What Is Unique About the Obstetrical Airway?




INTRODUCTION



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The ability to maintain a patent airway, provide adequate oxygenation, and place an endotracheal tube (ETT) remains a major concern for airway practitioners. Despite many equipment advances and the development of airway algorithms to guide care, management of the obstetric airway is still a cause for concern. Obstetrical anesthesia is a high-risk practice that is replete with medico-legal liability and laden with clinical challenges. On the obstetric service, the practitioner is required to provide safe anesthesia care to mother and baby, both of whom have unique and demanding anatomical and physiological requirements. The purpose of this chapter is to briefly review the status of maternal morbidity/mortality, highlight the principal reasons that airways of parturients might be difficult to manage, and review current guidelines and algorithms for the management of the obstetrical airway.



Underpinning all discussion is the critical importance of being prepared cognitively for the unexpected occurrence and being facile with appropriate emergency airway equipment. Early consultation for anesthesia intervention, and airway assessment of obstetric patients at high risk for operative intervention, particularly parturients who may be obese or have advanced maternal age, remain a key preventative pillar of care. Of equal importance is teamwork between the anesthesia practitioner, the labor and delivery nurses, and the obstetrician. Improved perioperative training of labor and delivery unit support staff (including anesthesia resources for airway management during and after general anesthesia) are important clinical care considerations. Practicing difficult airway scenarios is invaluable. Being unprepared will certainly guarantee failure.




MATERNAL MORBIDITY AND MORTALITY



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Discuss the Anesthetic-Related Morbidity and Mortality of Parturients



Women continue to experience preventable pregnancy-related deaths, with airway management being a significant contributor in developed countries.13 These anesthesia-related deaths are particularly catastrophic, because many of these anesthetics are elective and are administered to young otherwise well mothers.



In 1985, a unique perspective on anesthesia morbidity and mortality was unveiled with the institution of the American Society of Anesthesiologists (ASA) Closed Claims Project database. The data from this project are an accumulation of personal damage insurance claims filed against anesthesiologists and subsequently settled.4 Of the nearly 6500 cases in the database at that time, 12% were associated with obstetrical anesthesia care, and nearly three-fourths of these claims were associated with cesarean section. Critical events involving the respiratory system were the most common precipitating events in the obstetrical files. Trauma from repeated attempts at intubation was recognized as an issue of particular hazard.



Cases from the same ASA Closed Claims Project database between 1990 and 2003 were reviewed in 2009 and the review revealed 426 cases associated with obstetric anesthesia, with 58% of these claims associated with cesarean section. Maternal deaths and brain injury during this period occurred most frequently with high blocks during regional anesthesia, half of which occurred during placement for a vaginal delivery. While these may be attributable to change in anesthesia practice and general anesthesia avoidance strategies, failure to detect accidental intrathecal injection resulting in a high block, as well as delayed response to manage associated cardiorespiratory collapse, were major issues. The most common general anesthesia associated maternal deaths and brain injury claims were due to failed intubation and inadequate treatment of maternal hemorrhage, occurring in equal proportions.5



Obstetrical airway catastrophes occur most frequently during emergency cesarean sections. It is often in settings in which regional anesthesia is not an option because of either maternal condition or severe fetal distress. It is also in these settings that airway evaluation may be particularly hurried and harassed. Overall incidences of obstetrical airway problems are low (7.9%),6 but appear to be greater than in the non-obstetric patient (2.5%).7 Mask-ventilation can be difficult or impossible in approximately 0.02% of parturients, an incidence not dissimilar to other surgical populations.8 Consequences of a failed intubation, however, appear to be greater in the obstetric population.5,9,10



There is little prospective evidence and the literature is unclear as to the actual incidence of failed intubation under general anesthesia in obstetrical patients. While ranges have been given from 1 in 283 to 1 in 2130, a composite incidence of about 0.2%11 to 0.4%12 has been suggested. In a 2005 systematic review, Goldszmidt9 challenged the conventional wisdom and examined the evidence as to whether the obstetric airway is truly more difficult to intubate. In this review, difficult and failed intubation in the obstetric population was found to be rare, and there was no difference in the occurrence of difficult (1%–6%) or failed intubation (0%–0.7%) compared to general surgical populations.



In 2015, Kinsella et al.13 reanalyzed over 33 publications in the obstetric anesthesia airway literature from 1970 to 2014 using newly defined failed intubation criteria to allow for comparability across numerous obstetric airway studies. Using these criteria, their meta-analysis revealed an incidence of failed intubation of 1 in 443 general anesthetics for cesarean delivery (CD) that has remained unchanged over the time period studied.1315 While the actual incidence of difficult airways in the obstetrical population remains unclear, there are concerns that the rates of failed intubation in the obstetric population will increase with declining numbers of women requiring general anesthetics, and the potential loss of skills in managing the airway of an obstetric patient.12,1621




THE PARTURIENT AIRWAY



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Why Do Parturients Have More Airway Complications Compared to the General Population?



The parturient is at significantly greater risk for airway complications and possible difficult intubations than her nonpregnant counterpart.5,10,12,22 A wide range of both anatomical and physiological changes occur during pregnancy and many of these may impact the airway directly or indirectly (Table 51–1). 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 51–1.

Factors Affecting Management of the Parturient Airway





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:



Weight Gain


During pregnancy, average weight gain can be 12 to 20 kg over the parturient pre-pregnant 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 kg·m−2) 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.6 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 (greater than 140 kg or ∼300 lb, BMI ≥40 kg·m−2), the risks for diabetes, hypertension, preeclampsia, and primary CD are all increased. There is also a higher incidence of difficult labor resulting in instrumental deliveries, postpartum hemorrhage, or other conditions which may require anesthetic intervention.23



Morbidly obese parturients are at increased risks for anesthesia-related complications during CD, and increased risks for failed intubation and gastric aspiration if general anesthesia is required.24 The cesarean section rate in these patients can exceed 50%, with one-third of attempted tracheal intubations being difficult and 6% being failures.25 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.26



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, which results in an increased minute ventilation. Displacement of abdominal contents toward the chest due to the enlarged uterus, causes a reduction in functional residual capacity (FRC) and premature airway closure, with widening of the alveolar-arterial oxygen gradient. The reduction in FRC, which begins to decline as early as the fifth month and is reduced to 80% of nonpregnant values by term, and together with the increase in oxygen consumption would lead to exceedingly rapid desaturation with apnea. The tendency toward rapid desaturation is further aggravated by a decrease in FRC in relation to the supine position and obesity.



As a result of these changes, oxygenation of the mother and fetus may be compromised.27 Despite adequate denitrogenation, these physiological changes greatly reduce the time allowable for intubation post-induction.



Airway Changes


Generalized edema may affect the oropharynx and nasopharynx. 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).28



Numerous case reports suggest that edema of the pharyngeal and laryngeal structures (including vocal cords) may hinder visualization of the cords and passage of an ETT.29,30 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 of the trachea. An ETT one size smaller than might be usual (i.e., 6.0–7.0-mm ID) should be routinely used.



Excessive weight gain, even mild upper respiratory tract infections, pre-eclampsia, 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.6,14 This has been confirmed by acoustic reflectometry, which measures oropharyngeal volumes, and is likely a surrogate marker for ease of intubation of the trachea. It has demonstrated pharyngeal narrowing in pregnancy and labor, due to edema and an increase in localized fatty tissue volume.31 The Mallampati score may worsen even further as a consequence of bearing down, and the score may not return to the pre-labor state for a further 12 hours postpartum.32,33 Acoustic reflectometry also revealed decreased volumes both in women after delivery, and in women whose pregnancy was complicated by preeclampsia.28,33,34



Cardiovascular Changes


The supine position may result in compression of the aorta and 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 oxygen desaturation and compromised cardiac output is particularly lethal for the pregnant mother and fetus. This situation is further aggravated by obesity. 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 of the trachea.



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 sphincter 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 transsphincteric pressure. 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 this slowing may be further aggravated by the administration of opioids for labor pain management. Taken together, these gastrointestinal changes mandate that precautions for aspiration of gastric content be taken when a parturient undergoes general anesthesia.



Obstetrical Factors


There are a number of comorbid obstetrical factors that may put the parturient at risk for difficulties in airway management and related complications. Gestational hypertension, eclampsia, and preeclampsia aggravate mucosal and interstitial edema.28 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, together with dysphonia from uvular edema, should alert the practitioner to the possibility of difficult intubation of the trachea.35 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 all these have on the validity, and the positive and negative predictive values of the preoperative airway assessment, is unknown.



Massive peripartum hemorrhage (e.g., placenta previa, accreta, abruption) and acute fetal distress (e.g., 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,36,37 and 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.12,19 Endler et al.24 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.




AIRWAY EVALUATION



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Why Is It Important to Assess the Airway of Each Parturient?



Ideally, every pregnant patient admitted to the labor and delivery service should have a thorough preanesthetic airway evaluation. In the absence of this assessment, interdisciplinary education of our obstetric, general practice, midwifery and nursing colleagues in airway assessment and the risk factors for difficult airways in parturients is imperative. With the always-present risk of acute onset fetal distress, an essential and critical part of airway management is an accurate assessment of the patient’s airway.



A detailed discussion of the airway examination and those predictors associated with management difficulties can be found in Chapter 1. Most predictive studies have been conducted on general surgical populations, not parturients. Some 20 factors predicting difficult laryngoscopic intubation have been identified. The obstetrical patient presents unique assessment challenges, often the most important being a pressure of time.



How Do You Assess the Airway of a Parturient? What Are the Predictors or Risk Factors of a Difficult Airway for a Parturient?



The increasing use of regional anesthetic techniques for delivery has significantly decreased opportunity for clinical studies in patients undergoing general anesthesia. While parturients pose many unique airway challenges to anesthesia practitioners, assessment of the pillars of airway management (BMV, the use of extraglottic devices [EGDs], tracheal intubation under direct and indirect laryngoscopy, and establishment of a surgical airway) should not differ from the non-obstetrical population.



Difficult BMV


As discussed, BMV can be difficult to impossible in approximately 0.02% of parturients. However, this incidence is comparable to the general surgical patient.8 While the mnemonic MOANS (see section “Difficult BMV: MOANS” in Chapter 1) is a helpful reminder of the five patient characteristics associated with difficult BMV,38 many of these characteristics do not apply to the obstetrical population. For example, young and healthy pregnant women are typically not older than 55 years of age, or edentulous, and they do not generally have facial hair. Obesity (BMI ≥30 kg·m−2), however, is an important consideration and is becoming increasingly prevalent among pregnant women. It is noteworthy that 28% of pregnant patients and 75% of preeclamptic women reported snoring compared to 14% of nonpregnant women.28




  • Difficult Direct Laryngoscopy and Tracheal Intubation




    • Section “Difficult DL Intubation: LEMON” in Chapter 1 discusses in detail the current evidence in assessing the predictors of difficult direct laryngoscopy and intubation (LEMON). Dupont and colleagues39 conducted one of the early airway studies in the obstetrical population, and reported that the risk of difficult direct laryngoscopic intubation was eight times greater than in the general surgical population. This has since been disputed by Goldszmidt and others.9,15 Previously published increased rates of difficult and failed intubation in the parturient may in fact be related to anatomic abnormalities unrelated to pregnancy, augmented by emergency conditions, lack of preoperative airway assessment, or differences in intubation experience and expertise.9



    • The literature suggests a variety of clinical signs that can help determine the degree of difficult direct laryngoscopic intubation (Table 51–2), however, none of these has a high positive predictive value as a single tool, particularly in the obstetrical patient. A number of studies have suggested that, although the presence of risk factors was useful, they were not as reliable as the Mallampati examination. Benumof40 has frequently suggested that a patient’s relative tongue/pharyngeal size (Mallampati), degree of atlanto-occipital joint extension, and adequacy of the mandibular space, provide the clinician with three easy to perform and accurate predictors of difficulty in laryngoscopic intubation.



    • Rocke et al.6 conducted one of the sentinel studies specifically looking at the obstetrical population and difficult airway predictors. They prospectively evaluated the airways of 1500 parturients presenting for elective and emergency intubations, and found that a highly predictive sign for a difficult airway was a “neutral” to “extension” sternomental distance variation of less than 5 cm. In addition, the authors built a scale of predictive factors showing clearly that the greater the number of abnormal findings, the higher the prediction accuracy for a difficult intubation (Figure 51–1). The associated risk factors included short neck (SN), protruding maxillary incisors (PMI), receding mandible (RM), and Mallampati Class III and IV. The relative risk of experiencing a difficult intubation in comparison to an uncomplicated Class I airway assessment was as follows: Class II, 3.23; Class III, 7.58; Class IV, 11.3; SN 5.01; RM, 9.71; and PMI, 8.0. Using the probability index for a combination of risk factors, Rocke et al. showed that a combination of either Class III or IV, plus PMI, SN, and RM, correlated with a probability of difficult direct laryngoscopy of >90%. It was interesting that neither facial edema nor swollen tongue was associated with difficult laryngoscopic intubation.



    • Overall, the mnemonic LEMON (see section “Difficult DL Intubation: LEMON” in Chapter 1) examines almost all of the difficult direct laryngoscopic intubation characteristics (with the exception of the PMIs) and remains a useful guide for the obstetrical population. Obesity and BMI are not independent predictors of difficult intubation but an increased neck circumference is.41



    • In the obstetrical patient, obesity and large pendulous breasts often compound airway problems. It is important that the parturient be assessed in the recumbent position with left uterine displacement. Adjustments in the patient’s position should be made before induction of anesthesia, to make intubating conditions easier, but there are limits to the extent that these adjustments can be employed, because of the positioning required to reduce aortocaval compression. In the morbidly obese parturient, elevations (i.e., ramping) (see Figure 51–2 and Chapter 20) of the thorax, shoulders, and head may be necessary to bring the anatomical axes of the oral, pharyngeal, and laryngeal structures into alignment. Positioning on a ramp and use of a short “stubby” laryngoscope handle may also mitigate the problem of the laryngoscope handle abutting on the patient’s chest.




  • Difficult Indirect Laryngoscopy and Tracheal Intubation Using Video-Laryngoscopes




    • While section “Difficult VL Intubation: CRANE” in Chapter 1 discusses in detail the current evidence in assessing the predictors of difficult indirect video-laryngoscopy and intubation (CRANE), some of the predictors (e.g., radiation to the head and neck) may not be applicable to the healthy obstetric population. Furthermore, there is limited clinical information regarding the use of video-laryngoscopes in the obstetric population. In a retrospective review with 180 parturients, Aziz et al.43 showed that video-laryngoscopy resulted in 100% (18 out of 18) successful intubations on first attempt compared to 95% (157 out of 163 patients) with direct laryngoscopy. In a case series involving 27 obstetric patients undergoing general anesthesia, Shonfeld et al.44 reported that video display from the CMAC provided better Cormack–Lehane grade (100% Grade 1 view) compared to standard direct laryngoscopy (52% Grade 1, 44% Grade 2, and 4% Grade 3 and 4 view). Similarly in a randomized trial involving 80 obstetric patients, Arici et al.45 reported that the McGrath Series 5 video-laryngoscope provided better views during orotracheal intubation compared to Macintosh laryngoscope even though the intubation was longer with the video-laryngoscope. While there is a growing interest in the potential use of video-laryngoscopes in the obstetric population, more studies are needed to confirm their role in this patient population.46




  • Difficulty in Use of an EGD




    • The latest iterations of obstetric difficult airway algorithms advocate for even earlier use of an EGD as a part of a failed “Plan A” primary attempt at tracheal intubation. It also advocates that an EGD be used as an important backup maneuver in “Plan B,” and serve as a bridging attempt to reestablish gas exchange in a “can’t intubate, can’t oxygenate” (CICO) setting, while one prepares to perform a surgical airway (i.e., cricothyrotomy, tracheotomy, or “front-of-neck access [FONA]”) in parturients. RODS (see section “Difficult Use of an EGD: RODS” in Chapter 1) is a mnemonic that is intended to identify patients where the use of an EGD may be difficult.




  • Difficult Surgical Airway




    • While the necessity to perform a surgical airway (i.e., cricothyrotomy) in the obstetric population is exceedingly rare, all parturients requiring a general anesthetic ought to have an assessment of the feasibility of this procedure. The mnemonic SHORT (see section “Difficult Cricothyrotomy: SHORT” in Chapter 1) can be used to quickly assess the patient for features that may indicate a difficult cricothyrotomy. In addition, given that the most common complication of this procedure is misplacement due to failure to identify the cricothyroid membrane (CTM),47 it would seem advisable to identify and mark the CTM ahead of time while in supine position (at preanesthesia assessment or pre-induction) in patients who have risk factors for an anticipated difficult airway.48 Identification of the CTM using digital palpation is more challenging in females (independent of body habitus) and in obese patients.49 The use of ultrasound to identify the CTM has greatly improved accuracy, especially on obese patients, and given appropriate training, is potentially an invaluable tool in this setting.49 Most obstetricians do not have experience in performing a surgical airway, and it is incumbent upon the anesthesia practitioner to maintain the necessary skills for this procedure despite their infrequent use. It may be prudent to consult with an experienced surgical colleague for assistance when the need for a surgical airway is anticipated.



    • When considering the best technique, studies have shown that use of a narrow-bore (≤2 mm) cannula technique has the highest failure rate, particularly in obese patients. Greater success is achieved by using a wide-bore (≥4 mm) cannula technique (wire-guided Seldinger, or cannula-over trocar technique), with the highest and most consistent success achieved by using an open surgical method.47,50,51



    • While many anesthesiologists are not comfortable with an open technique, the skills required for the rapid four-step cricothyrotomy (palpation, horizontal incision through skin and CTM, retraction of cricoid cartilage with hook [or insertion of a gum-elastic bougie], and insertion of tube) are easy to master.47,51 Alternatively, there is an argument for using a 4 cm vertical incision for open cricothyrotomy in patients with a poorly identified CTM.14 Nonetheless, success depends on not just having the practical skills, the correct equipment readily available, and trained assistance, but also on timely decision making. Regardless of technique used, failure to recognize immediate need (CICO), and reluctance to perform cricothyrotomy, or other FONA can result in disastrous outcomes.47



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Jan 20, 2019 | Posted by in ANESTHESIA | Comments Off on What Is Unique About the Obstetrical Airway?
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