Anesthetic considerations in obesity

Preoperative Evaluation

The preoperative assessment  include consideration of comorbid conditions like hypertension, diabetes, heart failure, and obesity-hypoventilation syndrome. 

 The history of previous surgeries, their anesthetic challenges, need for ICU admission, surgical outcomes, and the weight of the patient at that time are noted.

 Recommended preoperative laboratory evaluations include fasting blood glucose, lipid profile, serum chemistries (to evaluate renal and hepatic function), complete blood count, ferritin, vitamin B₁₂, thyrotropin, and 25-hydroxyvitamin D. OSA by means of overnight oximetry or polysomnography, or both, if appropriate. If identified with OSA and CPAP is recommended, patients are encouraged to initiate therapy at home, and it should be continued throughout the perioperative period. AHI score greater than 30, implying severe sleep apnea, is a warning sign and a predictor for rapid and severe desaturation at induction. CPAP score greater than 30, implying severe sleep apnea, is a warning sign and a predictor for rapid and severe desaturation at induction.

Intraoperative Care

Obese patients present special challenges for the anesthesiologist in airway management, positioning, monitoring, choice of anesthetic technique and anesthetic agents, pain control, and fluid management. .

Patient Positioning

 Even in the supine position, rhabdomyolysis from pressure on the gluteal muscles leading to renal failure and death has been reported. For obese patients placed in the prone position, cushioning gel pads or other weight-bearing rolls may have excessive weight placed on them. Pressure points must be checked carefully,

Airway management of obese patients: Patients should be readily intubated by direct laryngoscopy if placed carefully in the ramped position. Obese patients must be examined for the common objective signs of potential difficult intubation, which include small mouth opening, large protuberant teeth, limited neck mobility, and retrognathia. Alternative airway management techniques such as awake, topicalized direct laryngoscopy with modest sedation can be used to assess the laryngoscopic view when deciding whether to proceed with induction of general anesthesia or awake, sedated fiberoptic intubation. Of course, the equipment for emergency airway management, including laryngeal masks and a fiberoptic bronchoscope, should be immediately available.

 Pulmonary physiology Obese patient's pulmonary physiology is especially important to appreciate techniques to maintain oxygenation and lung volume when caring for an obese patients as they  have multiple pulmonary abnormalities, including decreased vital capacity, inspiratory capacity, expiratory reserve volume, and functional residual capacity.A variety of intraoperative maneuvers to maintain lung volume and oxygenation are use of PEEP,the application of noninvasive modes of ventilation, including pressure support and bilevel support delivered by mask for preoxygenation, induction, and maintenance of anesthesia to maintain oxygenation and ventilatory mechanics in obese patients,to continue CPAP if already  exposed and right position of mild reverse trendelenburg position .

Thermal management in the operating room is best accomplished with forced-air warmers. Armboards may need extra padding to keep the patient from having the arm and shoulder out of an anatomic position. If the arms are to be tucked by the side of the patient, a wide, well-padded sled may be useful.

Fluid requirements may be greater than predicted, and in even a relatively short, 2- to 3-hour case, 4 to 5 L of crystalloid fluid may be needed to prevent acute tubular necrosis in the kidneys. Hypovolemia, which can cause a protracted prerenal state, can be prevented by appropriate hydration. 

Anesthetic Drugs and Dosing

Commonly used anesthetic drugs can be dosed according to total-body weight (TBW) or IBW based on lipid solubility. Lean body mass is a good weight approximation to use when dosing hydrophilic medications. The volume of distribution is changed in obese patients with regard to lipophilic drugs like benzodiazepines and barbiturates, among the commonly used anesthetic drugs. Three exceptions to this rule are digoxin, procainamide, and remifentanil, which even though highly lipophilic, have no relationship between properties of the drug and their volume of distribution_. Consequently, dosing of commonly used anesthetic drugs such as propofol, vecuronium, rocuronium, and remifentanil is based on IBW. In contrast, thiopental, midazolam, succinylcholine, atracurium, cisatracurium, fentanyl, and sufentanil should be dosed on the basis of TBW. Another caveat to this recommendation is that maintenance doses of propofol should be based on TBW and, conversely, on IBW for sufentanil. This implies that one can use, based on patient weight, larger amounts of benzodiazepines, fentanyl, or sufentanil, although these drugs are best titrated to the desired clinical effect. Conversely, based on real body weight, smaller amounts of propofol are needed to anesthetize the patient.The choice of volatile agents is based on the physical characteristics of tissue solubility, expressed as blood-gas partition coefficients and fat-blood partition coefficients. Some evidence suggests that desflurane may be the anesthetic of choice because of a more consistent and rapid recovery profile than is seen with sevoflurane and propofol.

Even though nitrous oxide provides some analgesic effect and is eliminated rapidly, we prefer to avoid it because of the high oxygen demand in the obese. 

Induction of Anesthesia

 Obesity itself does not increase the risk for aspiration,however, acid aspiration prophylaxis, including H₂ receptor agonists or proton pump inhibitors, must be considered in patients with identifiable risk for aspiration. Awake fiberoptic intubation may also be considered in such patients

Regional anesthesia, especially epidural and spinal, is safe and feasible in patients with large body habitus but technically more difficult because of the physical challenge of placing the catheters and the tendency of these catheters to migrate out of the epidural space. Special equipment, in terms of longer needles or special ultrasound probes, may be needed for correct placement of catheters in these patients. Care should be exercised in dosing these catheters because of the increased cephalad spread of the drug and the block as a result of a smaller epidural space than in normal-weight patients.

Indications for invasive monitoring depends on the comorbid conditions like  obesity-hypoventilation syndrome with pulmonary hypertension and cor pulmonale may require a pulmonary artery catheter or intraoperative use of transesophageal echocardiography,central venous access is useful if  peripheral access is difficult. Similarly, difficulty in noninvasive blood pressure measurements because of body habitus–related difficulty in appropriate cuff placement may be an indication for placement of an arterial catheter. Arterial blood gas analysis may help guide intraoperative ventilation and extubation.

In preparation for emergence from anesthesia, the neuromuscular blockade must be fully reversed before the patient is extubated.Pressure-support ventilation mode on many newer models of anesthesia machines,can be used to  maintained on pressure support during emergence as soon as spontaneous ventilation has resumed. When adequate muscle strength has returned, as demonstrated by sustained tetanus with the nerve stimulator and performance of a 5-second head lift, an awake patient who is following commands can be safely extubated. Pressure support or CPAP can be delivered immediately by mask applied to the face as is done during preoxygenation before induction of anesthesia. 

Postoperative pain management include  intravenous analgesia via patient-controlled analgesia (PCA) or thoracic epidural analgesia. 

Opioid-based PCA with local anesthetic infiltration of the wound and adjunctive non-narcotic medication is a reasonable approach for most patients. Injection of local anesthetic into the incision site before making the incision may result in preemptive analgesia. Adjunctive analgesia with non-narcotic medications, unless contraindicated, will decrease opioid requirements and thereby opioid-induced side effects as well.