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Incidence of aspiration of gastric contents during anesthesia has become far less common, yet there are certain populations that are still at increased risk. This is primarily due to changes in physiology of the gastroesophageal junction (GEJ) related to pathophysiology, normal body changes of pregnancy, or obesity. The combination of obesity and pregnancy in an urgent situation can prove to be a challenging case for the anesthesia provider. Unique considerations should be implemented to prevent the serious complication of aspiration pneumonitis.
Anatomy and Physiology
The esophagus begins just distal to the laryngopharynx at approximately the C-6 vertebral level.1 It passes through the mediastinum and makes its way through the diaphragm via an opening known as the esophageal hiatus. The skeletal muscle fibers here form a 2 cm thick continuous ring around the distal 4cm of the esophagus known as the crural diaphragm. The portion of the esophagus here consists of smooth muscle and maintains a continuous tone during the normal resting state.
These two muscular structures are collectively known as the lower esophageal sphincter (LES), and form the border between the esophagus and the cardia of the stomach. The LES functions to prevent reflux of acidic substances into the esophagus. During swallowing these muscles relax as a result of stimulation from receptors located in the pharynx. The pathways which mediate this action are via the dorsal vagal nucleus and nucleus ambiguus, and the right and left phrenic nerve which innervate the crural diaphragm. This control of tone and contraction occurs in the respiratory centers of the medulla.
Normally the LES maintains a pressure of 10-30mmHg, which is greater than the 7mmHg gastric pressure of the stomach.2 The difference in these pressures is known as the barrier pressure, and decreases in LES pressure or increases in gastric pressure can result in lowering of this barrier pressure, which could result in gastric contents entering the esophagus.
The loss of this protective barrier can result in movement of gastirc contents from the esophagus into the pharynx, resulting in aspiration in anesthetised patients.3 Multiple mechanisms related to anesthesia are implicated in reducing the LES pressures and increasing the risk of aspiration. Anesthetic gasses decrease LES pressures up to 14mmHg and in direct proportion to the dose of agent being used. Neuromuscular blockers relax the skeletal muscles of the crural diaphragm and inhibit the protective response of the airway. Opioids contribute by increasing gastric pH, decreasing motility, in addition to relaxing the LES. Cricoid pressure is thought to close the esophagus and prevent any upward movement of gastric contents, but it also stimulates pharyngeal receptors which lead to relaxation of the LES. Administration of Succinylcholine without pretreatment with a non depolarizing NMB can result in fasiculations great enough to increase gastric pressures.
Aspiration pneumonitis, or Mendelsons Syndrome, though a rare complication can result in massive lung injury or death.3 It is caused by inhalation of acidic particulates and cuases an inflammatory process in the alveolar endothelial cells. Patients with gastric volumes > 25ml and pH < 2.5 are at greatest risk. This includes patients with full stomachs, and those with conditions that are to be considered full stomachs. Examples are delayed gastric emptying, gastroparesis, hiatal hernia, pregnancy, and obesity. The latter two are of great interest due to the gastric physiology and anesthetic complications associated with both.
Obesity is defined as a BMI â°¥ 30, and morbid obesity as â°¥ 40.4 These populations are found to have increased volumes of hyperacidic gastric juices, in addition to the risk factors listed previously. Obesity presents its own set of problems for general anesthesia related to difficulty in airway management and intubation, OSA, decreased respiratory reserve, and difficulties with ventilatory management. Adverse events of aspiration are also greatly increased increased due to difficulties of respiatory management.
During pregnancy, barrier pressuers are known to be decreased.5 This is due to the upward and left displacement of the stomach from increased uterine size. The position of the esophagus is also altered, resulting in decreased LES tone.3 Elevated progesterone levels contribute to relaxation of the LES allowing transient opening, which leads to the GERD or heartburn associated with pregnancy. Although the proportions of women with increased gastric volumes and decreased pH are unchanged, during labor gastric emptying is delayed and gastric volumes are increased. Thus these two populations seem to carry a greater risk of adverse events due to both the physiology of the gastric contents and the inability of the sphincter to remain closed with the increased force pushing against it.
Because research has shown increased risks associated with general anesthesia for desarean delivery, it is reserved for populations in which neuraxial anesthesia is contraindicated or maternal or fetal well being is at risk.6 Cesarean is more likely to be needed in obese obstetric patients due to increased likelyhood of macrosmia and malposition of the fetus related to this.7 Unfortunately neuraxial anesthesia can prove to be more technically difficult in the obese.
Review of Current Literature
Recent studies have shown that intragastric pressures are infact much higher in the significantly obese populstion compared to those of normal weight.8 Each inch of increase of waist circumference (WC) increases the gastric pressure by 0.4mmHg, and each 1cm increase in WC resulted in an increase in expiratory intraespohageal pressures of 0.1mmHg, indicative of decreases in mean barrier pressure gradiants. Decreases in LES integritiy were also attributable to seperation of the GEJ caused by these continuous elevated pressures, augmenting the mobility of gastric juices into the espohagus.
The prevelance of obese parturients has increased dramatically in recent years.7 In the United States, up to 34.5% of women of childbearing age are obese, and 18.9% in England. Failed intubation in pregnancy has been shown to be approximately 8 times higher than the general population, as high as 15.5% in those with BMI greater than 35, and as high as 33% in morbidly obese patients. Airway changes in the obese due to excessive mucosal tissue, narrowed upper airway, and difficulty in proper positioning coupled with those in pregnancy such as airway edema due to capillary engorgement contribute to these alarming numbers.9 The higher risk of aspiration in combination with the possibiliity of difficult airway and increased amount of time that aspiration can occur before establishment of a protected airway increase the odds of aspiration dramatically.
In the United States, 35% of mothers who died during cesearean section between 1979 and 2002 were obese.7 Decreasing anesthesia associated risks, specifically aspiration is obligatory and must be addressed aggressively since cesareans done under GA are done as a last option. Assessment early in the labor process is key so that anesthesia providers are prepared should labor become complicated and necessitate cesarean.9 Neuraxial anesthesia should be attempted by the most experienced personell with adequate time given due to increased technical difficulties. Epidural anesthesia should be considered so that a titratable blockade can be achieved and bolus dosing is available as an option for unforseen prolonged surgical time. This also decreases the risk of high block as can be seen with spinal administration. It is also noted that finding the epidural space with a larger needle is less difficult than finding the subdural space with a small spinal needle, thereby increasing the chances of success.
If neuraxial anesthesia is unsuccessful or contraindicated, careful evaluation of the airway should be assessed. Intubation sholuld also be carried out by the most experienced personell as well as having another provider at the bedside for assistance.7 If the airway is anticipated to be difficult, awake fiberoptic intubation is the safest option for the mother should time allow. If GA must be preformed, whether scheduled or emergent, rapid sequence intubation is the safest method if fiberoptic is not to be used. Pretreatment with nondepolarizing neuromuscular blockers sholuld be followed by short acting depolarizing to reduce faciculations. Although there is controversy about whether cricoid pressure reduces the risk of aspiration by closing the esophagus, or increases the risk by relaxation of the LES, until conclusively proven otherwise is the standard of care and should be used. Administration of H2 antagonists to increase gastric pH and metaclopramide to increase gastric emptying is standard.7
As the incidence of obese and morbidly obese partuitents increases to rise, so does the increased risk of failure of the GEJ and aspiration contents leading to aspiration pneumonia and/or death. It is the responsibility of the anesthesia provider to anticipate the special needs of this patient population and take necessary actions to decrease morbidity and mortality.