Anesthetic Considerations for Bariatric Surgery
2002; Lippincott Williams & Wilkins; Volume: 95; Issue: 6 Linguagem: Inglês
10.1097/00000539-200212000-00061
ISSN1526-7598
AutoresBabatunde Ogunnaike, Stephanie B. Jones, Daniel B. Jones, David A. Provost, Charles W. Whitten,
Tópico(s)Pharmacology and Obesity Treatment
ResumoAccording to the National Institutes of Health, obesity is a major health problem with clearly established health implications, including an increased risk for coronary artery disease, hypertension, dyslipidemia, diabetes mellitus, gallbladder disease, degenerative joint disease, obstructive sleep apnea, and socioeconomic and psychosocial impairment (1). The risk of developing one or more of these obesity-related conditions is based on body mass index (BMI), with 25–30 kg/m2 being low risk and >40 kg/m2 being very high risk (2). The prevalence of obesity in the 18- to 29-yr-old group increased from 12% in 1991 to 18.9% in 1999 (3). Bariatric surgery encompasses a variety of surgical weight loss procedures used to treat morbid obesity. Obesity is clinically expressed in terms of BMI or Quetelet's index (4), which is derived by dividing weight by the square of height to estimate the degree of obesity. Thus, BMI = body weight (kg)/height2 (m2). Morbid obesity is a BMI more than 35 kg/m2, and super morbid obesity is BMI more than 55 kg/m2. The indications for surgical treatment of severe obesity, as outlined in the 1991 National Institutes of Health Consensus Development Conference Panel, include an absolute BMI more than 40 kg/m2 or BMI more than 35 kg/m2 in combination with life-threatening cardiopulmonary problems or severe diabetes mellitus (1). Patients seeking surgical weight loss must have proven attempts at medically supervised weight loss. Documentation of loss of <5% to 10% excess body weight or weight gain after at least 6 mo of diet modification, exercise, and medical therapy or nonimprovement in comorbid conditions during this period indicates failure. Studies have shown that weight loss of 5%–10% of initial body weight improves glucose intolerance and Type II diabetes, hypertension, and dyslipidemia (5–7). The average expenditure is approximately $7000 per year per patient on weight loss programs and equipment. Unfortunately, long-term weight loss is the exception, and most patients regain weight, sometimes more than they initially lost. Surgical Treatment of Obesity Surgical approaches designed to treat obesity can be classified as malabsorptive or restrictive (8,9). Malabsorptive procedures, which include jejuno-ileal bypass and biliopancreatic bypass, are rarely used at present. Restrictive procedures include the vertical banded gastroplasty (VBG) and gastric banding, including adjustable gastric banding (AGB). RYGB, the "gold standard" of bariatric operations, combines gastric restriction with a minimal degree of malabsorption. VBG, AGB, and RYGB can all be performed laparoscopically (10,11). At our institution, laparoscopic Roux-en-Y gastric bypass (RYGB) is routinely performed on patients weighing 180 kg (8,12). Gastric restriction, or gastroplasty, separates the stomach into a small upper pouch (15–30 mL), which restricts food intake. This pouch communicates with the remainder of the stomach through a narrow channel, or stoma. RYGB (Fig. 1), the most commonly performed bariatric procedure in the United States, involves anastomosing the proximal gastric pouch to a segment of the proximal jejunum, bypassing most of the stomach and the entire duodenum. It is the most effective bariatric procedure to produce safe short-term and long-term weight loss in severely obese patients (13). With RYGB, patients lose an average of 50%–60% of excess body weight and show a decrease in BMI of approximately 10 kg/m2 during the first 12 to 24 postoperative months. To the health care provider, the effect of weight loss on associated weight-related comorbidity is more important than absolute weight loss. Studies have shown that Type II diabetes resolves in up to 90% of patients (14).Figure 1: Roux-en-Y gastric bypass. A, A 15- to 30-mL gastric pouch with connected jejunal limb. B, Site of jejuno-jejunostomy.The variables used to measure surgical outcome include operative time, length of skin incision, estimated blood loss, number of patients requiring intensive care unit stay, length of hospital stay, early and late (>30 days) complications, early (<30 days) reoperation, and weight loss. Using these variables, Nguyen et al. (15) found that, with the exception of length of operative time, laparoscopic RYGB was generally associated with better outcomes and cost-effectiveness than open RYGB. The rate of anastomotic leakage is also slightly more frequent with the laparoscopic approach (8), but it becomes comparable once the learning curve has been mastered (approximately 70 cases) (16). Other advantages of the laparoscopic procedure include reduced hospital stay, more rapid return to normal activity, improved cosmesis, and a marked reduction in the incidence of incisional hernia and wound infection (8,15). There are also smaller postoperative pain medication requirements, less pain intensity during mobilization, and improved pulmonary function. Complications after RYGB include anastomotic leak, gastric pouch outlet obstruction, jejunostomy obstruction, deep vein thrombosis (DVT), pulmonary embolism (PE), respiratory failure, gastrointestinal (GI) bleeding, and wound infection. Late complications include prolonged nausea and vomiting, cholelithiasis, ventral hernia, anemia, and protein-calorie malnutrition. Nguyen et al. (15) discovered, in a prospective, randomized study, that these complications are more common after open RYGB than after laparoscopic RYGB, except for late anastomotic stricture, which was significantly more common after the laparoscopic approach. The more frequent leak rate with laparoscopic RYGB was thought to be related to the learning curve. RYGB induces an undesirable "dumping syndrome" if the patient ingests a high-sugar liquid meal (17), with potential side effects of iron and vitamin B12 malabsorption. Dumping syndrome consists of early postprandial abdominal and vasomotor symptoms resulting from fluid shifts and release of vasoactive neurotransmitters (the pathophysiology of which is peripheral) and splanchnic vasodilation, coupled with a relative hypovolemia, leading to diarrhea and abdominal cramps. It occurs in approximately 10% of patients postgastric bypass surgery. Late dumping symptoms are due to reactive hypoglycemia, which results from an exaggerated insulin and glucagon-like peptide 1 release. Symptoms can be relieved with dietary modifications to minimize the ingestion of simple carbohydrates and to exclude fluid intake during ingestion of the solid portion of the meal. Severe cases may respond to agents such as pectin and guar (plant polysaccharide bulking agents that increase the viscosity of intraluminal contents) or to acarbose, an α-glucosidase inhibitor that blunts the rapid absorption of glucose (18,19). Octreotide, a somatostatin analog that alters gut transit and impairs the release of vasoactive mediators, may also be useful in patients refractory to all other therapy (18). It acts through its inhibitory effects on insulin and gut hormone release, a delay of intestinal transit time, and inhibition of food-induced circulatory changes (19). The AGB (Fig. 2), recently approved by the Food and Drug Administration for use in the United States, is the newest gastric restrictive operation and is usually placed by a minimally invasive laparoscopic approach. It consists of an adjustable inflatable band placed around the proximal stomach to limit oral intake (8). It is a less dynamic operation than RYGB and has a learning curve of 30 operations. Up to 50% ± 28% average excess weight loss has been reported with AGB at 2-yr follow-up, with a complication rate of 19% and a mortality rate of 0.4%(20,21). Band erosion and erosive esophagitis were reported by Westling et al. (22) to be the most common complications requiring repeat surgery over 3 yr. Other complications include herniation of the stomach upward inside of the band and band migration from overfilling (23). In a series of 250 laparoscopic AGB patients by Nehoda et al. (24), the most significant complications were early pouch dilations occurring in the first week; however, the most common complications were disconnections at the portal site between the tube and reservoir. Specific contraindications to AGB include inflammatory diseases of the GI tract (such as severe esophagitis, gastric or duodenal ulcers, or specific inflammation, such as Crohn's disease), upper GI bleeding (such as esophageal or gastric varices), portal hypertension, congenital or acquired anomalies of the GI tract (e.g., atresias or stenoses), intraoperative gastric injury (e.g., gastric perforation at or near the location of the intended band placement), liver cirrhosis, chronic pancreatitis, and allergy to the materials used to make the band.Figure 2: Adjustable gastric banding. A, Proximal pouch. B, Adjustable band. C, Needle access port through which saline is injected or removed to vary the size of the adjustable band.Medical Therapy for Obesity Approved indications for drug treatment include a BMI of ≥30 kg/m2 or a BMI from 27 and 29.9 kg/m2 in conjunction with an obesity-related medical complication. The combination of phentermine and fenfluramine (Phen-Fen) was the most popular treatment for obesity until it became associated with valvular heart disease and pulmonary hypertension. As a result of this, Phen-Fen is no longer approved by the Food and Drug Administration and should never be used for this purpose. Sibutramine and orlistat are newer antiobesity medications approved for long-term use. Sibutramine inhibits the reuptake of norepinephrine, serotonin, and dopamine, thereby causing anorexia. These mechanisms act synergistically to increase satiety after the onset of eating rather than reduce appetite (25). It does not promote the release of serotonin, unlike fenfluramine and dexfenfluramine, which primarily increase the release of serotonin in brain synapses and also inhibit the reuptake, thereby causing anorexia (26). These differences in mechanisms of action may explain why there have been no reports thus far of sibutramine causing cardiac valvular lesions. Because sibutramine does not deplete the neural synapses of catecholamines, dangerous hypotension unresponsive to indirectly acting vasopressors (seen with fenfluramine and dexfenfluramine) does not generally occur (27). The most frequent adverse effects of sibutramine treatment include dry mouth, insomnia, anorexia, and constipation (28). Sibutramine also causes transient dose-related increases in both systolic and diastolic blood pressure by a mean of 2–4 mm Hg and induces a small increase in heart rate of 3–5 bpm (29). Although blood pressure decreases with weight loss, this stimulatory effect on blood pressure remains detectable as long as sibutramine is taken. A randomized, controlled trial (30) showed that 13% of those treated with sibutramine, compared with 4% of those treated with placebo, lost ≥10% of their initial weight. Si-butramine results in peak weight loss after approximately 6 mo that is maintained for at least 1 yr (30,31). Orlistat is a synthetic derivative of a product from Streptomyces toxytricini that inhibits mammalian lipase (32). It blocks digestion and absorption of dietary fat by binding lipases in the GI tract. Serum low-density lipoprotein cholesterol concentrations decrease in addition to the weight loss (33). GI complaints induced by fat malabsorption are the most common. A decrease in serum concentration of fat-soluble vitamins (A, D, E, and K) has been observed in approximately 5%–15% of patients (33–35). Some trials with orlistat achieved an average weight loss of 9% compared with 5% in the placebo group at the end of 1 yr (33–35). In isolated cases, orlistat has been blamed as a causative factor of aggravated hypertension in previously treated hypertensive and normotensive patients, but a cause-effect relationship has not been definitely proven (36–38). Warfarin's anticoagulant effect may increase because orlistat decreases the absorption of vitamin K (39). Both orlistat and sibutramine induce ≥5% to 10% weight reduction, with maintenance for up to 2 yr (40). There is a paucity of studies and literature on direct interactions between sibutramine or orlistat and anesthesia medications. However, anesthesiologists should be aware of the side effects of these drugs and their effects on body systems and tailor their anesthesia accordingly. Metabolism/Elimination Histological and liver function test abnormalities are relatively common in the obese, but clearance is usually not reduced. Up to 90% of morbidly obese patients show histological abnormalities of the liver, with one-third of them having fatty change involving more than 50% of hepatocytes (41). In a prospective study of 127 consecutive morbidly obese patients presenting for bariatric surgery, 75% had histological evidence of hepatic steatosis, which was severe and diffuse in 20%(42). Twenty to thirty percent of obese patients without evidence of concomitant liver disease have increased liver function tests. Increased alanine aminotransferase (ALT) is the most frequent hepatic abnormality in the obese population. For every 1% reduction in body weight, ALT activity improves by 8.1%(43). In a study of 198 patients awaiting gastric banding (44), 18.7% had increased liver enzymes. ALT increased by 14.1%, aspartate aminotransferase by 9.6%, and γ-glutamyl transpeptidase by 6.6% before surgery. ALT and aspartate aminotransferase returned to normal after surgery in all the patients in direct proportion to the extent of weight reduction after gastric banding. Palmer and Schaffner (43) showed that in overweight adults without primary liver disease, a weight reduction of ≥10% corrected abnormal hepatic test results, decreased hepatomegaly, and resolved some stigmata of liver disease. More recently (45), 75 morbidly obese patients who had intraoperative liver biopsies at the time of RYGB had an 84% rate of hepatic steatosis, with only approximately 20% having moderate to severe inflammation and fibrosis. Despite these histologic and enzymatic changes, no clear correlation has been found between routine liver function tests and the capacity of the liver to metabolize drugs (46). Renal clearance of drugs is increased in obesity because of increased renal blood flow and glomerular filtration rate (GFR) (47). Ribstein et al. (48) assessed the influence of obesity on renal function and urinary albumin excretion in normotensive and hypertensive subjects and found that the GFR and effective renal plasma flow were increased in overweight compared with lean subjects irrespective of the presence of hypertension. Brochner-Mortensen et al. (49) documented up to a 40% increase in GFR in obese patients; this may be an important contributing factor to proteinuria, the most cited renal abnormality in these patients (50). Other studies (51–53) have also independently shown increases in measured GFR in obese compared with normal-weight subjects. Preoperative Considerations Preoperative Evaluation Attention should focus on issues unique to the obese patient, particularly cardiorespiratory status and the airway. Patients presenting for bariatric surgery should be evaluated for systemic hypertension, pulmonary hypertension, signs of right and/or left ventricular failure, and ischemic heart disease. Signs of cardiac failure—such as increased jugular venous pressure, added heart sounds, pulmonary crackles, hepatomegaly, and peripheral edema—may be difficult to detect. The most common symptoms of pulmonary hypertension include exertional dyspnea, fatigue, and syncope, which reflect an inability to increase cardiac output during activity (54). Identification of tricuspid regurgitation with echocardiography is the most useful confirmation of pulmonary hypertension (55). An electrocardiogram may demonstrate signs of right ventricular hypertrophy, such as tall precordial R waves, right axis deviation, and right ventricular strain. The higher the pulmonary artery (PA) pressure, the more sensitive the electrocardiogram (56). Chest radiograph may show evidence of underlying lung disease and evidence of prominent pulmonary arteries (56). Mild to moderate pulmonary hypertension warrants avoidance of hypoxemia, nitrous oxide, and other drugs that may further worsen pulmonary vasoconstriction. Inhaled anesthetics may be beneficial because they cause bronchodilation and decrease hypoxic pulmonary vasoconstriction (57). With severe pulmonary hypertension, PA catheterization and monitoring may be necessary. Peripheral and central venous access and arterial cannulation sites should be evaluated during the preoperative examination, and the possibility of invasive monitoring should be discussed with the patient. Baseline arterial blood gas measurements will help evaluate carbon dioxide retention and provide guidelines for perioperative oxygen administration and possible institution of and weaning from postoperative ventilation. Patients scheduled for repeat bariatric surgery may confront the anesthesiologist days, months, or years after the initial surgery, so the anesthesiologist should be familiar with possible metabolic changes in these patients. Common long-term nutritional abnormalities include vitamin B12, iron, calcium, and folate deficiencies. Vitamin deficiency is uncommon in patients compliant with daily vitamin supplements, especially in patients followed up with regular postoperative visits. With rapid weight loss, patients may also be protein depleted. Electrolyte and coagulation indices should be checked before surgery, particularly if patient compliance has been poor or if the patient is acutely ill. Chronic vitamin K deficiency can lead to an abnormal prothrombin time with a normal partial thromboplastin time because of deficiency of clotting factors II, VII, IX, and X (58). For elective surgery, the administration of a vitamin K analog, such as phytonadione, can be used to correct the coagulopathy within 6–24 h. Fresh frozen plasma will be required for emergency surgery or active bleeding (59). Concurrent and Preoperative Medications It is recommended that the patient's usual medications, except insulin and oral hypoglycemics, be continued until the time of surgery. Antibiotic prophylaxis is important because of increased risk of postoperative wound infection. Published rates of wound infection after gastric operations for obesity are approximately 5%(60), and rates after clean contaminated GI surgery are 2%–3%(61). A metaanalysis of open bariatric surgery quoted the infection rate of restrictive procedures (VBG, silastic ring vertical gastroplasty, and AGB) as 3%–11%, whereas that of combination procedures (RYGB and extended RYGB) was 5.27%(62). Other authors have quoted wound infection rates of 11.7%–15.8% after open gastric bypass (63,64). In a prospective, randomized study, Nguyen et al. (15) found that open RYGB had an approximately 10 times more frequent incidence of wound infection (10.5% versus 1.3%) when compared with the laparoscopic approach. The increased incidence of wound infection is due to longer incisions, generally longer operative times because of obesity, tissue trauma from excessive traction, difficulty in dead-space obliteration, and inability of adipose tissue to resist infection (65). Antibiotic prophylaxis is, however, also recommended by many practitioners for the laparoscopic approach. Anxiolysis, analgesia, and prophylaxis against both aspiration pneumonitis and DVT should be addressed during premedication. Oral benzodiazepines are reliable for anxiolysis and sedation because they cause little or no respiratory depression. IV midazolam can also be titrated in small doses for anxiolysis during the immediate preoperative period. Pharmacologic intervention with H2-receptor antagonists (e.g., cimetidine, ranitidine, famotidine) and nonparticulate antacids (e.g., sodium bicitrate) and proton pump inhibitors (e.g., omeprazole, lansoprazole, rabeprazole) will reduce gastric volume, acidity, or both, thereby reducing the risk and complications of aspiration. Morbid obesity is a major independent risk factor for sudden death from acute postoperative PE (66,67). Heparin, 5000 IU subcutaneously, administered before surgery and repeated every 12 h until the patient was fully mobile, reduced the risk of DVT (68). Recently, low molecular weight heparins (LMWH) have gained popularity in thromboembolism prophylaxis because of their bioavailability when injected subcutaneously (69). Two recent studies examined two different doses of two different types of LMWH for use in the prophylaxis of DVT in patients undergoing bariatric surgery. Scholten et al. (70) found that 40 mg every 12 h rather than 30 mg every 12 h of enoxaparin resulted in a decreased incidence of postoperative DVT complications without an increase in bleeding complications. In the second study, Kalfarentzos et al. (71) evaluated two different doses of nadroparin (5700 IU versus 9500 IU) for DVT prophylaxis in patients undergoing RYGB and found that the smaller dose (5700 IU) given once daily is safe and well tolerated and has equal thromboembolism prophylaxis as the larger dose (9500 IU) in high-risk patients. Nadroparin is currently not commercially available in the United States. Studies have shown that unfractionated heparin 5000 IU given subcutaneously three times daily is equivalent to the LMWH enoxaparin given once daily for thromboprophylaxis (72). In a survey of members of the American Society for Bariatric Surgery regarding their current practices for thromboprophylaxis (73), small-dose heparin, 5000 U every 8–12 h, was the most preferred method (50% of members), followed by pneumatic compression stockings (33%), LMWH (13%), and other methods (4%). In combination with subcutaneous heparin, we favor placement of pneumatic compression devices on the feet because knee- or thigh-length devices tend to slip and fall off. Intraoperative Considerations Positioning Specially designed tables or two regular tables joined together may be required for safe anesthesia for bariatric surgery. Regular operating room tables have a maximum weight limit of approximately 205 kg, but operating tables capable of holding up to 455 kg, with a little extra width to accommodate the extra girth, are available. Electrically operated or motorized tables facilitate maneuvering into various surgically favorable positions. Bariatric surgical patients are prone to slipping off the operating table during table position changes; therefore, they should be well strapped to the operating table. The use of a bean bag is also recommended. Bean bags (Vac-Pac®; Olympic Medical, Seattle, WA) are soft pads available in various sizes and shapes that are filled with thousands of tiny plastic beads (74). The patient is positioned on the bean bag, which is then molded around the patient, and a suction line is attached to it, creating a vacuum inside the bean bag which allows outside atmospheric pressure to force the beads together so they cannot move. It is worthwhile to note that all materials used to manufacture Vac-Pac® are latex free. Particular care should be paid to protecting pressure areas, because pressure sores and neural injuries are more common in this group, especially in the super obese and the diabetic. Brachial plexus and sciatic nerve palsies have been reported (75). Stretch injuries may be caused by extreme abduction of the arms, thereby stretching the lower roots of the brachial plexus. The upper roots are most likely stretched by excessive rotation of the head to the opposite side (76). Sciatic nerve palsy may be caused by prolonged ischemic pressure from tilting the table sideways. Lateral femoral cutaneous nerve injury may occur if the lower limb falls and hangs freely. Ulnar neuropathy has been associated with increased BMI. A retrospective study by Warner et al. (77) documented such an association because 29% of patients with ulnar neuropathy in their series had a BMI ≥38 kg/m2, compared with only 1% of the control subjects. The extent and degree to which a nerve is injured should be well documented so that recovery and prognosis can be discussed with the patient. Electromyography and nerve conduction studies provide valuable clinical information in this respect. A mild degree of reversible neural insult that results in impulse conduction failure across the affected segment is termed neuropraxia(76). It is a focal conduction block from local myelin injury of primarily larger fibers, with recovery expected within weeks to months. Axonotmesis describes physical disruption of only the axon, with preservation of endoneurial and other connective tissue structures. There is loss of nerve conduction at the injury site and distally, with disruption of axonal continuity and Wallerian degeneration. Recovery of function depends on the time for the process of Wallerian degeneration and neural regeneration to occur. Prognosis is good because the original end organs are reached (76). Neurotmesis implies complete severance of the nerve, with complete disruption of all supporting connective tissue structures. It carries a poor prognosis for complete functional recovery (76). Gradings of this type help with the discussion of prognosis with patient and family. Despite careful positioning and appropriate padding, nerve injury may still occur in this at-risk population. Fortunately, most resolve with time. Laparoscopy and Anesthesia Pneumoperitoneum causes systemic changes during laparoscopy. The gas most often used for this purpose is carbon dioxide. Positioning, such as Trendelenburg, can worsen the systemic changes of pneumoperitoneum (78). Systemic vascular resistance is increased with increased intraabdominal pressure (IAP). The degree of IAP determines its effects on venous return and myocardial performance (79). There is a biphasic cardiovascular response to increases in IAP. At an IAP 20 mm Hg, with decreased venous return from the lower body and consequent decreased cardiac output (79). Increased renal vascular resistance at an IAP >20 mm Hg decreases renal blood flow and GFR (81). Femoral venous blood flow can be reduced by both pneumoperitoneum and Trendelenburg positioning, with an increased risk of lower-extremity thrombosis (82). Abdominal viscera further exert weight on the diaphragm during Trendelenburg positioning, causing a reduction in vital capacity, and placement of surgical packs and retractors in the upper abdomen may worsen the situation (83). Sprung et al. (84) studied the effect of morbid obesity, 20 mm Hg pneumoperitoneum, and body posture (30° head down and 30° head up) on respiratory mechanics, oxygenation, and ventilation during laparoscopy. In contrast however, they did not find body position to have any significant effect on respiratory mechanics during laparoscopy. They reported that, whereas arterial oxygen tension was adversely affected only by increased body weight, respiratory mechanics were affected by both obesity and pneumoperitoneum but varied little with body position. We have witnessed situations in which cephalad displacement of the diaphragm and carina from pneumoperitoneum caused a firmly secured endotracheal tube to be displaced into a bronchial mainstem. Hypercarbia and hypoxemia may be caused by ventilation-perfusion mismatch because of restriction of diaphragmatic mobility from pneumoperitoneum that leads to uneven distribution of ventilation to the nondependent part of the lung. Absorption of carbon dioxide can worsen hypercarbia and acidosis, which can be offset by hyperventilation. Catastrophic complications that should be kept in mind include massive gas embolism, pneumothorax, and mediastinal emphysema. Monitoring Invasive arterial monitoring should be used for the super morbidly obese with severe cardiopulmonary disease and for those with poor fit of the noninvasive blood pressure cuff because of severe conical shape of the upper arms or unavailability of appropriately sized cuffs. Blood pressure measurements can be falsely increased if a cuff too small for the arm is used (85). Cuffs with bladders that encircle a minimum of 75% of the upper arm circumference or, preferably, the entire arm, should be used (86). Comparable and accurate blood pressure readings can be obtained from the wrist (87) or ankle (88) with appropriately sized blood pressure cuffs in situations in which difficulty occurs with upper-arm noninvasive blood pressure measurement. We use central venous catheters in cases in which peripheral IV access cannot be obtained, whereas PA catheters are reserved for serious cardiopulmonary disease. Another strong indication for central venous catheterization is postoperative IV access, which can be problematic in this patient population and is probably more easily performed in the anesthetized patient. Induction, Intubation, and Maintenance of Anesthesia Preparation should be made for the possibility of a difficult intubation, and a surgeon familiar with surgical airways should be readily available. A towel or folded blankets under the shoulders and head can compensate for an exaggerated flexed position from posterior cervical fat (89). The object of this maneuver, known as "stacking," is to position the patient so that the tip of the chin is at a higher level than the chest, to facilitate laryngoscopy
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