Handbook of Clinical Anesthesia
- Upper Esophageal Sphincter.The cricopharyngeus muscle, one of the two inferior muscles of the pharynx, together with the circular fibers of the upper esophagus, acts as the functional upper esophageal sphincter (UES) at the pharyngo-esophageal junction (Ogunnaike BO, Whitten CW: Gastrointestinal disorders. In Clinical Anesthesia. Edited by Barash PG, Cullen BF, Stoelting RK, Cahalan MK, Stock MC. Philadelphia: Lippincott Williams & Wilkins, 2009, pp 1221–1229). The UES helps prevent aspiration by sealing off the upper esophagus from the hypopharynx in conscious, healthy patients. UES function is impaired during both normal sleep and anesthesia. Most anesthetic agents, except ketamine, reduce UES tone and increase the likelihood of regurgitation of material from the esophagus into the hypopharynx.
- The border between the stomach and esophagus is formed by the lower esophageal sphincter (LES) (Table 46-1). The LES is the major barrier to gastroesophageal reflux. The major physiologic derangement in patients with gastroesophageal reflux is a reduction in LES pressure. The difference between the LES pressure and gastric pressure is “barrier pressure” and is more important than the LES tone in the production of gastroesophageal reflux. Anesthetic agents that may reduce the barrier pressure, thereby reducing LES pressure, include thiopental, propofol, opioids, anticholinergics, and inhaled anesthetics. Antiemetics, cholinergics, antacids, and succinylcholine (Sch) increase LES pressure. Nondepolarizing muscle relaxants and H2-receptor antagonists have no effect on LES pressure. In both conscious and
unconscious patients, cricoid pressure decreases LES tone because of a significant reduction in esophageal barrier pressure while gastric pressure remains normal. The evidence that Sch increases LES tone while cricoid pressure decreases LES tone makes the necessity for application of cricoid pressure during a rapid sequence induction questionable.
Table 46-1 Factors Affecting Lower Esophageal Sphincter Tone
The stomach is very distensible with the capacity to store large amounts of material (≤1.5 L of fluid) without a significant increase in intragastric pressure. There is a dose–response relationship in the severity of aspiration pneumonitis for both gastric volume and acidity that directly reaches the lung. Human breast milk predisposes individuals to an increased severity of aspiration pneumonitis when compared with other types of milk.
III. Protective Airway Reflexes
Protective Airway Reflexes include apnea with laryngospasm, which causes closure of both the false and true vocal cords and coughing. Premedicated and anesthetized patients and elderly individuals have reduced airway reflexes, putting them at an increased risk for perioperative aspiration pneumonitis.
- Reducing Perioperative Aspiration Risk (Table 46-2)
- Control of gastric contentsinvolves minimizing intake; increasing gastric emptying with prokinetics; and reducing gastric volume and acidity with a nasogastric (NG) tube, antacids, H2-receptor antagonists, and proton pump inhibitors (PPIs). Clear liquids can be administered to children and adults up to 2 and 3 hours, respectively, before anesthesia without an increased risk for regurgitation and aspiration. Altered physiological states (pregnancy and diabetes mellitus) and gastrointestinal (GI) pathology (bowel obstruction and peritonitis) may adversely affect the rate of gastric emptying, increasing the risk of
aspiration. The extent of delayed gastric emptying with diabetes mellitus correlates well with the presence of autonomic neuropathy. The American Society of Anesthesiologists recommends a fasting period of 4 hours for breast milk and 6 hours for non-human milk; infant formula; and light, solid meals. The presence of an NG tube does not guarantee an empty stomach and may impair the function of the LES and UES, but it does not diminish the effectiveness of cricoid pressure. An NG tube also provides a direct connection to the outside for passive drainage of gastric contents and is best left in place and open to freely drain during induction of anesthesia.
Table 46-2 Methods for Reducing the Risk of Regurgitation and Pulmonary Aspiration
- Prevention of Pulmonary Aspiration
- Cricoid pressuremay be recommended (its effectiveness is controversial) to occlude the upper end of the esophagus to prevent passive regurgitation of gastric contents and decrease the risk of pulmonary aspiration during a rapid-sequence induction–intubation technique.
- Application of cricoid pressure reduces LES tone and may cause the esophagus to be displaced to the side rather than to be compressed.
- The force applied to the cricoid cartilage should be sufficient to prevent aspiration but not so great as to cause airway obstruction or allow the possibility of esophageal rupture if vomiting occurs. The recommended force is estimated to range between 20 and 44 N; however, cricoid deformation occurs at 44 N with associated cricoid occlusion, vocal cord closure, and difficult ventilation. Awake patients experience pain, coughing, and retching with pressures greater than 20 N, so this amount of force should be applied only after loss of consciousness. A reasonable approach is to apply 10 N of force to the cricoid in the awake state and increase to 30 N after loss of consciousness. Cricoid pressure by itself, before laryngoscopy and intubation, in-creases the incidence of hypertension and tachycardia during induction of anesthesia.
- Airway Protection.Use of a cuffed endotracheal intubation tube is the mainstay of prevention of regurgitated material from reaching the trachea and lungs. Of the other airway devices used, the laryngeal mask airway reduces barrier pressure at the LES with an increased incidence of reflux compared with the cuffed endotracheal tube.
- The Intestines
The small intestine (SI) is the site of most of the absorption of fluids and nutrients from the GI tract. Parasympathetic stimulation increases the activity of the SI, and antagonism decreases this activity. Whereas suppression of sympathetic activity increases SI activity, stimulation results in a decrease. Hypokalemia, peritonitis, and laparotomy all suppress intestinal activity for up to 48 hours. Neostigmine increases colonic activity, and morphine and other opioids decrease both activity and tone.
- Splanchnic Blood Flow.Splanchnic blood flow is influenced predominantly by the autonomic nervous system. α-Adrenergic stimulation leads to vasoconstriction, and β2-adrenergic stimulation causes vasodilation. Splanchnic vascular resistance increases with severe hemorrhage, which helps to divert blood flow to other vital organs. Hypocapnia significantly reduces splanchnic blood flow, and hypercapnia does the opposite. Neostigmine reduces mesenteric blood flow because of inducing exaggerated contraction. Atropine partially offsets the blood flow reduction.
- Postoperative Anastomotic Leakage.Anastomotic leakage after colon surgery may be related to patient factors (anemia, comorbidity), surgical factors (bowel preparation, operative expertise), or factors related to anesthesia and pain management related (morphine, epidural analgesia, neostigmine).
- The theoretical mechanism by which anesthesia-related factors may increase the incidence of anastomotic dehiscence is by increasing intestinal motility and intraluminar pressure.
- Sugammadex, unlike neostigmine, does not stimulate bowel peristalsis.
- Clinical observations and animal studies have largely discounted the suggestion that neostigmine has a deleterious effect on bowel anastomosis.
- Prokinetics such as metoclopramide have been associated with colonic anastomotic dehiscence in animals during the early postoperative period.
- Postoperative Ileus.Multiple mechanisms contribute to ileus after intestinal surgery (Table 46-3).
- Epidural analgesia that includes local anesthetics has been most effective in minimizing postoperative ileus.
Minimally invasive surgery (including laparoscopy), early enteral nutrition, and early mobilization reduce inflammatory responses, thereby reducing ileus.
Table 46-3 Mechanisms of Ileus after Abdominal Surgery
- Sham feeding by chewing gum is thought to accelerate bowel function by increasing vagal cholinergic stimulation of the gut. Gum chewing also increases gastric fluid pH and volume.
- Mesenteric traction syndromeconsists of sudden tachycardia, hypotension, and decreases in PaO2. Nonsteroidal anti-inflammatory drugs and aspirin, which inhibit cyclooxygenase, significantly ameliorate these clinical features, suggesting a prostacyclin-mediated cause. Prophylactic administration of H1 and H2 antihistamines also reduce the incidence of dysrhythmias from the mesenteric traction syndrome.
- Nitrous Oxide and the Bowel.Because nitrous oxide is 30 times more soluble than nitrogen in blood, nitrous oxide diffuses into gas-containing body cavities from the bloodstream faster than the nitrogen in those cavities can diffuse out into circulation. This may contribute to excessive distention of gas-containing bowels, possible bowel ischemia, and increased difficulty with surgical exposure.
- Factors that determine the extent of distention include the amount of gas within the bowel, the duration of nitrous oxide administration, and the concentration of nitrous oxide used. Whereas use of 80% nitrous oxide may potentially result in a fivefold increase in bowel gas, use of a 50% concentration may result in no more than a doubling of bowel gas.
- Nitrous oxide is best avoided in situations when the bowel is distended. On the other hand, it is reasonable to use low concentrations of nitrous oxide during elective abdominal operations in which no significant amount of gas is present in the bowels.
- Carcinoid Tumors
Carcinoid Tumors are usually asymptomatic, although nonspecific symptoms such as abdominal pain, diarrhea, intermittent intestinal obstruction, and GI bleeding are occasionally seen. Nonmetastatic carcinoid tumors secrete hormones that are usually transported to the liver through the portal vein, where they are subsequently inactivated. Carcinoid tumors, especially those arising in the midgut, secrete a variety of hormones, mediators, and biogenic amines, including large quantities of serotonin, that produce increased platelet serotonin levels and increased urinary levels of 5-hydroxy-indole-acetic-acid (5-HIAA), a metabolite of serotonin. Other secreted substances include histamine, substance P, catecholamines (including dopamine), bradykinin, tachykinin, motilin, corticotrophin, prostaglandins, kallikrein, and neurotensin. Bradykinin may produce cutaneous flushing, bronchospasm, and hypotension, and serotonin may cause hypertension or hypotension.
- Carcinoid Syndrome.Metastatic carcinoid tumors release vasoactive peptides into the systemic circulation, which leads to signs and symptoms such as bronchoconstriction, hypotension, hypertension, diarrhea, and carcinoid heart disease. There is no correlation between the blood level of serotonin and the severity of symptoms.
- Carcinoid heart diseaseis seen in up to 60% of patients with carcinoid syndrome. Cardiac involvement is usually right sided, affecting the tricuspid (tricuspid regurgitation) and pulmonary valves. The predominance of right-sided cardiac lesions suggests that the substances secreted from liver metastasis into the hepatic vein never reach the left side of the heart because of pulmonary metabolism.
- Perioperative Management of Carcinoid Patients.Complete surgical excision is the most effective treatment for carcinoid tumors. Management should focus on blocking histamine and serotonin receptors and avoiding drugs that facilitate mediator release from tumor cells. Mediator release may be triggered by opioids and muscle relaxants that release histamine. Patients with carcinoid may have diarrhea and high gastric output. Therefore, fluid resuscitation may be required, and serum electrolytes and glucose should be measured at regular intervals.
- Somatostatinis a GI regulatory peptide that reduces the amount of serotonin released from carcinoid tumors. Somatostatin has a very short half-life of about 3 minutes and must therefore be given as infusion.
- Octreotideis a synthetic somatostatin analogue with a half-life of approximately 2.5 hours. Preoperative preparation should include 100 mg of octreotide subcutaneously three times daily in the preceding 2 weeks, and if necessary, it should be weaned slowly over 1 week postoperatively. Preoperative anxiolytics should be administered to prevent stress-triggered release of serotonin, and patients receiving octreotide preoperatively should continue with their normal dose on the morning of surgery. Octreotide also effectively treats intraoperative patients with carcinoid crises.
- Anesthetic Management.Any of the currently available induction agents and muscle relaxants, including propofol, etomidate, synthetic opioids, vecuronium, cisatracurium, and rocuronium, can be used successfully. Caution should be exercised with drugs such as thiopental and Sch that can release histamine. All current inhalation agents can be used successfully, but desflurane may be the best choice in patients with liver metastasis because of its low rate of metabolism. Administration of octreotide before manipulation of the tumor attenuates adverse hemodynamic responses. The sympathetic blockade produced by epidural or spinal anesthesia may worsen hypotension, which can be minimized by dosing the epidural catheter with opioids or dilute local anesthetic solutions. Intraoperative hypotension from sympathetic blockade should be treated with volume expansion and intravenous infusion of octreotide rather than with sympathomimetics such as ephedrine, which can trigger release of vasoactive substances from carcinoid tumors. Intense hemodynamic monitoring and octreotide administration should continue into the postoperative period because secretion of vasoactive substances may still occur from residual tumor or metastasis.
Editors: Barash, Paul G.; Cullen, Bruce F.; Stoelting, Robert K.; Cahalan, Michael K.; Stock, M. Christine
Title: Handbook of Clinical Anesthesia, 6th Edition
Copyright ©2009 Lippincott Williams & Wilkins
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