5.4.1 What Is the Relevance of the Issues of Gastric Volume, pH, and Constituency of the Gastric Contents?
Roberts and Shirley3 concluded that a pH of less than 2.5 and a gastric volume of 25 mL (or 0.4 mL·kg−1) correlated with aspiration and resultant pneumonitis. In their study, an acid solution was injected directly into the bronchus of a monkey and extrapolations were made in regard to the volume and pH that would place humans at risk. These conclusions have been challenged by numerous investigators.25,26 Schreiner26 in 1998 pointed out that over 30% to 60% of patients have a gastric fluid volume of greater than 0.4 mL·kg−1 (median 0.3, but as high as 4.5 mL·kg−1), yet the incidence of aspiration is quite rare. Indeed, it has been demonstrated that the incidence of gastroesophageal reflux (GER) is not associated with residual gastric volume (RGV).27 Rather, it has been shown that GER during anesthesia is related to episodes of straining on an endotracheal tube when inadequate anesthesia has been provided.28
Maltby25 argues that the risk of aspiration is due to loss of the barrier pressure at the gastroesophageal sphincter (GES), also referred to as the lower esophageal sphincter (LES). Normally, stomach contents are prevented from refluxing into the esophagus by the pressure exerted by the LES. The difference between LES pressure and intragastric pressure is the barrier pressure. The stomach is a very compliant structure and intragastric pressure can remain stable until volumes greater than 1000 mL are present.29 Indeed, as intragastric pressure rises, because of its anatomical design, so does LES pressure, maintaining the barrier pressure. In one study, measurements of the intragastric pressure and LES pressure during laparoscopy demonstrated that a rise in mean gastric pressure from 5.2 to 15.7 was matched by a rise in LES tone from 31.2 to 47.0 cm H2O.30
There is a clear association between aspiration and vomiting or gagging.1,6-8,10 With active vomiting, or gagging, the sudden onset of high intragastric pressure is associated with relaxation of both the lower and upper esophageal sphincter mechanisms. This combination enhances the risk of pulmonary aspiration.
The higher the baseline intragastric pressure, the greater the tendency for GER and pulmonary aspiration. With an intestinal obstruction, for example, the intragastric pressure is high in association with the large RGV. This accounts for the high incidence of aspiration in this patient population and the finding that it is one of the most common factors associated with aspiration in most publications. By the same reasoning, patients with a documented hiatal hernia, or a history of GER disease (GERD), are also exposed to a higher risk of regurgitation and aspiration.
Active vomiting, in association with an unprotected airway, is most likely to occur during induction of anesthesia, with airway manipulation prior to placement of the endotracheal tube, and at the end of a procedure as the patient is awakening and the airway is no longer protected. Inadequate levels of anesthesia at these times, as well as difficulty securing an airway, are the essential elements favoring the occurrence of aspiration. Two-thirds of aspiration events are reported during induction and extubation, equally divided between the two periods.6
5.4.2 How Important Is a History of Heartburn? Acid Taste or Burping? a History of GERD? How Much Reflux Is Significant?
Reflux occurs when the barrier pressure fails to prevent gastric contents moving from the stomach into the esophagus. Intuitively, those with a clear history of reflux should be at greater risk of aspiration. Kluger10 found that a history of reflux and hiatal hernia were the ninth and tenth most common predisposing factors for aspiration, representing 7 and 6 cases, respectively, in the database of 133 total cases of aspiration. The patient with a history of acid reflux, with complaints of acid taste or choking at night, represents a more significant risk than one with only complaints of heartburn. The latter may simply suggest gastric mucosal pathology. However, no specific data are available to indicate that one symptom is more helpful than another in identifying who is at greater risk. Again the larger the volume of reflux, the more significant is likely to be the risk of aspiration.
5.4.3 What Clinical Situations and Characteristics Predispose to Aspiration?
188.8.131.52 Emergency Surgery
Emergency surgery is the most significant risk factor associated with aspiration in the studies outlined earlier, increasing the incidence of aspiration by four- to sixfold.6
184.108.40.206 ASA Physical Status
When Warner6 compared the ASA status to the risk of aspiration in elective situations, the risk or aspiration increased by almost sevenfold as ASA status rose from I to IV or V. In emergency situations, the occurrence of aspiration increased from 1:2949 for ASA I patients to 1:343 for ASA IV and V patients, or almost a ninefold increment (Table 5-1). Olsson8 also reported an increased risk of aspiration and increased morbidity with increasing ASA status. Most, if not all, of the reported aspiration-associated deaths occur in ASA IV and V patients.
Table 5-1 Risk of Pulmonary Aspiration in Elective and Emergency General Anesthetics by ASA Physical Status Classification ||Download (.pdf)
Table 5-1 Risk of Pulmonary Aspiration in Elective and Emergency General Anesthetics by ASA Physical Status Classification
|ASA PHYSICAL STATUS||ELECTIVE||EMERGENCY||P|
|I||4/36,916 (1:9,229)||1/2,949 (1:2,949)||.319|
|II||11/82,436 (1:7,494)||3/5,036 (1:1,679)||.043|
|III||31/74,301 (1:2,397)||8/4,413 (1:552)||Less than .001|
|IV and V||6/8,409 (1:1,401)||3/1,029 (1:343)||.066|
|Total||52/202,061 (1:3,886)||15/13,427 (1:895)||Less than .001|
220.127.116.11 Airway and Intubation Difficulties
Difficult intubation is associated with an increased risk of aspiration. Vomiting during airway interventions is frequently associated with aspiration, far more so than passive regurgitation. In Olsson's paper,8 out of 15 cases of aspiration in elective patients in which no risk factors predisposing to aspiration could be identified from the chart review, 10 (67%) had difficulty with intubation preceding the vomiting and aspiration. In total, 58 out of the 87 patients who aspirated did so due to difficulty with intubation or, with airway manipulation. Warner6 described aspiration in 69% of his patients in whom active vomiting or gagging occurred during intubation or extubation. Mort demonstrated that when the number of intubation attempts went from ≤2 to greater than 2, a significant increase in complications occurred. The incidence of regurgitation rose from 1.9% to 22% and aspiration from 0.8% to 13%, directly correlated with an increase in the number of intubation attempts.19 Sakai et al9 reported that 5 of 16 reported aspirations occurred during laryngoscopy or airway interventions including the exchange of airway devices in at-risk patients.
In summary, there are multiple studies describing morbidity and mortality associated with airway interventions and difficulties.19,31-33
There was no correlation between obesity and aspiration in the studies by Warner et al6 or Mellin-Olson et al.7 Olsson,8 however, did find obesity to be a contributing factor for aspiration risk. Obesity is frequently listed as an aspiration-associated factor in many other references. The association of obesity with an elevated risk of aspiration may relate to a high incidence of pertinent comorbidities. For example, delayed gastric emptying is known to be associated with diabetes, which is a more frequent finding in the obese. Other factors related to the obese include GER, difficult intubation, and inadequate anesthesia at the time of induction. This may account for the larger number of obese patients reported in aspiration populations.10
Obese patients have the same gastric emptying rate for liquids as nonobese patients. Depending on the meal content, the gastric emptying in obese patients for solids may be faster, slower, or the same as in the nonobese patients.25,34-37 Maltby25 reported that obesity did not slow gastric emptying in the absence of other predisposing comorbid conditions and suggested that fasting guidelines should be applied to obese patients using the same criterion as for the nonobese. In their paper, obese patients, with no comorbid conditions, were randomized into fasting and nonfasting groups. The later received a 300-mL clear-fluid challenge preoperatively, with no difference in RGV demonstrated postintubation between the two groups. A study reviewing anesthesia for electroconvulsive therapy in 50 obese patients reported no cases of aspiration in 660 procedures.38
It has been well accepted that the obstetrical population is at increased risk of aspiration. This has been felt to be secondary to a number of factors. Hormones, particularly progesterone, cause relaxation of the LES and impair gastric emptying. Mechanical effects of the gravid uterus alter the position of the stomach and, as term approaches, create a gastric pinchcock partially obstructing the gastroduodenal junction. The gravid uterus also increases intra-abdominal pressure, which then increases intragastric pressure. It has been demonstrated that the intragastric pressure in pregnancy is increased to 17.2 cm H2O from the nonpregnant level of 7.3 cm H2O. Women experiencing heartburn in pregnancy have a drop in the LES tone from the normal in pregnancy of 44 cm H2O to 24 cm H2O. Heartburn in pregnancy is reported in some series to be between 45% and 70%, with 27% of these patients having hiatal hernias. The onset of labor with pain and stress, coupled with the presence of opioid analgesics, are independent factors associated with a reduction in gastric emptying. Increased difficulty with intubation occurs in the parturient related to hormonally induced mucosal edema and increased breast mass. For many reasons, the parturient is at an increased risk for aspiration.3,39-41
Interestingly, however, this risk has significantly decreased since Mendelson reported a maternal death rate from aspiration during C-section of 1:667.2,5 This may well be due to the increased use of regional anesthesia, as well as the application of rapid-sequence induction (RSI) techniques, with cricoid pressure and cuffed endotracheal tubes. The use of pharmacologic interventions, although not proven to alter the incidence, may also be a contributing factor. The adoption of difficult airway practices that discourage persistent failing attempts at intubation may be an important factor, as is the increased use of regional anesthesia.
A number of recent publications support the contention that the risk of aspiration has become a much smaller contributor to maternal morbidity and mortality. Mhyre et al42 reviewed all reported maternal deaths in the state of Michigan, USA, between 1985 and 2003. Of the recorded 855 deaths over that time, 15 were felt to be associated with anesthesia in either a related or contributing form. Of the 15 cases, only 1 was felt to be due to aspiration. This occurred in the PACU following caesarian delivery of a stillbirth. The rest of the deaths had no association with aspiration. Interestingly, all anesthesia-related deaths from airway issues occurred during emergence from general anesthesia or in the recovery room. None occurred during induction.
In a recent prospective observational study, McDonnell et al43 reported on the incidence of problems associated with airway management in the parturient, during the period 2005 to 2006, in 13 hospitals with just under 50,000 deliveries per year. During that period, 1095 general anesthetics were performed. In that series, eight cases reported regurgitation (0.7%), with one case of aspiration confirmed (0.1%). Two of the regurgitation cases occurred in elective caesarian sections. Of the eight regurgitation cases, four occurred during induction, and five at emergence (one patient regurgitated at both times). Interestingly, the incidence of difficult intubation was 3.3%, with failed intubation of 0.36%. The later were managed with laryngeal mask airway (LMA).
The low incidence of aspiration is supported by the Closed Claims Analysis of the ASA44. From 1990 onward, only two cases of aspiration were implicated in a maternal death or permanent brain damage, with one of these being in association with general, the other with regional anesthesia.
In conclusion, the incidence of aspiration in obstetrics continues to decline as a source of morbidity and mortality. Increased use of regional anesthesia, a larger number of skilled practitioners comfortable with airway management options, better monitoring, and the use of prophylaxis may all be contributing factors.
According to Warner,6 age was not found to be an independent risk factor for aspiration. Olsson,8 however, did find that extremes of age increased the risk of aspiration. Warner12 reviewed 63,180 anesthetics in children under the age of 18. The incidence of aspiration in that population was not dissimilar to adults, except that there was an increased incidence of aspiration in patients less than 3 years of age. Over 91% of the aspirations in this population had either a bowel obstruction or ileus perhaps skewing the incidence in young children, although there is some uncertainty as to the effectiveness of the LES in this population. Distended stomachs from both fluids and air entrained during crying or using a pacifier predisposes to gastric reflux when these infants cry or gag. The efficacy and method of application of cricoid pressure during RSI in small children has not been defined.
Borland found that the incidence of aspiration in the pediatric population was 10.2:10,000 higher than Warner's reported 3.8:10,000.12,45
18.104.22.168 Decreased Levels of Consciousness and Neurological Disease
It is recognized that the incidence of aspiration of extraglottic (eg, blood) and lower GI tract (eg, stomach contents) contaminants is increased in patients with a reduced level of consciousness.8,13 In these patient populations, loss of function of the LES and upper esophageal sphincter and delayed gastric emptying combine with a reduction of upper airway protective reflexes to promote both regurgitation and aspiration.46,47 This has relevance for the postanesthetic period as well, as it has been shown that patients left in the supine position with a reduced level of consciousness have an increased incidence of aspiration.
Patients with other underlying neurological diseases, such as Parkinson disease and multiple sclerosis, are also at increased risk of aspiration due to impairment of their protective airway reflexes.48 The diabetic with autonomic neuropathy has been demonstrated to have delayed gastric emptying, sometimes manifested by early postprandial satiety, but it is usually asymptomatic. Diabetics have a theoretically increased incidence of difficult laryngoscopy and intubation due to glycosylation of collagen in the cervical vertebrae. Inspite of speculation that diabetics ought to be at increased risk of regurgitation and aspiration,49,50 no studies have found diabetes to be an independent risk factor for aspiration.1,6-8,10-12,45
22.214.171.124 Bowel Obstruction or Other Gastrointestinal Pathology
As discussed in Section 5.4.1 in this chapter, increased gastric volume predisposes patients to an increased risk of aspiration. Gastric obstruction, and or ileus, is one of the commonest associations with aspiration.8,10,12 The incidence of aspiration in esophageal endoscopy is 1:188, and appendectomy is 1:751.
Even in the absence of bowel pathology, recent ingestion of a meal has been documented to be a risk factor for aspiration.6,12 Guidelines for fasting have been developed for the elective population. However, fasting does not guarantee an empty stomach and RGVs can be quite variable.
5.4.4 Is There a Difference in Gastric Emptying in Emergency Patients or Those Who Have Received Opioids?
Trauma patients have been shown to have delayed gastric emptying up to a week after injury. Patients who are critically ill, in ICU, also have significantly delayed gastric emptying.51 Neurological injury, either head or spinal cord, is associated with significant delays in gastric emptying related, in part, to catecholamine surge.52
Opioids, irrespective of the manner of administration, have been shown to decrease gastric emptying significantly.53-55