Chapter 35 The Patient with a Full Stomach
II. Definition of Pulmonary Aspiration
III. Timing of Pulmonary Aspiration
IV. Physiologic Risk Factors in the Perioperative Patient
V. Patients at Risk for Pulmonary Aspiration
VI. Perioperative Anesthesia Considerations in Full-Stomach Patients
VII. General Anesthesia Management
VIII. Management of the Difficult Airway in the Full-Stomach Patient
I Introduction
Anesthetic administration in a patient with a full stomach poses three challenges to the anesthesia provider: prevention of gastric regurgitation, prevention of pulmonary aspiration, and institution of appropriate airway management. The accepted safe anesthesia management plan for reducing pulmonary aspiration of gastric contents in these patients involves identifying those at risk for gastric regurgitation and pulmonary aspiration by using practice guidelines for preoperative fasting, implementing prophylactic pharmacologic therapies, and applying appropriate airway management techniques that may reduce the risk of pulmonary aspiration. A review of the relevant literature suggests that the incidence of perioperative pulmonary aspiration is declining.1 Mortality rates for this dreaded complication have dramatically decreased over the past few decades.1
II Definition of Pulmonary Aspiration
Perioperative pulmonary aspiration of regurgitant gastric contents is defined as the presence of bilious secretions or particulate matter in the tracheobronchial tree. The term bronchoaspiration has been used to describe the same phenomenon, but the previous nomenclature for pulmonary aspiration is used in this chapter.2 Pulmonary aspiration can occur at any time preoperatively until 2 hours after terminating anesthesia. Diagnosis is made by direct examination of the airway, bronchoscopic assessment of the tracheobronchial tree, or postoperative imaging that demonstrates lung infiltrates not previously identified on the preoperative radiograph.3 The incidence, morbidity, and mortality from pulmonary aspiration are considered in Chapter 12.
III Timing of Pulmonary Aspiration
Patients with a full stomach are considered to be at high risk for pulmonary aspiration, which may occur before or during induction of anesthesia or at emergence from anesthesia. Most adult cases of pulmonary aspiration occur during induction of anesthesia (i.e., before laryngoscopy and tracheal intubation) (50%), during laryngoscopy (29%), or during and after emergence from anesthesia.1,4 In the pediatric population, most cases of pulmonary aspiration occur during induction with an inhalation anesthetic or tracheal intubation without a muscle relaxant, and more than 30% take place during emergence and extubation.3
Although most instances of pulmonary aspiration coincide with anesthetic induction, laryngoscopy, or surgery, they can also occur postoperatively.5 Patients who are at risk before surgery are also at risk during the postoperative period because the residual effects of anesthetic agents, muscle relaxants, and narcotic analgesics decrease protective airway reflexes.
IV Physiologic Risk Factors in the Perioperative Patient
Several physiologic risk factors predispose to aspiration pneumonitis (Table 35-1):
1. Increased gastric fluid volume (GFV) with acidic pH, increased bacterial count, or solid material
3. Impaired protective physiologic mechanisms, which include reduction in lower esophageal sphincter (LES) and upper esophageal sphincter (UES) pressure
4. Loss of protective airway (laryngeal-pharyngeal) reflexes
TABLE 35-1 Physiologic Risk Factors for Regurgitation and Aspiration of Gastric Contents
| Physiologic Effect | Risk Factors |
|---|---|
| Increased gastric volume, pressure, and acidity |
CNS, Central nervous system; LES, lower esophageal sphincter; NPO, nil per os (no oral intake); UES, upper esophageal sphincter.
A Gastric Volume and pH
The widely cited criteria for aspiration pneumonitis (GFV = 0.4 mL/kg and pH < 2.5), which were generated from the study of a single Rhesus monkey and extrapolated to humans, have been refuted.6 Current evidence supports a dose-response relationship for gastric volume instilled directly into the lung and gastric acidity. The lethal dose for acid pulmonary aspiration has been studied in numerous animal models.5,7,8 James and colleagues demonstrated that hydrochloric (HCl) acid instilled directly into rat tracheas resulted in a high mortality rate.9 Late mortality rates were 90% with a volume of 0.3 mL/kg at a pH of 1.0 and 14% with a volume of 1 to 2 mL/kg at a pH less than 1.8. An investigation involving primates demonstrated that aspiration of large volumes (0.8 to 1.0 mL/kg) of acid at a pH of 1 was associated with severe pneumonitis. Instillation of smaller volumes (0.4 to 0.6 mL/kg) produced mild radiologic and clinical changes but no deaths. The median lethal dose for acid aspiration into the lungs was 1.0 mL/kg. Extrapolation of these data to humans provides a critical volume of approximately 50 mL for severe pulmonary aspiration in adults.10
The effect of aspiration of milky products into the lungs has also been studied in animals.11 Acidification of human milk to a pH of 1.8 with HCl acid increased the severity of aspiration pneumonitis compared with 5% dextrose acidified to a pH of 1.8 with HCl acid. Acidification of human breast milk with gastric juice instead of HCl acid did not increase the severity of lung injury. Instillation of soy-based formula or other dairy milk formula into the lungs caused a less severe form of acute injury.12 Human milk is particularly noxious when aspirated compared with other types of milk.
In normal, healthy children, the range of residual gastric volume at the time of anesthetic induction is broad. Cote and coauthors reported values of 0.11 to 4.72 mL/kg, which were higher than the values (0.01 to 4.08 mL/kg) reported by Splinter and associates.13,14 Children in clinical studies typically have average gastric volumes greater than 0.4 mL/kg and acidic pH levels,14–22 but they rarely have (1 in 10,000 anesthetics) aspiration pneumonia associated with anesthesia.23 Instead of focusing on GFV at induction, the emphasis to prevent pulmonary aspiration should be on patients’ comorbidities, their risk factors, type of anesthesia, and characteristics of the aspirate.
C Impaired Protective Physiologic Mechanisms
1 Lower Esophageal Sphincter Tone
The LES forms a border between the stomach and the esophagus, with the lower esophagus creating a sling around the abdominal esophagus.24 Barrier pressure is the difference between LES pressure and gastric pressure. Intragastric pressure is normally less than 7 mm Hg. Normal resting LES pressure in conscious individuals is 15 to 25 mm Hg higher than intragastric pressure. An incompetent LES reduces barrier pressure and increases the risk of regurgitation of gastric contents.
LES tone reduction is the major problem in patients with gastroesophageal reflux (GER) during anesthesia and in disease states. In patients presenting with hiatal hernia, the maximum pressure at the gastroesophageal junction was lower (17.1 mm Hg) than the pressure in healthy volunteers (28 mm Hg).25 Intragastric pressure increases to 35 mm Hg when the stomach is distended.26 Gastric distention with increased intragastric pressure and reflex relaxation of the LES causes spontaneous GER. When esophageal pressure equals gastric pressure, the development of a common cavity leads to spontaneous GER.27
Patients with coexisting gastroesophageal pathology presenting for anesthesia are susceptible to GER. The mechanism for reflux is a transient relaxation of the LES.27,28 Anesthetic agents and techniques relax the LES, reduce barrier pressure, and predispose the patient to GER.29 Cricoid pressure application and laryngoscopy during anesthesia also lower LES tone.30 Drugs that lower LES tone include anticholinergics, benzodiazepines, dopamine, sodium nitroprusside, ganglion blockers, thiopental, tricyclic antidepressants, β-adrenergic stimulants, halothane, enflurane, opioids, and propofol.29 Inhalation agents can reduce the LES pressure below the intragastric pressure, depending on the degree of relaxation.29,31 Propofol has no effect on barrier pressure except for a transient decrease in LES tone and gastric pressure at 1 minute, which return to baseline values later.32 Drugs that increase LES pressure include antiemetics, cholinergic drugs, succinylcholine, pancuronium, metoclopramide, domperidone, edrophonium, neostigmine, metoprolol, metoprolol, α-adrenergic stimulants, and antacids.29,32
2 Upper Esophageal Sphincter Tone
The cricopharyngeus muscle acts as the functional UES. It is one of the two inferior constrictor muscles of the pharynx. It extends around the pharynx from one end of the cricoid arch to the other and is continuous with the circular, muscular coat of the esophagus.24 In the conscious, healthy patient, the UES helps to prevent pulmonary aspiration by sealing off the upper esophagus from the hypopharynx.33
Various anesthetic techniques and agents (except ketamine) reduce the UES tone and predispose the patient to regurgitation of material from the esophagus into the hypopharynx. Residual neuromuscular blockade (even with train-of-four ratio of 0.7) puts the patient at risk for pulmonary aspiration because of a reduction in UES tone and impaired swallowing.34–38
D Loss of Protective Laryngeal-Pharyngeal Airway Reflexes
Four well-defined reflexes in the upper airway protect the lungs from aspiration. They are apnea with laryngospasm, coughing, expiration, and spasmodic panting that involves the glottic area and the true or false vocal cords.39 Impaired laryngeal-pharyngeal function usually results from an altered consciousness level. Patients with central nervous system disorders, cerebrovascular injuries, head trauma, alcohol intoxication, and neuromuscular disorders (particularly myotonia dystrophica and scleroderma) have an increased pulmonary aspiration risk due to diminished pharyngeal sensation and diminished protective airway reflexes.40
Anesthetic agents that result in the loss of UES tone impair protective reflexes.41 Prevention of protective laryngeal closure permits entry of this foreign matter into the tracheobronchial passages, causing regurgitation of gastroesophageal contents into the pharynx.
V Patients at Risk for Pulmonary Aspiration
Recognition of patients who have any of the aforementioned risk factors for pulmonary aspiration is the first step toward minimizing the incidence of perioperative pulmonary aspiration. These patients can be categorized in two groups: those with a full stomach (i.e., history of ingestion of a meal with less than 6 hours fasting time)42 and those designated as having a full stomach despite a prolonged preoperative fast.
Patients who are at high risk for pulmonary aspiration are at the extremes of age, have altered consciousness, have ingested solids or liquids despite orders to take nothing by mouth (nil per os [NPO]), are pregnant (particularly those in labor), or have sustained trauma. Trauma, even if not abdominal, delays gastric emptying.43,44 Trauma patients, especially those in acute pain who are scheduled for emergency surgery, have decreased gastrointestinal motility and increased gastrointestinal secretion despite fasting preoperatively.45 The incidence of pulmonary aspiration increases markedly after trauma because of recent ingestion of food, depressed consciousness, diminished or absent airway reflexes, or gastric stasis induced by raised sympathoadrenal influx of catecholamines. In 53 adults with Glasgow Coma Scale scores less than 8 who were intubated by the London Helicopter Emergency Medical Service, the incidence the of gross pulmonary aspiration was 38%.46,47
Other factors increase the risk of pulmonary aspiration. Medications that delay gastric emptying affect the protective physiologic mechanisms. Patients receiving narcotics are expected to have delayed gastric emptying,48,49 but it has also been demonstrated that administration of opioids in a single dose to healthy patients did not delay gastric emptying or affect acidity.50–52
Long-standing diabetes and gastroparesis are risk factors. Diabetic gastroparesis impairs gastric emptying and may compromise LES function.1,53–55 Patients who are morbidly obese can have delayed gastric emptying, increased abdominal pressure, and a difficult airway, all of which are potential risks for pulmonary aspiration. The gastric volumes in 71% of these patients are in the at-risk range compared with the levels found in normal subjects.56 The risk of aspiration also is increased for patients with stress and acute pain, raised intracranial pressure, and neuromuscular disorders.
Certain types of surgery are associated with pulmonary aspiration. The incidence of silent gastric regurgitation is higher in esophageal,1,4 upper abdominal,57 and emergency laparoscopic operations.58,59 Comorbidities are more likely in patients with American Society of Anesthesiologists (ASA) IV or V physical status. Emergency surgery, especially when performed at night, is a risk factor for aspiration.
Failed tracheal intubation of a patient with a difficult airway can lead to hypoxia associated with gastric regurgitation and pulmonary aspiration. General anesthesia administration in patients with a full stomach necessitates tracheal intubation to protect the airway from pulmonary aspiration. If tracheal intubation that is undertaken to isolate the airway and avoid pulmonary aspiration proves to be difficult, the procedure itself can lead to pulmonary aspiration. A difficult intubation can be predicted in only two thirds of cases. Problematic anesthetic inductions compounded by difficult tracheal intubation, difficult mask ventilation, inadequate anesthesia with coughing or straining, difficult emergence, or difficult extubation constitute prime conditions for gastric regurgitation and pulmonary aspiration, and they have been reported in up to 77% of these cases.60–63
Regional anesthesia may be followed by complications. Regional anesthesia is usually considered safe in patients at risk for pulmonary aspiration because they are awake and have intact protective airway reflexes.64 However, after administration of spinal anesthesia, some patients develop extensive sympathetic block followed by hypotension, vomiting, difficulty swallowing, or impaired cough reflex.1 The risk of pulmonary aspiration is exacerbated by difficulty in swallowing because of a high level of sympathetic block. Concomitant administration of narcotics or sedatives can obtund the protective airway reflexes, leading to gastric regurgitation and pulmonary aspiration.39,65,66 Vomiting associated with sympathetic blockade–induced hypotension requires turning the patient’s head to the lateral position and placing the patient in the Trendelenburg position to avoid pulmonary aspiration of gastric contents.
VI Perioperative Anesthesia Considerations in Full-Stomach Patients
A Preoperative Fasting
Historically, adult patients have fasted 8 to 12 hours before surgery to reduce the volume of gastric contents and the aspiration pneumonitis risk. The National Confidential Enquiry into Perioperative Deaths highlighted the issue of preoperative starvation.67 NPO after midnight is an accepted preoperative order. Long fasting before an elective operation is uncomfortable and creates detrimental effects by causing thirst, hunger, irritability, noncompliance, and resentment in adult patients.57 Prolonged fasting is especially deleterious in children because it produces dehydration or hypoglycemia.21,68 During the past 20 years, several scientific papers have challenged the traditional practice of preoperative fasting for more than 8 hours. The current understanding of gastric emptying physiology has generated revised preoperative fasting policies.3,21,50,69–77
Despite the knowledge that the stomach handles emptying solids and liquids differently, physicians traditionally lumped consideration of them together in the standard preoperative order: NPO after midnight the day before surgery. After an extensive review, the ASA task force revised policies and published specific practice guidelines for preoperative fasting.42,78
The Fourth National Audit Project (NAP4) was conducted by the Royal College of Anaesthetists and the Difficult Airway Society. The large-scale review of airway-related complications was published in 2011 after analyzing the events included in NAP4 from September 1, 2008, to August 31, 2009.63 The NAP4 examined records for major complications among the 2.9 million general anesthetics in the operating rooms, airway management procedures in intensive care units (ICUs), and airway management techniques in the emergency departments across the United Kingdom.63 The data revealed 184 serious airway-related complications that led to death, brain damage, emergency invasive airway access through the neck, unanticipated admission to the ICU, or prolonged ICU stay.63 The most frequent fatal complication from general anesthesia was pulmonary aspiration of regurgitated gastric contents, which was implicated in 50% of the deaths.63 Cook and colleagues thought that pulmonary aspiration was a well-recognized problem for patients under general anesthesia and that it should be preventable in most cases. However, in some cases reported in the study, proper precautions were not taken.63 The incidence in NAP4 was three to five times higher than that found in the analysis of the ASA Closed Claims Project database reported from the United States.79
1 Liquids
Residual gastric volume is directly related to regurgitation and pulmonary aspiration. However, the ASA Task Force on Preoperative Fasting, despite extensive scrutiny of the existing data, has been unable to establish a link between residual gastric volume and pulmonary aspiration.78,80
a Clear Liquids
Clear liquids in healthy patients empty exponentially. The gastric emptying half-life (t1/2) of clear liquids is 12 minutes, which is considerably faster than for solids. Theoretically, for a patient who consumes 500 mL of clear liquid, almost 97% of the liquid is eliminated from the stomach after five half-lives (i.e., 60 minutes).81–83 The rate of gastric emptying after a liquid meal has been well studied in adults.74,76,80 The studies demonstrated that after drinking 750 mL of pulp-free orange drink, the mean t1/2 ranged from 10 ± 7 to 20 ± 11 minutes. The fastest t1/2 for an individual subject was 2.9 minutes, and the slowest t1/2 was 41.6 minutes, indicating that almost complete gastric emptying could be accomplished in 2 hours (approximately five half-lives) after a clear liquid drink was consumed. Even in individuals with the slowest emptying rates, only 10% or less of the original liquid was retained in the stomach after 2 hours.72,75,84–86
The ASA Committee on Standards and Practice Parameters supports a fasting period of 2 hours after the ingestion of clear liquids for all patients.78 Clear liquids include water, fruit juices without pulp, carbonated beverages, clear tea, and black coffee. The clear liquid category does not include alcohol. The volume of liquid ingested is less important than the type of liquid.
b Milk
In term and preterm infants, breast milk leaves the stomach more rapidly than formula milk. The gastric emptying t1/2 for breast milk is approximately 25 minutes; for formula milk, it is twice as long.87,88 Consultants and the ASA Committee on Standards and Practice Parameters recommend fasting 4 hours after breast milk and 6 hours after infant formula.78
2 Solids and Nonhuman Milk
Gastric emptying for solids occurs in a linear pattern with time, and 10% to 30% of ingested solids may remain in a patient’s stomach after 6 hours.89 Gastric emptying is inhibited when the duodenum is distended; when the chyme contains a high concentration of acid, proteins, or fats; or when the osmolarity is not iso-osmolar. Gastric emptying after solid ingestion also depends on body posture after intake, exercise, meal weight, size of food particles, amount of food, and the type of food.90,91 Lack of readily available, appropriate methodology makes the assessment of stomach contents in the perioperative period difficult.
Miller and coworkers investigated patients who ate a light breakfast of a slice of buttered toast and a cup of tea or coffee with milk 2 to 4 hours before surgery.92 Gastric contents were measured by inserting a gastric tube after anesthetic induction. There was no significant difference in gastric volume or pH between the control group (fasting) and the study group. Soreide and associates investigated a group of healthy, female volunteers who ingested a standard hospital breakfast of one slice of white bread with butter or jam, one cup (150 mL) of coffee without milk or sugar, and one glass (150 mL) of pulp-free orange juice.93 Gastric contents were measured by repeated ultrasonography and paracetamol absorption techniques. No solid food was detected in any volunteer 240 minutes after breakfast. The latter test determined that at least 4 hours between eating and surgery are needed for solid foods to empty from the stomach.
It is appropriate to fast at least 6 hours after intake of a light meal or nonhuman milk before elective procedures. The ASA Committee on Standards and Practice Parameters revised the guidelines because the members found that intake of fried or fatty foods or meat could prolong gastric emptying time. In these situations, 8 hours or more may be needed for preoperative fasting.78
3 Patients with Diabetes
Radioisotopic techniques and electrical impedance tomography have been used to show that type 1 diabetic patients have delayed gastric emptying.55 After ingestion of a semisolid meal, the mean t1/2 of gastric emptying was 54.8 ± 26.6 minutes in diabetic patients compared with 40.4 ± 8.6 minutes in nondiabetic control subjects. Diabetic patients require a longer fasting period (8 hours) than nondiabetic patients.
4 Ambulatory Patients and Anxiety
Anxiety delays gastric emptying and increases acid secretion.94,95 Ambulatory surgery increases anxiety. A background study showed that the mean gastric volume was 69 mL in outpatients and 33 mL in inpatients, with an average pH of less than 2.5 in both groups.96
To test the hypothesis that preoperative stress affects residual GFV and pH in pediatric patients, children between the ages of 3 and 17 years were randomly assigned to three groups: outpatients, inpatients, and patients who had multiple operations.13 There were no differences in the residual GFV between the three groups and no differences in gastric contents between inpatients and outpatients. The relationships among oral premedication, preoperative anxiety, and gastric contents showed that premedication reduces anxiety. However, there were no correlations among the type of premedication, level of anxiety, gastric volume, and gastric pH.97
B Fasting Guidelines Summary
Almost 80% of elective operations are scheduled for ambulatory patients or those with same-day admittance. The ASA Task Force on Preoperative Fasting recommends avoiding anesthetizing a patient with a full stomach to reduce the risk of pulmonary aspiration.80 The ASA practice guidelines about minimum fasting period for ingested material include the following recommendations: clear liquids, 2 hours; breast milk, 4 hours; infant formula, nonhuman milk, and a light meal, 6 hours. A fast should precede elective procedures requiring general anesthesia, regional anesthesia, or sedation or analgesia (i.e., monitored anesthesia care). The guidelines also recommend a fasting period for a meal that includes fried foods, fatty foods, or meat of 8 hours or more before elective procedures. Diabetic patients need 8 hours or more for gastric emptying after ingesting semisolid material.
Most anesthesiologists practicing outpatient anesthesia in the United States have conformed to the recommendation of the ASA practice guidelines for preoperative fasting time.77 Similar opinions from associations linked to the World Federation of Societies of Anesthesiologists and from the current literature have led to changes in preoperative fasting guidelines worldwide.69–71,98,99
C Role of Preoperative Ultrasonography
Ultrasonography has been proposed as a useful, noninvasive, bedside tool to determine gastric content and volume in the perioperative period based on studies in parturients,100 a pilot phase study enrolling 18 healthy adult volunteers,101 and a follow-up phase II study enrolling 36 adult volunteers that suggested the gastric antral cross-sectional area was a reliable indicator of the quantitative assessment of gastric volume in the right lateral decubitus position.101 Another prospective trial performed an ultrasonographic qualitative and quantitative analysis of the gastric antrum in 200 fasted patients undergoing elective surgery by assigning a grade to each patient on a 3-point grading scale. Eighty-six patients were categorized as grade 0, suggesting an empty antrum (0 mL); 107 patients as grade 1, suggesting minimal fluid volume (16 ± 36 mL) detected only in the right lateral decubitus position; and 7 patients as grade 2, suggesting a distended gastric antrum with fluid 180 ± 83 mL visible in supine and right lateral decubitus positions.102 Results of this study of elective surgical patients indicates that preoperative ultrasonography may be able to help identify patients who are at higher risk of pulmonary aspiration. Because certain patient populations, such as trauma patients undergoing surgery, are known to be at high risk for pulmonary aspiration with associated morbidity and mortality,46 it seems prudent to use ultrasonography as a screening tool preoperatively for these patients.
D Pharmacotherapy
The ASA practice guidelines do not recommend the routine preoperative use of gastrointestinal stimulants, medications that block gastric acid secretion, antacids, prokinetics, antiemetics, or anticholinergics to reduce the risk of pulmonary aspiration in patients who have no apparent increased risk.78,80
Proton pump inhibitors (PPIs) reduce intragastric acidity and gastric juice volume, and they have been advocated for preanesthetic use to prevent pulmonary aspiration.103 Peptic ulcer bleeding is common, and recurrent bleeding is an independent risk factor for mortality. A PPI infusion prevents recurrent upper gastrointestinal bleeding after endoscopic therapy.104 A study from Japan showed that rabeprazole may be a suitable alternative to standard H2-blocker prophylaxis against acid aspiration pneumonia.105
Surveys on pulmonary aspiration prophylaxis in the full-stomach, nonobstetric population are rare. The accepted pharmacologic regimens include an attempt to manipulate the gastric pH, volume, and barrier pressure.106–114 Other surveys of antacid prophylaxis are limited to obstetric anesthesia.115–120 Most studies suggest improved safety from reduced gastric volume or an increase in gastric pH, or both.1,121–131 However, no data show evidence of improved outcome after the use of antacids, H2-receptor blockers, PPIs, or prokinetics. Because of the paucity of data and a lack of evidence to prove the value of pharmacologic therapies for preventing pulmonary aspiration, it is not possible to provide a cost-benefit ratio.78,80
E Preoperative Gastric Emptying
1 Preoperative Gastric Tube Insertion Before Emergency Surgery
Preoperative gastric emptying is not without hazards. Preoperative insertion of a gastric tube and subsequent gastric emptying in an awake, unsedated patient elicits a profound sympathetic response and oxygen desaturation.132 This response, which is similar to the cardiovascular response after tracheal intubation without analgesia,133,134 puts the patient with cardiac problems at risk for cardiac ischemia and dysrhythmias. Routine preoperative nasogastric tube placement is not recommended except in selected patients with small bowel or gastric outlet obstruction. Although the gastric tube helps to reduce intragastric volume and pressure, it does not guarantee that the stomach is completely empty. Preoperative nasogastric tube insertion and stomach decompression are recommended only in patients with a distended stomach (e.g., bowel obstruction).
2 In Situ Gastric Tube During Induction
The disposition of an existing gastric tube during induction of general anesthesia is a debatable issue. After the stomach is decompressed, the gastric tube can be removed or left in situ during a rapid-sequence induction (RSI). Although the presence of the gastric tube may impair the function of the UES and LES,135 studies with cadavers have shown that the efficacy of CP application during induction of anesthesia is not impaired.136,137 If the patient has a nasogastric tube, it need not be withdrawn before induction of anesthesia because it acts as an overflow valve and prevents pressure buildup in the stomach; it also allows drainage during anesthesia induction.138
3 Effects of Gastric Tube Placement in Mechanically Ventilated Patients
The third concern was tested in mechanically ventilated ICU patients to determine the effect of the nasogastric tube size on the incidence of GER.139 The concern about GER is that it can result in pulmonary aspiration and bacterial pneumonitis. Investigators found no significant difference in GER and pulmonary aspiration with the use of small-gauge and large-gauge nasogastric tubes.
4 Sealing the Esophagus by Inflatable Cuffs
Inflating a cuff at the gastroesophageal junction to prevent gastric reflux is considered to be unsafe. A newly designed, but similar nasogastric device with an inflatable balloon to occlude the gastric cardia (Aspisafe, Braun, Melsungen, Germany) was first studied in pigs (Fig. 35-1).140 After gastric filling with large volumes, despite maneuvers and drugs used to promote regurgitation, the new nasogastric device did not produce GER.140 The same experiment was duplicated in healthy volunteers and surgical patients considered to be at risk for pulmonary aspiration, and findings were similar.140 Use of this device is considered safe because there was no test evidence of GER after RSI.39
The device was subsequently studied in conjunction with the use of a laryngeal mask airway (LMA). A dye indicator injected into the stomach revealed that the balloon tube prevented GER.141 Future clinical studies should decidedly prove balloon tube safety and usefulness in preventing regurgitation during induction and emergence.
VII General Anesthesia Management
A Awake Tracheal Intubation in Patients at High Risk for Pulmonary Aspiration
A full-stomach patient with a difficult airway warrants an awake tracheal intubation. The potential for managing a difficult airway (i.e., difficult intubation or difficult mask ventilation) may be self-evident because of a preexisting or acquired condition. However, normal individual anatomic variation may contribute to the difficulty with tracheal intubation or mask ventilation. Various physical characteristics are associated with a difficult airway, including a small mouth, limited mouth opening, short interincisor distance, prominent upper incisors with overriding maxilla, short neck, limited neck mobility, receding mandible or mandibular hypoplasia, high-arched and narrow palate, temporomandibular joint dysfunction, rigid cervical spine, obesity, and congenital anomalies found in infancy. Morbidly obese patients and infants particularly are at risk for a difficult airway and pulmonary aspiration. No single feature on physical examination accurately predicts a difficult intubation, but a variety of simple diagnostic tests have been suggested to identify patients with difficult airways.142–154
Intubation of the trachea while the patient is awake is a useful technique with a high degree of acceptance by patients, and it is considered a fail-safe method of choice when gastric regurgitation and pulmonary aspiration are likely.155 Awake intubation is useful in situations of anticipated difficulties in tracheal intubation and in patients with intestinal obstruction, gastrointestinal hemorrhage, or upper airway obstruction; in seriously ill or moribund patients; and in those with respiratory failure.156
Efficacy and safety of awake intubation in full-stomach patients can be accomplished with the use of minimal sedation, administration of oxygen by nasal cannula during intubation, and application of local anesthetic to the pharynx, larynx, and trachea. The premise of conscious sedation is to balance the comfort of the patient and tolerance of the procedure and still have a patient responsive to commands. Judicious sedation with small amounts of short-acting benzodiazepines and narcotics helps allay anxiety and helps the patient who is awake tolerate any discomfort that occurs during tracheal intubation. Ovassapian and colleagues presented the concept of awake fiberoptic intubation using judicious sedation and topicalization of upper and lower airways in patients at high risk for pulmonary aspiration.157 An accompanying editorial stated that this study of 121 patients was too small to accept the safety of the technique of awake intubation, sedation, and topicalization of the airway.
B Rapid-Sequence Induction
RSI of anesthesia to protect the airway from pulmonary aspiration of gastric contents has evolved since the introduction of succinylcholine in 1951 and the first description of CP by Sellick in 1961.135 The primary objective of RSI is to minimize the time interval between loss of consciousness and tracheal intubation. The essential features of RSI include preoxygenation with 100% oxygen, administration of a predetermined induction dose, application of effective CP, and avoidance of positive-pressure ventilation until the airway is secured with a cuffed tracheal tube.158,159 Many emergency physicians use RSI of anesthesia as their technique of choice to facilitate orotracheal intubation in patients presenting to the emergency room.160–165 Having specialized equipment for management of failed tracheal intubation is an integral part of RSI.42,166–168
RSI plays a major role in emergency anesthesia and is used almost universally for obstetric general anesthesia in the United Kingdom and United States158; however, it is practiced less widely in mainland Europe.169 A survey of French anesthetists showed that only 23% used a full complement of measures to prevent pulmonary aspiration, and CP was rarely used,1,170 but the incidence of fatal aspiration in France is lower than in other countries (1.4 per 10,000 versus 4.7 per 10,000 episodes of anesthesia).1,170
Preoxygenation is a standard component of RSI.159 Preoxygenation with fresh gas flows of 100% oxygen through a mask with a good fit for 3 to 5 minutes is recommended. Alternatively, a series of four vital capacity breaths of 100% may be used in an emergency.159 Thiopental remains the most popular induction agent for RSI,159 although propofol also is used extensively. Other intravenous induction agents include ketamine and etomidate.171,172
Succinylcholine remains the muscle relaxant of choice for use in RSI.159 Certain conditions may preclude the use of succinylcholine, as in burns and spinal cord injuries, because administration of succinylcholine can result in adverse effects such as hyperkalemia and dysrhythmias. Several studies have demonstrated that rocuronium in adequate doses (0.8 to 1.2 mg/kg) produces intubation conditions rapidly comparable to those with succinylcholine 1 mg/kg.173,174 However, this large dose leads to prolonged duration of action of up to 1 hour.175 In a patient with a difficult airway, this drug may allow less margin for error than succinylcholine.176 Fifty percent of cases of difficult intubation occur without preoperative predictive signs and can increase the risks of gastric regurgitation and pulmonary aspiration in full-stomach patients.177
1 Modified Rapid-Sequence Induction
RSI is a well-established technique in anesthesia practice. During standard RSI, patients are made apneic and unconscious without establishing the ability to ventilate the lungs. Positive pressure usually is avoided to prevent gaseous distention of the stomach and subsequent pulmonary aspiration in RSI. However, avoidance of ventilation of the lungs during RSI precludes the ability to test the airway and verify that a patient’s lungs can be ventilated by mask before the administration of a muscle relaxant. The technique of RSI therefore involves inherent risks to the patient, including the possible inability to secure an airway or to ventilate the lungs and oxygenate the patient who is unconscious and apneic. During RSI, failure to secure the airway can lead to many attempts at tracheal intubation with subsequent trauma to the airway, and failure to ventilate the lungs can lead to hypoxia, hypercarbia, and alterations in heart rate and blood pressure, resulting in significant morbidity. These risks may not be warranted or appropriate, particularly in patients who are at risk for gastric regurgitation and pulmonary aspiration. For them, appropriate management can include modification of the standard RSI technique consisting of preoxygenation, laryngoscopy using an elevated-head position,178 induction of anesthesia, application of CP, apneic diffusion oxygenation,179 and the added step of gentle positive-pressure ventilation of the lungs before tracheal intubation.135,180
