Anesthesiology

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What the Anesthesiologist Should Know before the Operative Procedure

Bariatric surgical procedures have traditionally been grouped as either malabsorptive (biliopancreatic diversion, for example) or restrictive (including gastric banding and sleeve gastrectomy) depending on the presumed mechanism for weight loss.

The most commonly performed bariatric procedure in the United States, the Roux-en-Y gastric bypass (RYGB), does not fall in either category as the weight loss is thought to result from hormonal changes affecting satiety, culminating in decreased caloric intake. The rest of this chapter will focus mainly on the RYGB.

The RYGB consists of two anastomoses: a proximal gastrojejunostomy and a distal jejunojejunostomy. Because the upper stomach is completely closed off, >95% of the stomach, all of the duodenum and 15-20 cm of proximal jejunum are excluded from digestive continuity. The procedure can be done either laparoscopically or via laparotomy.

Patients presenting for weight loss surgery are obese (BMI>30) to severely obese (BMI>40) and can have significant comorbidities (such as cardiorespiratory derangements, diabetes), potential for airway complications, as well as postoperative adverse events.

1. What is the urgency of the surgery?

What is the risk of delay in order to obtain additional preoperative information?

Bariatric surgery is an elective procedure. In general, prior to this surgery, patients have undergone several months of evaluation by a multidisciplinary team: psychologist, nutritionist, internist, and surgeon. A pre-anesthetic evaluation should take place before the operation.

2. Preoperative evaluation

  • Severe obesity is a condition which affects all systems. Many patients presenting for weight reduction surgery will have limited mobility and, thus, may be asymptomatic at rest despite significant cardiorespiratory derangements.

  • Medically unstable conditions warranting further evaluation include: myocardial infarction and active ischemia, heart failure, unstable arrhythmias, stroke, TIA, COPD exacerbation, significant hypertension, pulmonary hypertension, and hyperglycemia. They warrant further assessment, consultation, workup, and stabilization.

  • Delaying surgery may be indicated if: the patient is not medically optimized for this elective procedure.

3. What are the implications of co-existing disease on perioperative care?

b. Cardiovascular system

Perioperative evaluation

I. Obesity is associated with hypertension, diastolic and systolic dysfunction, coronary artery disease, and arrhythmias. In patients with severe obstructive sleep apnea, pulmonary hypertension is a concern. Valvular dysfunction may be associated with prior use of amphetamine-based appetite suppressants. Limited functional capacity may challenge the assessment of cardiovascular function. It may be necessary to utilize noninvasive testing in the presence of multiple risk factors (such as history of ischemic heart disease, heart failure, CVA, insulin- dependent diabetes) or limited mobility.

II. Baseline coronary artery disease or cardiac dysfunction—Goals of management— good clinical history (functional status, changes in exercise tolerance, fatigue), prior EKG or noninvasive stress testing, and consider speaking with patient’s cardiologist or primary care physician.

Perioperative risk reduction strategies

Monitoring—Noninvasive blood pressure monitoring is recommended unless otherwise dictated by the patient’s medical condition. The forearm should be considered as placement of a blood pressure cuff in a conically shaped arm can be difficult.

Goals—optimize myocardial oxygen supply and demand ratio

  • Supply (increase)

    • i. FIO2-consider 80-100%

    • ii. Heart rate—consider reducing with beta blockers in order to increase time in diastole. Left ventricle is perfused only in diastole.

    • iii. Diastolic perfusion pressure—Coronary perfusion=Diastolic perfusion pressure-left ventricular pressure.

  • Demand (reduce)

i. Heart rate - beta blockers reduce rate and oxygen consumption

ii. Contractility

iii. Afterload - Increased afterload (SVR) increases the tension that the heart needs to pump against, and hence myocardial oxygen consumption (pressure work). However, if afterload (SVR) is too low, it may jeopardize diastolic perfusion pressure.

iv. Preload - need to optimize to maintain adequate stroke volume, without pushing too far along Frank-Starling curve (avoid increasing pressure work). Patients presenting for RYGB may be fairly hypovolemic due to a dehydrating bowel prep and weight loss before the surgery. However, an increasing number of bariatric centers are implementing enhanced recovery after surgery (ERAS) pathways allowing the ingestion of clear liquids up to 2 hours prior to the procedure. These may mitigate the risk of hypovolemia.

c. Pulmonary

Sleep Apnea (OSA)

i. Perioperative evaluation: Careful clinical history (from patient as well as bed partner) including loud snoring, episodes of apnea/hypopnea, daytime somnolence –consider use of the STOP-Bang questionnaire for screening (Chung 2008), oxygen requirements, use of non-invasive positive pressure ventilation (NIPPV) at home (such as CPAP), current medication regimen, and available studies (polysomnography). A review of prior anesthetic records as well as a thorough airway exam (Mallampati classification, thyromental distance, size/condition of the teeth, neck diameter/mobility etc.,) are imperative to estimate the probability of difficult mask ventilation and intubation. Equally important is an assessment of the patient’s ability to lie flat as this may reveal significant compromise of oxygenation and airway patency.

ii. Perioperative risk reduction strategies:

  • Continue current medications and NIPPV preoperatively; ask the patient to bring his own equipment for postoperative use while in the hospital.

  • Pursue maneuvers intraoperatively to avoid airway obstruction such as use of a nasopharyngeal airway, semi-recumbent or reverse Trendelenburg position during induction of and recovery from general anesthesia

  • Use preoperative sedatives judiciously as patients with OSA have increased sensitivity to the respiratory depressant effects of opioids, hypnotics and sedatives. Consider use of sedatives with minimal respiratory depression such as clonidine.

  • Consider awake fiber optic intubation when faced with the potential for difficult airway management.

  • Maintenance—VT around 6 ml/kg of IBW; recruitment maneuver followed by 7-10 cm H2O PEEP as tolerated by hemodynamics. Add PEEP (reduce atelectasis and shunting) before flattening patient from semi-recumbent position. Consider use of PCV.

  • Extubate patient when fully awake after full recovery from neuromuscular blockade. Consider use of nasopharyngeal airway if high risk for airway obstruction.

  • Postoperative—Semi-recumbent position (should avoid the supine position), NIPPV if needed.

ii. Reactive airway disease (Asthma)

i. Perioperative evaluation: Assess severity of disease including frequency of exacerbations, triggers (cold weather, environmental, medications, upper respiratory infections), control and rescue medications, recent oral or IV steroid therapy, ER visits, hospitalizations, ICU admission or intubations.

ii. Perioperative risk reduction strategies:

  • Continue control medications perioperatively and consider course of nebulizer or steroid therapy if patient appears to have moderate to severe disease.

  • Ensure a deep state of anesthesia and adequate muscle relaxation prior to instrumentation of the airway in order to avoid coughing, gagging, eliciting bronchospasm. For severe intraoperative bronchospasm, consider a low dose intravenous infusion of epinephrine (approx. 0.25 mcg/min).

  • Maintenance—keep patient deep to avoid bronchospasm, adjust I:E ratio to allow for adequate expiratory time (prevent small airway closure), PEEP (reduce atelectasis and shunting), humidify airway gases.

  • Postoperative—Appropriate pulmonary toilet, incentive spirometry, nebulizer and steroid therapy if appropriate

d. Renal-GI:

Renal—the elevated baseline intraabdominal pressure in the obese patient is transmitted to the kidney and renal vasculature and has been implicated in dilation of afferent arteriole as well as elevation in RBF and GFR. Patients are often dehydrated from bowel prep and weight loss just prior to surgery.

i. Perioperative evaluation:

I. Dehydration

II. Assessment of volume status can be difficult in the obese patient. History of oral intake, vital signs, mucus membranes, urine output, lab values should be reviewed.

III. Laboratory testing. Baseline creatinine, BUN, sodium, hemoglobin, should be performed.

IV. Chronic kidney disease (CKD): arterial and abdominal hypertension, hyperlipidemia as well as insulin resistance are risk factors for CKD. Hypotension, hypoxia, inadequate fluid status, or factors that lead to inadequate renal perfusion can cause further damage to the kidneys.

ii. Perioperative risk reduction strategies: Avoid potential nephrotoxic drugs (aminoglycosides), adequate volume repletion intraoperatively.

GI—a positive correlation exists between obesity and frequency of gastroesophageal reflux (GERD) symptoms and presence of hiatal hernia, thus consider the possibility of full stomach and the need for rapid sequence induction. Non-alcoholic steatohepatitis (NASH), a more advanced form of non-alcoholic fatty liver disease (NAFLD) associated with severe obesity, can progress to cirrhosis.

i. Perioperative evaluation: Careful clinical history regarding frequency and severity of GERD symptoms as well as review of studies such as UGI endoscopy and radiographic abdominal evaluation. NAFLD tends to be asymptomatic.

i. Laboratory testing. Baseline liver function tests should be performed.

II. Perioperative risk reduction strategies: Make preparations to reduce risk of pulmonary aspiration: consider promotility, gastric acid reducing, gastric volume reducing, and antacid medications as appropriate, prior to GA. An orogastric tube should be placed after intubation. It must be removed prior to gastric stapling.

e. Neurologic:

N/A

f. Endocrine:

Endocrine - Diabetes mellitus, most commonly Type II (resistance to endogenous insulin), has a strong association with obesity.

I. Perioperative evaluation: Metabolic control and medication regimen should be carefully reviewed.

i. Laboratory testing. Baseline creatinine, BUN, glucose, hemoglobin A1C, EKG should be performed.

II. Perioperative risk reduction strategies: Blood glucose should be monitored throughout the perioperative period, particularly in insulin-dependent patients, to minimize the risk of hypoglycemia.

III. Chronic disease: A long history of diabetes may predispose the patient to stiff joints, nephropathy, peripheral neuropathy, CAD, PVD as well as autonomic dysfunction (orthostatic hypotension, gastroparesis and intraoperative hemodynamic instability). Limitations in cervical spine and temporomandibular joint mobility mandate a thorough evaluation of the airway. Consider placement of an arterial line to assist with glucose and blood pressure monitoring. Phenylephrine infusion (alpha agonist) may be helpful for blood pressure maintenance with changes in intraoperative patient position. If gastroparesis, may consider full stomach precautions.

g. Additional systems/conditions which may be of concern in a patient undergoing this procedure and are relevant for the anesthetic plan (e.g., musculoskeletal in orthopedic procedures, hematologic in a cancer patient)

i. Perioperative evaluation: Careful clinical history considering a prior history of embolic events (e.g., DVT), family history as well as coexistence of other risk factors such as use of oral contraceptives.

ii. Perioperative risk reduction strategies: Perioperative chemoprophylaxis is recommended as mechanical DVT prophylaxis may be limited by inadequate equipment size.

4. What are the patient's medications and how should they be managed in the perioperative period?

h. Are there medications commonly seen in patients undergoing this procedure and for which should there be greater concern?

Cardiac medications

I. Beta blockers—patients on beta blockers should be continued perioperatively, watching for hypotension from reduced contractility and bronchospasm.

II. Statins—patients on statins should be continued perioperatively. Discontinuation is associated with increased risk of adverse cardiac events and mortality.

III. Aspirin—restart as soon as appropriate postoperatively. Not a contraindication for neuraxial techniques, even at high dose. Is a coronary stent the indication for aspirin? If so, increased vigilance for acute myocardial ischemia and infarction.

IV. Angiotensin-converting enzyme (ACE) inhibitors and angiotensin II receptor antagonists—long-term therapy with these medications has been associated with intraoperative hemodynamic instability and hypotension, especially in the hypovolemic patient. Consider holding dose 24 hours prior to surgery.

Endocrine

I. Oral hypoglycemic agents—hold on day of surgery

II. Insulin—On day of surgery, patients should be instructed to take one-half to two-thirds of their total insulin dose as intermediate acting (NPH) insulin. Blood glucose levels should be assessed periodically in the intraoperative period.

i. What should be recommended with regard to continuation of medications taken chronically?

I. Cardiac: see above for beta blockers, statins, aspirin,

II. Pulmonary: recommend continuing control medications (inhaled beta agonists, leukotriene inhibitors, inhaled steroid, and oral therapy) perioperatively. If moderate or severe, may consider additional therapy (inhaled beta agonists as a nebulizer, steroid course).

III. Endocrine: see above for oral hypoglycemic agents and insulin.

IV. Gastrointestinal: continue proton-pump inhibitors and other anti-reflux treatment preoperatively.

V. Psychiatric: continue anti-depressant and anti-anxiety medications perioperatively

j. How To modify care for patients with known allergies -

N/A

k. Latex allergy—If the patient has a sensitivity to latex (e.g., rash from gloves, underwear, etc.) versus anaphylactic reaction, prepare the operating room with latex-free products.

N/A

l. Does the patient have any antibiotic allergies?

Given the high risk of perioperative wound infections, obese patients presenting for gastric bypass receive 2 g of cefazolin as antibiotic prophylaxis. If the patient's weight exceeds 120 kg, the cefazolin dose is increased to 3 g. If allergic to penicillin, then 900 mg clindamycin should be administered.

m. Does the patient have a history of allergy to anesthesia?

Malignant hyperthermia

  1. Documented—avoid all trigger agents such as succinylcholine and inhalational agents:

    1. Proposed general anesthetic plan:

    2. Insure MH cart available: [MH protocol]

  2. Family history or risk factors for MH:

  3. Local anesthetics/muscle relaxants:

5. What laboratory tests should be obtained and has everything been reviewed?

  1. Hemoglobin levels: patients are frequently dehydrated and thus hemoglobin must be evaluated in light of volume status (hypovolemic, euvolemic, or hypervolemic). Polycythemia is commonly seen in patients with obesity hypoventilation syndrome.

  2. Electrolytes:

    • i. Potassium—patients on potassium wasting diuretics, or hemodialysis may have abnormalities

    • ii. Sodium—dehydration

    • iii. BUN/Creatinine—ratio greater than 20:1 often suggests hypovolemia

    • iv. glucose

Intraoperative Management: What are the options for anesthetic management and how to determine the best technique?

a. Regional anesthesia

Not an option intraoperatively, but may be considered for postoperative analgesia in patients scheduled to undergo an open gastric bypass in the absence of low molecular weight heparin for DVT prophylaxis.

  1. Neuraxial—for postoperative analgesia

    1. Benefits

      1. Pain control; excellent pain control may preclude need for IV narcotics and their side effects

      2. Potentially lower incidence of MI and postoperative hypoxia

    2. Drawbacks

      • contraindicated or limited in patients taking low molecular weight heparin, warfarin, or have coagulopathy. Follow ASRA guidelines.

      • Positioning of the obese patient may be difficult and landmarks may be difficult to palpate. Consider use of ultrasound.

      • High failure rate

      • May delay patient mobilization and removal of urinary catheter

      • e. Sympatholysis may not be tolerated (CAD, hypovolemia).

    3. Issues—need to plan postoperative anticoagulation therapy with surgical team.

      Patients can be started on LMWH, however removal of catheter needs to be timed appropriately.

      If BID dosing need to wait 12 hours from prior dose, remove catheter, and hold repeat dosing for 2 hours.

b. General Anesthesia-

  1. Benefits

    • i. secure airway, positive pressure ventilation and PEEP in patient with high probability of OSA.

    • ii. Need to mechanically ventilate during pneumoperitoneum (for laparoscopic gastric bypass) or laparotomy (open gastric bypass).

  2. Drawbacks: swings in hemodynamics and, potentially, postoperative respiratory depression in OSA patients.

  3. Other issues:

  4. Airway concerns: A review of prior anesthetic records as well as a thorough airway exam (Mallampati classification, thyromental distance, size/condition of the teeth, neck diameter/mobility etc.) are imperative to estimate the probability of difficult mask ventilation and intubation. Equally important is an assessment of the patient’s ability to lie flat as this may reveal significant compromise of oxygenation and airway patency. Consider awake fiber optic intubation when faced with the potential for difficult airway management. Patient should be extubated in semi recumbent position when fully awake, after full recovery from neuromuscular blockade.

c. Monitored Anesthesia Care- not an option

6. What is the author’s preferred method of anesthesia technique and why?

For this intra-abdominal procedure, most commonly performed laparoscopically, the preferred anesthetic is a general anesthetic with endotracheal intubation as it allows for a secure airway and control of ventilation. Occasionally placement of the intravenous cannula can be challenging and an ultrasound can be useful in locating the vein.

Obesity can alter the tissue distribution and elimination of drugs. Obese people have larger absolute lean body mass as well as fat mass than non-obese individuals of the same age, gender and height. However, at present our knowledge of the influence of obesity on pharmacokinetics is limited. In practice it seems that these patients require more than would be estimated based on ideal body weight. Bouillon and Shafer (Anesthesiology. 1998) suggest that for a person larger than ideal body weight, the dose of anesthetic drugs be based on IBW or IBW plus some fraction of the difference between TBW and IBW.

If we take into account not only the altered pharmacokinetics but the fact that obese patients are more sensitive to the respiratory effects of anesthetics, it seems sensible to use short acting drugs (like propofol) and predictable muscle relaxants like cisatracurium. Of the inhalational anesthetics, sevoflurane and desflurane have very low lipid solubility.

Their use is associated with a rapid emergence and potentially reduced depressant effects postoperatively.

Given the increased sensitivity to the respiratory depressant effects of sedatives in patients with OSA, a narcotic-sparing multimodal approach to pain control is recommended. This may include local wound infiltration with local anesthetics, acetaminophen, and NMDA receptor antagonists among other adjuncts. At our institution most patients receive postoperative PCA (no basal infusion) with either morphine or hydromorphone for breakthrough pain. Given the association between nonsteroidal anti-inflammatory agents and marginal ulcers, most surgeons prefer to avoid their use in patients undergoing Roux-en Y gastric bypass.

  • What prophylactic antibiotics should be administered? - Based upon recommendations published in October 2009, antibiotic prophylaxis gastric bypass is Cefazolin. If ß-lactam allergy: Clindamycin. Antibiotics should be reviewed to determine if there were recent changes in the recommendations since October 2009.

  • What do I need to know about the surgical technique to optimize my anesthetic care? -

    • i. Types of surgical procedures

      • Roux-en Y gastric bypass (laparotomy)—Patient position will be supine, with bed in steep reverse Trendelenburg throughout most of the procedure; thus it is important to adequately pad and secure the upper extremities to prevent injury. In general, blood loss is minimal. Hemodynamic changes may take place with position change. These may be minimized with adequate hydration and use of a pressor such as ephedrine or phenylephrine. Towards the end of the procedure, the surgeon may request instillation of a methylene-blue solution into the stomach (via orogastric tube) to check integrity of the anastomosis.

      • Roux-en Y gastric bypass (laparoscopy)—Patient position will be supine, with bed in steep reverse Trendelenburg throughout most of the procedure; thus it is important to adequately pad and secure the upper extremities to prevent injury. In general, blood loss is minimal. Hemodynamic changes may take place with position change and insufflation of the abdomen. These may be minimized with adequate hydration and use of a pressor such as ephedrine or phenylephrine. It is critical that the orogastric tube as well as any tubing that may be in the stomach (e.g., esophageal stethoscope, temperature probe) be removed prior to gastric stapling. Consider monitoring axillary temperature (instead of esophageal). Towards the end of the procedure, the surgeon may insert a gastroscope orally to evaluate the gastrojejunostomy.

      • Laparoscopic gastric banding: Patient position / changes in hemodynamics same as b. In this restrictive surgical option the stomach is divided into two sections by placement of an adjustable silicone band around its uppermost portion. The pouch, located above the band, is small (approximately 10-20 ml capacity) compared to the rest of the stomach below the band. The band is connected to an infusion port which can be easily accessed postoperatively. Injection of saline into the port decreases the band’s diameter and, thus, the size of the communication (stoma) between the two gastric compartments. Compared to other surgical alternatives for weight loss, this is relatively simple and reversible. This procedure is generally shorter than the RYGB with faster recovery. NSAIDS may be a suitable option for pain relief.

      • Laparoscopic sleeve gastrectomy: Patient position / changes in hemodynamics same as b. A procedure that can be either the first step in a staging weight loss surgery strategy or a stand alone procedure, this option consists of removing most of the stomach and creating a tubular stomach without interruption in gastrointestinal continuity.

  • What can I do intraoperatively to assist the surgeon and optimize patient care?

    • Muscle relaxation will allow for adequate abdominal insufflation and facilitate mechanical ventilation.

    • Adequate positioning and careful padding of pressure points to prevent nerve injuries, pressure sores and rhabdomyolysis.

  • What are the most common intraoperative complications and how can they be avoided/treated? Prioritize them by urgency.

    • a. Cardiac complications—optimize myocardial oxygen supply/demand ratio perioperatively. Assess the need for invasive monitoring. Continue beta-blockers and statins if previously on these medications.

    • b. Pulmonary embolism—use of pneumatic compression boots (if appropriate fit available), anticoagulation efforts with LMWH or heparin and early ambulation

    • c. Airway complications

    • 1. Desaturation: ventilatory strategies incorporating the use of PEEP, high FIO2 and recruitment maneuvers as needed. Ensure adequate placement of ETT as it may migrate into the right main stem bronchus with the cephalad mediastinal shift triggered by pneumoperitoneum.

    • 2. Bronchospasm: maintain a deep level of anesthesia, use inhalational agents, consider a low dose intravenous epinephrine infusion for refractory bronchospasm.

    • 3. Permissive hypercapnia is acceptable, particularly in patients with OSA.

a. Neurologic:

N/A

b. If the patient is intubated, are there any special criteria for extubation?

Ensure the patient is in semi recumbent or reverse Trendelenburg position, fully awake, alert and recovered from neuromuscular blockade.

c. Postoperative management

  1. What analgesic modalities can I implement? PCA

  2. What level bed acuity is appropriate? The choice of floor, telemetry, step down, or ICU admission depends on the patient's preoperative comorbidities (particularly severity of OSA), intraoperative course, and hospital resources.

What are common postoperative complications, and ways to prevent and treat them?

Postoperative hypoxemia

  • i. Semi-recumbent position. Unless contraindicated, severely obese patients should not be supine after weight reduction surgery.

  • ii. Supplemental oxygen

  • iii. Use of non-invasive positive pressure ventilation (e.g., CPAP) which has not been associated with an increase rate of anastomotic leaks

  • iv. R/O (rule out) other causes of hypoxemia (i.e.,) PE, hypotension, respiratory depression

  • v. Minimize use of respiratory depressants

Postoperative DVT/PE

  • i. Prophylactic measures include use of pneumatic compression boots (if appropriate fit available), anticoagulation efforts with LMWH or heparin and early ambulation.

  • ii. High index of suspicion needs to be maintained. Tachycardia, hypoxia, shortness of breath, P wave strain, hypotension may be signs.

  • iii. Prompt treatment if DVT suspected

Anastomotic leak

  • i. Presenting symptoms are non-specific: tachycardia, chest pain, shortness of breath, anxiety, abdominal pain may or may not be present

  • ii. Can be fatal if not promptly recognized and treated. If high clinical suspicion and hemodynamic instability, proceed with emergent exploration and control of leak.

  • iii. Operative mortality: ranges from approximately 0.1% for adjustable gastric banding to 0.5% with open RYGB.

  • Mortality rates after bariatric surgery tend to be lower for experienced surgeons, high volume centers and for patients with few comorbidities.

What's the Evidence?

Ortiz, VE, Vidal-Melo, MF, Walsh, JL. "Strategies for managing oxygenation in obese patients undergoing laparoscopic surgery". Surgery for Obesity and Related Diseases. vol. 11 . 2015. pp. 721-728.

(A review of the impact of obesity, particularly abdominal obesity, on respiratory physiology. The authors provide suggestions on intraoperative ventilatory strategies to maintain oxygenation in the severely obese patient undergoing pneumoperitoneum.)

Ortiz, VE, Kwo, J. "Obesity: Physiologic changes and implications for preoperative management". BMC Anesthesiology. vol. 15 . 2015.

(This manuscript presents a review of the major obesity-related alterations in physiology and addresses the considerations for the preoperative evaluation of the severely obese patient.)

Bouillon, T, Shafer, SL. "“Does size matter?”". Anesthesiology. vol. 89. 1998. pp. 557-60.

(A small editorial reviewing the pharmacokinetics in obese patients.)

Dorman, RB, Miller, CJ, Leslie, DB, Serrot, FJ, Slusarek, B, Buchwald, H, Connett, JE, Ikramuddin, S. "Risk for hospital readmission following bariatric surgery". PLoS One. vol. 7. 2012. pp. e32506.

(A prospective observational study of 24,662 patients undergoing RYGB and 26002 undergoing AGB in US between 2007-09. Readmission rates are low and mortality is very rare.)

Cadi, P, Guenoun, T, Journois, D, Chevallier, J-M, Diehl, J-L, Safran, A. "“Pressure-controlled ventilation improves oxygenation during laparoscopic obesity surgery compared with volume-controlled ventilation.”". BJA. vol. 100. 2008. pp. 709-716.

(A study of 36 patientsrandomized to two different ventilator strategies intraoperatively.)

Chung, F, Yegneswaran, B, Liao, P. "“STOP questionnaire: A tool to screen patients for obstructive sleep apnea.”". Anesthesiology. vol. 108. 2008. pp. 812-21.

(STOP questionaire given to 2467 patients, with about 30% at high risk for sleep apnea. The questionaire was then validated in perioperative patients and has a high sensitivity if used in conjunction with neck circumference, bmi and other features associated with sleep apnea.)

Isono, S. "Obstructive sleep apnea of obese adults. Pathophysiology and perioperative airway management". Anesthesiology. vol. 110. 2009. pp. 908-21.

(Pathophysiology of sleep apnea in obese adults and review of airway management.)

Ortiz, VE, Wiener-Kronish, J. "Perioperative Anesthetic Care of the Obese Patient". Informa Healthcare USA Inc. 2010.

(Book with multiple chapters on sleep apnea and perioperative issues in obese patients.)

Reinius, H, Jonsson, L, Gustafsson, S, Sundbom, M, Duvernoy, O, Pelosi, P, Hedenstierna, G, Fredén, F. "“Prevention of atelectasis in morbidly obese patients during general anesthesia and paralysis: a computerized tomography study.”". Anesthesiology. vol. 111. 2009 Nov. pp. 979-87.

(Investigation into strategies for prevention of atelectasis in obese patients.)

Schumann, R, Jones, SB, Cooper, B, Kelley, SD, Vanden Bosch, M, Ortiz, VE, Connor, KA, Kaufman, M, Harvey, AM, Carr, DB. "“Update on Best Practice Recommendations for Anesthetic Perioperative Care and Pain Management in Weight Loss Surgery, 2004–2007”". Obesity. vol. 17 5. 2009. pp. 889-894.

(Review of perioperative management in obese patients undergoing weight loss surgeries.)
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