Anesthesia for morbid obesity

Abstracts

JUSTIFICATIVA E OBJETIVOS: A obesidade mórbida é uma doença muito freqüente em nosso meio, enquanto nos EUA já assumiu caráter epidêmico. O paciente obeso apresenta uma série de alterações fisiopatológicas, além de importantes comorbidades, o que exige do anestesiologista pleno conhecimento da fisiopatologia da doença. O procedimento cirúrgico de redução gástrica tem sido cada vez mais realizado e o período perioperatório apresenta características únicas com alterações cardiovascular e pulmonar que o tornam um verdadeiro desafio para os profissionais envolvidos. O hospital também deve estar preparado para receber esses pacientes, com equipamentos adequados, equipe multidisciplinar e cuidados pós-operatórios. O objetivo deste estudo foi demonstrar que o paciente obeso mórbido não é apenas um paciente com excesso de peso, e, portanto, procurou-se nortear as principais condutas a serem observadas. CONTEÚDO: São apresentadas neste artigo as principais alterações fisiopatológicas do obeso mórbido, bem como dados de epidemiologia e doenças correlacionadas. É realizada uma revisão das doses dos medicamentos usados na anestesia, bem como a melhor abordagem pré-, intra- e pós-operatória pelo anestesiologista. CONCLUSÕES: A abordagem do paciente com obesidade mórbida exige um planejamento minucioso que se inicia na seleção dos pacientes, tem continuidade com pré-operatório detalhado e intra-operatório individualizado, e se estende até o pós-operatório, quando a incidência de complicações pulmonar, cardiovascular e infecciosa é maior que na população não-obesa. Para que os resultados sejam favoráveis é extremamente importante o envolvimento de uma equipe multiprofissional que inclui Clínica Geral, Anestesiologia, Cirurgia Geral, Enfermagem, Psicologia, Fisioterapia, Nutrologia e Terapia Intensiva.

CIRURGIA, Abdominal; DOENÇAS, Obesidade

REVIEW ARTICLE

Anesthesia for morbid obesity^*

Anestesia para obesidad mórbida

Michelle Nacur Lorentz, TSA^I; Viviane Ferreira Albergaria, TSA^I; Frederico Augusto Soares de Lima^II

^IAnestesiologista do Biocor Instituto e do Hospital da UNIMED

^IIME₃ de Anestesiologia da Fundação Hospitalar do Estado de Minas Gerais

^{Correspondence to}

SUMMARY

BACKGROUND AND OBJECTIVES: Morbid obesity is very frequent in our society, having achieved the level of an epidemic in the United States. Obese patients present several physiopathologic changes and important comorbidities, which the anesthesiologist must be aware of. Gastric reduction surgery is increasingly more frequent, and the perioperative period has unique characteristics, with cardiovascular and pulmonary changes that make it a real challenge for the professional involved. The hospital should also be prepared to receive those patients, with adequate equipment, a multidisciplinary team, and postoperative care. The objective of this study was to demonstrate that the patient with morbid obesity is not only a person with weight excess and, therefore, we attempted to describe the main conducts to be followed.

CONTENTS: Here we present the main physiopathologic changes in the patient with morbid obesity, as well as the epidemiological data and correlated diseases. We review the doses of the drugs used in anesthesia, and the best pre, intra, and postoperative approach.

CONCLUSIONS: The care of the patient with morbid obesity demands careful planning, which begins with patient selection, continues with a detailed preoperative and individualized intraoperative periods, and extends through the postoperative period, when the incidence of pulmonary, cardiovascular, and infectious complications is greater than in the non-obese population. The involvement of a multidisciplinary team, including Internal Medicine, Anesthesiology, General Surgery, Nursing, Psychology, Physical Therapy, Nutrition, and Intensive Care, is extremely important for good results.

Key Words: DISEASES, Obesity: morbid; SURGERY, Abdominal: bariatric.

RESUMEN

JUSTIFICATIVA Y OBJETIVOS: La obesidad mórbida es una enfermedad muy frecuente en nuestro medio, mientras que en los EUA ya llegó a tener un carácter epidémico. El paciente obeso presenta una serie de alteraciones fisiopatológicas, además de importantes comorbidades lo que exige del anestesiólogo un pleno conocimiento de la fisiopatología de la enfermedad. El procedimiento quirúrgico de reducción gástrica ha sido cada vez más realizado y el período perioperatorio presenta características únicas con alteraciones cardiovascular y pulmonar que lo convierten en un verdadero desafío para los profesionales involucrados. El hospital también debe estar preparado para recibir a esos pacientes con equipos adecuados, un equipo multidisciplinario y cuidados postoperatorios adecuados. El objetivo de este estudio fue demostrar que el paciente obeso mórbido no es solamente un paciente con exceso de peso, y por tanto se buscó guiar las principales conductas a ser observadas.

CONTENIDO: En este artículo presentamos las principales alteraciones fisiopatológicas del obeso mórbido, como también datos epidemiológicos y enfermedades correlativas. Se realiza una revisión de las dosis de los medicamentos usados en la anestesia, como también el mejor de los abordajes pre, intra y postoperatorio por partes del anestesiólogo.

CONCLUSIONES: El abordaje del paciente con obesidad mórbida exige una planificación minuciosa que se inicia en la selección de los pacientes, tiene una continuidad con el preoperatorio detallado y intraoperatorio individualizado, y se extiende hasta el postoperatorio, cuando la incidencia de complicaciones pulmonar, cardiovascular e infecciosa es mayor que en la población no obesa. Para que los resultados sean favorables es extremadamente importante la involucración de un equipo multiprofesional que incluye Clínica General, Anestesiología, Cirugía General, Enfermería, Psicología, Fisioterapia, Nutrología y Terapia Intensiva.

INTRODUCTION

The prevalence of morbid obesity has increased in the past few years, with serious consequences for the health care system. Approximately 7% of the world population (250 million people) ¹ and 30% of the North-American people are obese ^2,3. Sixty percent of the adults in the United States are overweight, and 1:16 North-American women fulfill the criteria for morbid obesity. In Australia, the number of obese individuals more than doubled in the last 20 years ⁴. Another alarming datum is the number of obese children and adolescents (about 11%) ⁵, and 60% to 85% of those will become obese adults ⁶.

Childhood obesity has also become an international crisis in public health, with more than 22 million overweight children below the age of 5 ⁷. The increase in the prevalence of obesity in children and adolescents, associated with the knowledge that several comorbidities secondary to obesity start during childhood, makes it an important issue in health care prevention systems ⁸. An important factor regarding this increase in prevalence is that the patient becomes refractory to medical treatment and diets, and he/she cannot maintain the weight off ⁸.

As a result, surgeons have been performing bariatric surgeries that were, in the past, done only in adults and adolescents. The correlation between body mass index (BMI) and other measurements of obesity are less adequate in children, since it varies with age, gender, and race. However, it is agreed that the BMI provides a reasonable evaluation of the amount of fat in children. Body mass index correlates with childhood morbidity, such as hypertension ⁹, hyperlipidemia, non-insulin dependent diabetes mellitus (NIDDM), asthma ¹⁰, liver changes, and decreased survival. Sinha et al. found glucose intolerance ¹¹, a precursor of diabetes and risk factor for the development of coronary artery disease, in 25% of obese children between the ages of 4 and 10. Other important aspect of childhood obesity is the emotional impact, leading to isolation and social stigmatization. A recent study compared the quality of life of obese adolescents and adolescents with cancer ¹². The risk of overweight in children occurs when their weight is between the 85^th and 95^th percentiles; they are considered overweight when their weight is above the 95^th percentile ⁸. Medical literature on bariatric surgery in adolescents is limited. Patients must have a BMI > 40 kg.m^-2 associated with severe comorbidities (type 2 diabetes mellitus, sleep apnea, or pseudotumor cerebri) or BMI > 50 kg.m^-2 without comorbidities ⁷.

The indication of this surgery in severely obese adolescents is controversial. This is an aggressive procedure, and there are several questions about its safety and outcome that should be answered, such as whether it promotes sustained, long-term weight loss or if it reduces the comorbidities, which are seen in adults; whether the adolescent is ready for the changes in diet and the medical follow-up imposed by the procedure; and the long-term nutritional consequences, especially on his/her growth, bone density, and reproductive potential. Further prospective studies on the outcome of this procedure are necessary to answer those questions.

The approach of a patient with morbid obesity is a real challenge for the anesthesiologist, since comorbidities are frequent, venous access is more difficult, positioning the patient has its particularities, and ventilation should be individualized. Mortality is 12 times greater in the obese than in the non-obese adult ¹³. The perioperative mortality rate of gastrointestinal surgeries is 6.6% in obese patients, and 2.6% in non-obese patients ¹⁴. Nowadays, obesity has surpassed smoking as a cause of death and disease ¹⁵.

One should also remember that obesity in men is different from obesity in women. In women, fat tends accumulate in the abdomen and hips (gynecoid pattern), and in men, in the thorax (androgenic pattern).

In normal adults, adipose tissue corresponds to 15% to 18% of the weight in men and 20% to 25% in women ¹⁶. Direct consequences of this pattern of fat distribution in androgenic obesity include greater difficulty to access the airways and to ventilate, and a greater incidence of diabetes mellitus, hypertension, and cardiovascular disease, when compared to gynecoid obesity ^17,18.

Bariatric surgery is considered an effective method to treat refractory obesity and, according to the consensus of the 1996 conference of the National Institutes of Health (NIH), it remains the only effective treatment of severe obesity that leads to long-term weight reduction. This occurred especially with patients who failed the clinical treatment, usually based on restricting the ingestion of high caloric foods ¹⁹; however, it carries significant morbidity, limitations, and is restricted to severely obese patients ^20-29.

There are two categories of surgical techniques: procedures that decrease absorption, rarely used, such as the ileum-jejunum bypass and ileum-pancreatic bypass, and restrictive: vertical banding gastroplasty (VGB), gastric banding, adjustable gastric banding (AGB), and Roux-en-Y gastric bypass (RYGB). The last procedure is considered the golden standard and combines gastric restriction with a minimal degree of decreased absorption. They can all be performed by laparotomy or laparoscopy. Laparoscopic bariatric surgery is difficult to perform in patients weighing more than 180 kg ^30,31. There are several studies comparing open and laparoscopic bariatric surgery ^32-34, and the latter seems to have better postoperative evolution. Provost and Jones observed a slightly greater incidence of leakage through the anastomosis in laparoscopic surgeries ³⁰, but it seems that when the learning curve (70 cases) is achieved, the incidence is similar in both techniques ³⁴. Juvin compared the intra and postoperative period after conventional and laparoscopic gastroplasty in obese patients; there were no significant differences in the duration of the surgical procedure; length of ICU and hospital stay was greater in the conventional technique, as well as the need for analgesics. Besides, institution of liquid diet and ambulation occurred earlier with the laparoscopic technique ³⁵. A study evaluating 4,047 obese patients treated surgically compared with obese patients treated conventionally showed that, after bariatric surgery, there was an improvement in life quality, diabetes mellitus, hypertriglyceridemia and increased uric acid, but not in hypercholesterolemia, and the results regarding control of hypertension were inconclusive ³⁶. Evaluation of the surgical procedure focusing in parameters such as weight loss, improvement of comorbidities, changes in life quality, and patient satisfaction is yet to be done ^37,38.

DEFINITION AND CLASSIFICATION

Obese patients have a high proportion of body fat. The term morbid obesity refers to the obesity that will cause a significant reduction in life expectancy, if left untreated ³⁹.

The BMI is a convenient method to define and quantify obesity. It is calculated by relating the weight, in kilograms, and height, in square meters (BMI = weight (kg) . height^-2 (m)).

Morbid obesity is defined as a BMI > 40 kg.m^{-2 40}.

Some authors propose substituting the term morbid obesity for clinically significant obesity, defined by a BMI > 40 or a BMI > 35 with significant comorbidities ⁴¹ (Table I).

The American Society of Anesthesiology includes, in its classification of morbid obesity, the superobese whose BMI is greater than 50 kg.m^-2 and the super superobese whose BMI is greater than 60 kg.m^-2 (Table II).

CO-EXISTING DISEASES

Patients with clinically severe obesity have a higher incidence of comorbidities. Hypertension is the most common, affecting about 60% of the patients; for each 10 kg increase in weight, the systolic blood pressure increases by 3 to 5 mmHg and the diastolic pressure by 2 mmHg; non-insulin dependent diabetes mellitus is also frequent. Among the cardiovascular diseases, patients may have heart failure, coronary ischemia, cardiomyopathy, cor pulmonale, deep venous thrombosis (2.6% of the patients), arrhythmias, and sudden death ^44,45.

Gastroesophageal reflux, esophagitis, and increased incidence of aspiration during anesthesia (there is an increase in the secretion of gastric secretions) should always be remembered before the anesthetic induction of a patient with morbid obesity.

The incidence of infection is also increased: approximately 20% in abdominal surgeries: cholelithiasis, non-alcoholic hepatic steatosis, hiatal hernia, esophageal carcinoma, and other types of cancer, Cushing syndrome, hypothyroidism, and vitamin deficiency.

Respiratory diseases, such as sleep apnea (5% of the patients), Pickwick syndrome, aspiration pneumonia, restrictive pulmonary disease, respiratory failure, pulmonary thromboembolism (0.95%), hypoventilation syndrome, pulmonary hypertension, and asthma are also common.

The main syndromes associated with morbid obesity are: metabolic, characterized by visceral fat, dyslipidemia, hypertension, and insulin resistance; obstructive sleep apnea, in which the patient experiences 30 episodes of apnea lasting more than 20 seconds in 7 hours; hypoventilation syndrome of obesity, with reduced ventilatory response to CO₂ and O₂, resulting in sleep apnea, hypoventilation, hypercapnia, pulmonary hypertension, and increased somnolence; and Pickwick syndrome, which includes the symptoms of the hypoventilation syndrome of obesity, hypoxemia, polycythemia, and heart failure.

PHYSIOPATHOLOGY

The central regulation of satiety and appetite is altered. There is an elevation in the activity of the sympathetic nervous system that predisposes the patient to develop insulin resistance, dyslipidemia, and hypertension. The immunologic response is decreased, predisposing the individual to postoperative infections ⁴⁶. Coagulation factors are altered, with a reduction in fibrinogen, and factors VII and VIII, leading to a hypercoagulable state. The change in body composition is extremely important for the anesthesiologist, since there is an increase in body mass and fat, decreased muscle mass and percentage of water, and histologic and enzymatic changes in the liver ^47,48 that do not correlate clearly with changes in liver function or the capability of the liver to metabolize drugs ⁵¹. Drug renal clearance is increased due to the increase in renal blood flow ⁵². Binding to plasma proteins is altered, changing the pharmacodynamics and pharmacokinetics of several drugs ⁴⁴.

There is an increase in oxygen consumption and production of carbon dioxide secondary to an increase in the metabolic demand and energetic expenditure necessary to supply a large body mass. Functional residual capacity (FRC) is decreased and the airways close during normal ventilation ⁵³. Perfusion of the non-ventilated alveoli results in a reduction in the partial pressure of oxygen (PaO₂), which is below the adequate level for a non-obese individual. Arterial blood gases frequently show alveolar hyperventilation and relative hypoxemia, with PaO₂ varying from 70 to 90 mmHg and partial pressure of carbonic gas (PaCO₂) from 30 to 35 mmHg. The preoperative work up of every obese patient should include arterial blood gases analysis, preferentially in room air with the patient on dorsal decubitus. This reflects the basal state and will help the anesthesiologist in the intra and postoperative periods. The use of positive end-expiratory pressure (PEEP) associated with large tidal volumes in patients with morbid obesity can worsen the hypoxemia. The presence of polycythemia suggests chronic hypoxemia. Pulmonary hypertension is a frequent complication of hypoxemia ⁵⁴.

PREOPERATIVE EVALUATION

Preoperative evaluation should be as detailed as possible, with a careful history and physical exam. The indication for pharmacological treatment of obesity include: BMI ³ 30 or BMI from 27 to 29.9 associated with medical complications secondary to obesity. It is important to inquire about the prior use of appetite suppressant drugs. The association of phentermine and fenfluramin (Phen-fen®) is related to valvular heart disease and pulmonary hypertension and, for that reason, its use has decreased considerably. However, isolated phentermine is still being used and should be discontinued two weeks prior to surgery. The drugs currently used are sibutramine and orlistat. Sibutramine inhibits the reuptake of norepinephrine, serotonin, and dopamine, increasing satiety ⁵⁵; although this drug does not have significant systemic effects or interaction with anesthetics, it has been implicated as the cause of tachycardia and hypertension. Orlistat blocks the digestion and absorption of fat by binding to lipases in the gastrointestinal tract, decreasing the absorption of liposoluble proteins (A, D, E, and K) in 5% to 15% of the patients ⁵⁶.

Among laboratory exams, hemogram, ionogram, coagulogram, urea, creatinine, urine, liver function tests, chest X-ray, and electrocardiogram (ECG) are necessary. Electrocardiographic changes suggesting ischemia or arrhythmias are common. Patients considered "high risk" should have a complete cardiologic evaluation. Risk stratification should be based in the number of comorbidities, cardiac history, and ECG. Low risk patients have none or one risk factor; moderate risk patients have two to three risk factors; and high risk, more than three risk factors. Patients with intermediate or high risk should undergo non-invasive cardiologic tests before surgery ⁵⁷. Kral investigated the ventricular function of 16 patients with morbid obesity, candidates for bariatric surgery, without cardiac symptoms, with preoperative cintilography. In this study, 12 of the 16 patients had right ventricular dysfunction and 5 of those had important right ventricular dilation. Eight of the 16 patients had left ventricular dysfunction, and five of those had an ejection fraction below 50% ⁵⁸. According to these concepts, even without classical symptomatology, obese patients should have a cardiovascular evaluation before any elective surgery.

PREANESTHETIC MEDICATION AND PREPARATION FOR ANESTHESIA

Preanesthetic medication and adequate preparation of the patient with morbid obesity should be done very carefully. Preanesthetic medication should not be administered to patients with sleep apnea. Low doses of benzodiazepines should be carefully administered to the other patients to avoid respiratory depression. One should avoid the intramuscular route, since its result varies because they are often injected in the fatty tissue. Every measure should be taken to prevent regurgitation. Metoclopramide should be administered 30 minutes before anesthetic induction to decrease the volume of gastric secretion and increase gastric pH; H₂ blockers, such as cimetidine and ranitidine, are also recommended, but since the mechanism of action of ranitidine is longer and it does not interfere with the hepatic metabolism of other drugs, it is preferred over cimetidine ¹⁷. The administration of antibiotics should be started at least 30 minutes before the surgery. Non-fractionated heparin, 5,000 IU, should be administered before the surgery and twice a day until the patient starts to ambulate to prevent deep venous thrombosis and pulmonary emboli. If low molecular weight heparin is used, it should be started 12 to 24 hours after the surgery, especially if the patient undergoes neuroaxis block. In this case, subcutaneous heparin sodium is a good choice, and it should be administered subcutaneously two hours after the blockade, and half of the dose of enoxiheparin 12 hours after, completing the total dose 24 hours after the blockade. Pneumatic compression of the lower limbs also helps to prevent deep venous thrombosis ⁵⁹.

The surgical table should be adequate to support the weight of the patient and, in the absence of a table with the proper size, two regular surgical tables can be joined together. Protection against excessive pressure in vulnerable areas is a source of concern for the anesthesiologist, since lesions of the brachial plexus, sciatic nerve, lateral femoral cutaneous nerve, and ulnar nerve (associated with increased BMI) are relatively common.

The proper equipment should be available and checked before induction of anesthesia; one should also be ready for a difficult intubation, which occurs in 13% of the patients. The algorithm for difficult airways should be followed, including careful inspection of the mouth and cervical movements, and laryngoscope blades of different sizes, laryngeal mask, and a fibroscope to help intubation should be available ^17,60,61. BMI, by itself, is not predictive of a difficult tracheal intubation ⁶². The Mallampati classification is less valuable in obese patients. Neck circumference (wider) and the presence of sleep apnea are risk factors for difficult intubation.

The sphygmomanometer should be wide and adequate to the circumference of the arm (1/3 of the width of the arm or 75% of its length). If it is not wide enough, a standard, or even a pediatric, sphygmomanometer may be used in the forearm, close to the wrist, or in a lower limb, but only in procedures lasting less than one hour.

Adequate positioning is extremely important. For some patients, the supine position may lead to fatal cardiorespiratory instability (fatal syndrome of the obese patient in the supine position) ⁵⁴. The supine position increases the pressure of the viscus on the diaphragm, hindering ventilation, leading to hypoxemia and hypercapnia; it increases cardiac output and the pressure in the pulmonary artery. The ventral decubitus is also not tolerated because it increases the abdominal pressure. The left lateral decubitus can be used to decrease the compression of the inferior vena cava ⁶³. The semi-sitting position associated with the lateral decubitus has a lower degree of cardiorespiratory compromise. A 30° to 45° elevation of the thorax improves cardiorespiratory function and increases the safe period of apnea. The use of cushions, towels, or pillows under the head and trunk of the patient is also indicated ^63-67.

MONITORING

Electrocardiogram, temperature, pulse oxymeter, capnograph, non-invasive blood pressure, vesical catheter, and peripheral nerve stimulator are essential. Surface electrodes might be inadequate due to the excess of fat over the nerves; therefore, the use of electrodes with needles is recommended.

The BIS ⁴⁴ has been recommended because it allows the individualization of the doses of the anesthetics, avoiding intraoperative awakening, as well as overdose of anesthetics.

Invasive blood pressure is indicated in severely obese patients with advanced cardiopulmonary disease, when non-invasive blood pressure readings are not accurate, or in prolonged surgeries.

Central venous pressure is not routinely used. When indicated, a long needle should be used for catheter positioning (18G spinal tap needle, 9 cm long, 2 cm longer than the standard needle). Usually, the impossibility to place a peripheral venous access is an indication for a central venous access ⁶⁸. Ideally, the central venous puncture should be followed by ultrasound.

ANESTHETIC TECHNIQUE

The anesthetic induction depends on the degree of obesity. For morbid obesity, the induction in rapid sequence is satisfactory. In superobese patients or those with probable difficult tracheal intubation, awake induction with local anesthesia and maintaining spontaneous ventilation, or with small doses of propofol without a neuromuscular blocker can be used ⁴⁴. Total intravenous anesthesia (TIVA) and balanced general anesthesia are equally accepted ⁶⁹. Epidural block can be combined with general anesthesia because it allows the use of opioids for postoperative analgesia. The puncture should be done with an 11 cm long 16G needle.

The dose of inhalational anesthetics is not weight related: it varies according to the sensitivity of the patient, age, and presence of comorbidities. Inhalational anesthetics with low gas-blood solubility, such as desflurane and sevoflurane, are indicated for bariatric surgery due to the short awakening time. In a study with adult, obese patients, comparing desflurane with sevoflurane, Strum demonstrated that the awakening in bariatric surgery was faster with desflurane ⁷⁰. In another study, Sollazzi demonstrated that the awakening of obese patients was much faster with sevoflurane than isoflurane ⁷¹. Obese patients seem to have an increased biotransformation rate of inhalational agents when compared to non-obese patients ^72,73. Nitrous oxide (N₂O) can be used because it is not liposoluble, has a fast onset of action, and early awakening; sometimes its use in obese patients is limited due to the increased O₂ consumption. Brodsky et al. used general anesthesia with nitrous oxide and verified that this gas does not disturb the surgeon during videolaparoscopic bariatric surgery ⁷⁴.

Most intravenous anesthetics are strongly lipophilic, with an increased volume of distribution (VD) in obese patients. Thus, their doses should be based on the corrected body weight, with the exception of procainamide, digoxin, and remifentanil that, even though they are lipophilic, their dose can be calculated using the ideal body weight. The corrected body weight (CBW) is calculated by the following formula: CBW — ideal body weight + [0.4 × excess weight].

The ideal body weight can also be obtained from tables that correlate it with the height, or subtracting 100, for men, or 105, for women, from the height of the patient in centimeters ⁷⁵.

The pharmacodynamics of the neuromuscular blockers (NMB) is altered in obese patients: the clearance of vecuronium is decreased due to fat infiltration of the liver; the pharmacokinetics of rocuronium is similar to vecuronium, but with a smaller VD and it does not have active metabolites, but its awakening is more prolonged. The dose of vecuronium and rocuronium should be based on the ideal body weight. There are controversies on the dose of atracurium and cisatracurium, regarding whether it should be based on the real or corrected body weight ⁷⁶. The plasma levels of pseudocholinesterase and the extracellular volume are increased in obese patients; therefore, higher doses of succinylcholine are necessary. The appropriate dose of succinylcholine in patients with morbid obesity has not been defined yet. Some authors suggest 1.5 to 2 mg.kg^-1 based on the ideal body weight; however, in a recent study comparing 45 patients with morbid obesity receiving 1 mg.kg^-1 based on the ideal body weight, 1 mg.kg^-1 based on the lean body weight, and 1 mg.kg^-1 based on the real body weight, Lemmens concluded that the dose based on the real body weight provided better conditions for intubation, and recommended this dose for patients with morbid obesity ⁷⁷.

Atracurium and cisatracurium seem to be the NMBs of choice in bariatric surgery. The overdose of NMB is a common problem in obese patients, being related to the efforts to improve respiratory mechanics and, especially, surgical conditions. Poor abdominal conditions, such as the increase in intra-abdominal pressure, excess fat in the abdominal wall, and difficulty in exposing the surgical field (large retractors) are characteristic, and larger doses of NMB will not improve conditions or facilitate the surgical technique. On the other hand, larger doses are associated with residual NMB at the end of the surgery with possible complications.

Remifentanil associated with postoperative analgesia is beneficial and improves the recovery of the patients. Multimodal analgesia (infiltration of the surgical wound with local anesthetic and PCA) is a simple, safe, and low cost alternative to pain control with epidural opioids ⁷⁸.

Hydration should be done with balanced electrolytic solutions, using the ideal body weight and the size of the surgery as guides ⁷⁹. Babatunde et al. considered that fluid requirements to prevent postoperative acute tubular necrosis are increased, and recommended reposition of the volume administered during the fast period plus twice the same volume to the calculated volume based on the lean body mass ⁸⁰.

VENTILATORY MECHANICS

Ventilatory mechanics is considered a separate chapter of the approach to the obese patient. The tidal volume to be used during anesthesia has been the center of the debate in the medical literature. Bardoczky et al. compared ventilation to tidal volumes between 13 and 22 mL per kilogram of the ideal body weight and demonstrated that the use of large volumes during anesthesia leads to hypocapnia, although it was not accompanied by a significant elevation in partial oxygen pressure ⁸¹. Auler et al., using ventilator settings based on P_ETCO₂ and SpO₂, showed that the intraoperative determination of the proper respiratory rate and tidal volume in morbidly obese patients could be similar to that of patients with smaller BMI, since the ideal body weight is used as a reference ⁸². Using high oxygen concentrations in the gaseous mixture during anesthesia causes atelectasis ⁸³. A larger safety margin in anesthesia in morbidly obese patients could be obtained by using positive end-expiratory pressure (PEEP), which recruits alveoli, at the end of extubation ⁸⁴. Thus, the use of PEEP would allow the use of even smaller inspired oxygen fractions, which is desirable regarding the intra and postoperative prevention of atelectasis. The increase in TV or respiratory rate did not improve ventilatory mechanics ^85-88. The Thompson retractor increases the diaphragmatic tension in the same way laparoscopy does. Elevating the head of the patient favors ventilation. By keeping alveoli opened, PEEP can increase oxygenation, although this is not a consensus in the literature ⁶⁴, as well as the ideal tidal volume to be established during anesthesia ^89-92. Some authors defend ventilation on the pressure mode, remembering that it is important to avoid hypocapnia (keeping P_ETCO₂ between 35 and 45 mmHg). If ventilation is not satisfactory, the O₂:AIR mixture (1:1) can be used, and total intravenous anesthesia (TIVA) can replace inhalational anesthesia.

One should be careful to avoid migration of the endotracheal tube and selective intubation after the pneumoperitonium, which is very common ⁹³. General anesthesia causes atelectasis more often in obese patients, which is a common intra and postoperative problem ⁹⁴. It is important to remove completely the nasogastric tube before dividing the stomach to avoid the accidental transection of the tube after creating the RYGB. The nasogastric tube should not be inserted blindly to avoid rupturing anastomosis. One should be careful to obtain maximal stability before extubating the patient and do reverse adequately the NMB.

POSTOPERATIVE PERIOD

The incidence of atelectasis after surgery of the upper abdomen is increased in obese patients (approximately 45%), and CPAP can be a good option to decrease its incidence ^95,96. The use of CPAP did not increase the incidence of anastomotic lesions ⁹⁷. It has not been shown that spirometry is useful to prevent atelectasis ⁹⁸. The drop in oxygen saturation in the recovery room is usually due to atelectasis.

Postoperative continuous epidural analgesia does not seem to decrease the incidence of thrombophlebitis and pulmonary embolism ^99,100, but decreases O₂ consumption and left ventricular work, and has advantages over patient controlled analgesia with morphine, with better quality of analgesia and decrease in the duration of the paralitic ileus ¹⁰¹.

Nausea, vomiting, and tremors should be treated because they can increase O₂ consumption and exert traction on sutures. In the recovery room or in the ICU, the head of the bed should be elevated and the patient should receive O₂ by nasal canula, since most of the patients have a PaO₂ below 60 mmHg in the first 24 hours after a gastroplasty when oxygen is not administered. Some authors recommend the use of the spyrometer and early ambulation.

If there is a suspicion of neurological damage, the patient should be evaluated immediately by the neurologist.

Prophylaxis of deep venous thrombosis and pulmonary embolism should be maintained.

High-risk patients should be transferred to the ICU. Some services recommend that every patient should be in the ICU for the first postoperative hours, since the first 6 hours are critical regarding pulmonary complications ^102,103.

Metabolic complications of the jejunum-ileum anastomosis include hypopotassemia, hypocalcemia, hypomagnesemia, nephrolithiasis, gout, and liver changes. Weight loss after bariatric surgery is dramatic and progressive until it reaches a plateau ¹⁰⁴. Patients usually undergo plastic surgery to remove excess skin after weight loss. Special considerations should be made in the plastic surgery of the abdomen in these patients due to the decreased vascularization and the persistent volume of subcutaneous tissue ¹⁰⁵.

REFERENCES

Submitted em 07 de abril de 2006

Accepted para publicação em 11 de dezembro de 2006

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*

Received from Biocor Instituto, Belo Horizonte, MG

Publication Dates

• Publication in this collection
  15 Mar 2007
• Date of issue
  Apr 2007

History

• Accepted
  11 Dec 2006
• Received
  07 Apr 2006