weight. This weight gain is related to increases in total body water, interstitial fluid (generalized body edema), blood volume, deposition of new fat and protein, uterine size and contents, and enlargement of the breasts.
Obesity (BMI >30) has become much more frequently encountered in the general population over the past decade. Mask ventilation is often difficult in obese patients because of reduced chest compliance and increased intra-abdominal pressure. The incidence of partially obliterated oropharyngeal structures in obese parturients is double that of non-obese parturients.4 In addition, weight gain may create a short neck, a large tongue, and large breasts, all of which contribute to difficult laryngoscopy. In the morbidly obese parturient (>140 kg or approximately 300 lb, BMI ≥40), the risks for diabetes, hypertension, preeclampsia, and primary cesarean delivery are all increased. There is also a higher incidence of difficult labor resulting in instrumental deliveries, postpartum hemorrhage, or other conditions that require anesthetic intervention.17
Morbidly obese parturients are at increased risks for anesthesia-related complications during cesarean delivery, and increased risks for failed intubation and gastric aspiration if general anesthesia is required.18 The cesarean section rate in these patients can exceed 50%, with one-third of attempted tracheal intubations being difficult and 6% being failures.19 In the ASA closed claims obstetrical files, damaging events related to the respiratory system were significantly more common among obese (32%) than non-obese (7%) parturients.20
184.108.40.206 Respiratory Changes
Respiratory changes during pregnancy are of special significance to the anesthesia practitioner. Over the course of a normal gestation, the parturient experiences a 30% to 60% increase in oxygen consumption, because of the metabolic demands of the growing fetus, uterus, and placenta. This, in combination with a reduction in functional residual capacity (FRC), which begins to decline as early as the fifth month and is reduced to 80% of nonpregnant values by term, invites exceedingly rapid desaturation with apnea. The tendency toward rapid desaturation is further aggravated by the supine position.
Displacement of abdominal contents toward the chest, as a result of the enlarged uterus, causes a reduction in FRC and premature airway closure, with widening of the alveolar-arterial oxygen gradient. As a result of these changes, oxygenation of the mother and fetus are easily compromised.21
Generalized edema may affect the oropharynx, nasopharynx, and trachea. These changes are aggravated by elevated estrogen levels that stimulate the development of mucosal edema and hypervascularity in the upper airways. Capillary engorgement of the nasal and oropharyngeal mucosa begins early in the first trimester and increases progressively throughout pregnancy. Accordingly, the parturient frequently appears to have symptoms of upper respiratory infection and laryngitis, with nasal congestion and voice changes due to swelling of the false vocal cords and arytenoids. Nasal obstruction from vascularity and edema may complicate bag-mask-ventilation (BMV).22
Numerous case reports suggest that edema of the pharyngeal and laryngeal structures, and vocal cords, may hinder visualization of the cords and passage of an endotracheal tube.23,24 Tongue edema may make retraction of the tongue into the mandibular space during laryngoscopy difficult. The increased engorgement and vascularity present special challenges in manipulating the nasopharynx (nasal trumpets, nasogastric tubes) or when considering repeated attempts at intubation. An endotracheal tube one size smaller than might be usual (ie, 6.0-7.0 mm ID) should be routinely used.
Excessive weight gain, even mild upper respiratory tract infections, preeclampsia, fluid overload, and bearing down, can all exacerbate airway edema potentially leading to a severely compromised airway. The classical Mallampati classification (Samsoon and Young modification) of mouth opening has been reported to advance by one or two classes during pregnancy.15 This may change even further as a consequence of bearing down, the score may not return to the pre-labor state for a further 12 hours postpartum.25,26 Acoustic reflectometry, which measures oropharyngeal volumes, and is likely a surrogate marker for ease of intubation, revealed decreased volumes both in women after delivery, and in women whose pregnancy was complicated by preeclampsia.22,26,27
220.127.116.11 Cardiovascular Changes
The supine position may result in compression of the aorta, the inferior vena cava, or both by the enlarged pregnant uterus. Compression of the aorta decreases uterine blood flow, impairing fetal oxygenation. Vena caval compression decreases venous return, cardiac output, and ultimately uterine blood flow. A combination of respiratory desaturation and compromised cardiac output is particularly lethal for the pregnant mother. It is therefore imperative that the parturient be positioned with a wedge under the right hip, creating left lateral displacement of the uterus, away from the great vessels. Unfortunately, such displacement may hinder adequate preoperative airway evaluation and the creation of an optimum position for intubation.
18.104.22.168 Gastrointestinal Changes
The risk of aspiration in the parturient impacts how the anesthesia practitioner approaches and manages the parturient's airway. Several factors increase the risk of aspiration in these patients. While intragastric pressure increases steadily during pregnancy, as the gravid uterus enlarges, a concomitant decrease in lower esophageal tone occurs as circulating levels of progesterone increase.
The enlarging uterus distorts esophageal and gastric anatomy. The cephalad pressure of the abdominal uterus decreases the obliquity with which the esophagus contacts the stomach, permitting reflux of gastric contents at lower than usual trans-sphincter pressures. Gastric emptying appears to be unaffected by pregnancy, though intestinal transit time and gastric acidity are increased. With the onset of labor, gastric emptying slows and may be further aggravated by the administration of opioids for labor pain management. Taken together, these gastrointestinal changes mandate that precautions be taken when a parturient undergoes general anesthesia.
22.214.171.124 Obstetrical Factors
There are a number of comorbid obstetrical factors that may put the parturient at risk for airway management difficulties and related complications. Gestational hypertension, eclampsia, and preeclampsia aggravate mucosal and interstitial edema.22 Concomitant proteinuria, with reduced intravascular plasma protein levels, leads to increased edema of the upper airway, an enlarged and less mobile tongue, and soft tissue deposition in the neck.
Preeclampsia is frequently accompanied by coagulopathy and edema, both of which may exaggerate bleeding with repeated attempts at direct laryngoscopy. Airway and laryngeal edema can develop exceedingly rapidly in preeclamptic patients and neck and face edema, and dysphonia from uvular edema, should alert the practitioner to the possibility of difficult intubation.28 In these patients, extreme caution should be exercised not only at intubation, but at the time of extubation as well.
Maternal knee-chest and left lateral positioning, as part of intrauterine fetal resuscitation for non-reassuring fetal heart tracings, may also limit ability to conduct adequate preoperative airway evaluation. The impact that this maternal positioning has on the validity, and the positive and negative predictive value of the preoperative airway assessment, is unknown.
Massive peripartum hemorrhage (eg, placenta previa, accreta, abruption) and acute fetal distress (eg, abruption, cord prolapse) are frequently encountered obstetrical emergencies occurring acutely and unannounced. The visual impact of profuse vaginal bleeding, or the slow ominous sound of the tocodynamometer with fetal distress, frequently pushes obstetricians and anesthesia practitioners to urgently proceed to general anesthesia, without taking the time to adequately assess the patient's airway. General anesthesia in the obstetric population is most frequently conducted for emergency clinical indications.29,30 Most airway catastrophes occur when the difficult airway is not recognized before the induction of anesthesia. Indeed, retrospective publications have reported poor ability to predict difficulty in the obstetrical population, and poor documentation of preoperative airway evaluation.8,13 Endler et al found that emergency surgery was implicated in up to 80% of maternal deaths with general anesthesia, and difficult or failed intubation was associated with 4 of 15 deaths.18