Obesity and Anesthesia: Challenges in the Perioperative Period

Abstract

The obesity epidemic has significant implications for all aspects of healthcare. The physiological changes of obesity affect every area of perioperative medicine. In this article, we discuss several anesthetic concerns regarding obesity. We will specifically discuss preoperative evaluation, perioperative challenges, and postoperative pain control and monitoring.

Introduction

Obesity is a common comorbidity that affects all areas of medicine including anesthesiology. According to the Missouri Department of Health and Senior Services “almost two out of every three adult Missourians are overweight or obese, impacting all genders, all races and ethnicities, and all socioeconomic groups.” (Figure 1).¹ The Centers for Disease Control categorizes obesity as anyone who has a body mass index (BMI) of greater than 30, and further subdivides obesity into three categories. Those individuals with the most severe form of obesity (type III obesity) have a body mass index (BMI) of 40 or greater.² The American Society of Anesthesiologists (ASA) recognizes that severe obesity is a “severe systemic disease with substantive functional limitations”. ³ Evaluation and understanding how obesity interacts with other physiological, pharmacological, and anatomical factors is necessary to appreciate and assess perioperative risk more fully.³ Appropriate perioperative plans and potential interventions can then be made to minimize risks in the perioperative period for these patients.

Figure 1.

Trends in age-adjusted obesity and severe obesity prevalence among adults aged 20 and over: US 1999–2000 to 2017–2018.

¹ Significant linear trend.

NOTES: Estimates were age adjusted by the direct method to the 2000 U.S. Census population using the age groups 20–39, 40–59, and 60 and over.

DATA SOURCE: NCHS, National Health and Nutrition Examination Survey, 1999–2018

REFERENCE: Hales CM, Carroll MD, Fryar CD, Ogden CL. Prevalence of obesity and severe obesity among adults: United States, 2017–2018. NCHS Data Brief, no 360. Hyattsville, MD: National Center for Health Statistics. 2020

Physiologic Changes with Morbid Obesity

Cardiovascular

Perioperative cardiovascular risk is increased in patients with class III obesity. Physiological changes include increased cardiac output of 20–30 ml/kg of excess body fat.⁴ This is accomplished by LV dilation and increased stroke volume, which can lead to LV hypertrophy, diastolic dysfunction, elevated LVEDP and pulmonary edema.⁴ Additionally obese patients frequently have co-morbidities associated with cardiovascular disease such as HTN, dyslipidemia, myotonic dystrophy type 2, and sleep disorders.⁵ Some studies suggest an increased risk of cardiovascular disease in obesity even after accounting for these comorbidities. ^6,7 A meta-analysis of over 300,000 patients with 18,000 coronary artery disease (CAD) events show a five-unit increase in BMI was associated with 29% increased risk of coronary heart disease (CHD).⁸ Even after adjustment for blood pressure and cholesterol levels, there was a 16% increased risk of CHD relative to normal BMI 20–25.⁸ Visceral adiposity is associated with the systemic and vascular inflammatory processes that lead to atherosclerosis.⁹ The full extent of cardiovascular disease is often masked due to asymptomatic cardiovascular disease present due to limited exercise tolerances.^6,7

While the 2014 ACC/AHA Guideline for Perioperative Cardiovascular Evaluation and Management of Patients undergoing Noncardiac Surgery does not specify different preoperative testing for patients with class III obesity, it is important to understand the limitations of common cardiovascular testing modalities in these patients.^5,10 ECG has a lower sensitivity and specificity in obesity, possibly due to underdiagnoses of left ventricular hypertrophy (LVH).⁵ Treadmill stress testing may be non-diagnostic as many patients with BMI> 40 are unable to achieve the 80–85% age-predicted heart rate needed for diagnostically valid results.^5,11,12 Single photon emission CT (SPECT) may require two-day protocols for weights 250–350 lbs and is limited by table weight limitations above 350 lbs.⁵ Stress echocardiography may be incomplete due to poor acoustic windows secondary to obesity, though contrast does improve sensitivity and specificity in obese patients preparing for bariatric surgery. ^5,13,14 Stress cardiac MRI had 89% diagnostic quality in a study of 285 participants with average BMI of 34 kg/m²; however, there are still table weight and waist circumference limitations.^5,14,15 As MRI tables have weight limits, CT calcium scans and cardiac CT coronary angiography are similarly limited with obesity.⁵ Image quality also degrades as BMI increases due to increased background noise and decreased signal-to-noise ratio.⁵

Respiratory

Obesity is known to decrease chest wall and lung compliance which leads to decreased functional reserve capacity (FRC), vital capacity (VC), and total lung capacity (TLC).⁴ The decrease in FRC is mostly due to a reduction in expiratory reserve volume (ERV). With general anesthesia, patients with obesity may have up to a 50% reduction in FRC compared to a 20% reduction in FRC in non-obese patients.⁴ Excess fat is metabolically active leading to increases in oxygen consumption and carbon dioxide production in patients with obesity.⁴ The decrease in FRC and increase in oxygen consumption reduces the tolerated apnea time during induction of anesthesia.⁴ Surgical techniques such as pneumoperitoneum can negatively combine with morbid obesity further worsening underling pulmonary compliance by up to 30% compared to normal-weight individuals with increase in inspiratory resistance.¹⁶

Obesity is a risk factor for obstructive sleep apnea (OSA).¹⁷ Untreated OSA can lead to chronic hypoxemia, hypercapnia, pulmonary hypertension, systemic hypertension, and polycythemia.¹⁷ These physiologic changes increase risk of cardiovascular disease, cerebrovascular disease, and right ventricular failure. While OSA screening and diagnosis has increased, nonadherence to positive airway pressure treatment is common, either due to discomfort of the CPAP mask, insurance issues, or various other reasons.¹⁷ Patient’s with OSA and obesity are at higher risk for difficult airway management and postoperative pulmonary complications.⁴ Obesity hypoventilation syndrome (OHS) is chronic hypoventilation secondary to obesity and is often underdiagnosed. OHS occurs in 5–10% of patients with class 3 obesity and increases risk of postoperative complications and death.^4,18 These patients are more sensitive to the respiratory depressant effects of anesthesia than even patient’s with OSA and should have close post-operative monitoring.^4,18

Airway Challenges

Mask ventilation and tracheal intubation are more likely to be difficult in patients with class III obesity.⁴ This is especially concerning in patients with OSA and/or a short, thick neck. Even with good preoxygenation, these patients are more likely to desaturate with apnea due to their decreased FRC. This can be improved by positioning the patient in reverse Trendelenburg. Mean arterial oxygen levels improve with 25-degree ramp, which can also help minimize the risk of aspiration of gastric contents.⁴ The ramp positioning with head extended helps to align the oral, pharyngeal, and laryngeal axes while improving FRC to optimize intubation. Awake intubation should be considered in patients with a history of, or anticipated, difficult mask ventilation and difficult intubation. Accessing a surgical airway may still be particularly challenging for obese patients in an emergent ‘can’t ventilate, can’t intubate’ situation despite optimal positioning.

Close monitoring of ventilation and oxygenation is critical in obese patients undergoing sedation. Obese patients with OSA are more susceptible to the respiratory depressant effects of benzodiazepines, opioids, and propofol; and it may be difficult to mask ventilate or intubate them as a rescue.⁴ Closed claims analysis shows an increase in malpractice claims related to adverse events during monitored anesthesia care (MAC) compared to respiratory complications during general anesthesia.⁴

Other Organ Systems

Obesity is associated with increased residual gastric volume (>25 mL despite 8 hours NPO) and low gastric fluid pH (<2.5) compared to non-obese individuals.⁴ Abdominal obesity increases intragastric pressure, increasing risk of GERD and/or hiatal hernia formation, both of which increase the risk of aspiration. Fatty liver is commonly associated with obesity which is often a precursor to liver disease such as nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH). Up to 33% of obese patients may have abnormal liver function tests; however, drug clearance is usually not reduced.⁴

Metabolic syndrome is common in obese patients and increases risk of type 2 diabetes, doubles risk of death from myocardial infarction and stroke, and increases risk of perioperative complications including cardiovascular, pulmonary, renal injury, and wound infection.^4,19 Type 2 diabetes, hyperglycemia, and insulin resistance predispose patients with obesity to surgical site infection.^4,20 Obesity is also associated with increased renal blood flow due to increased cardiac output and increased glomerular filtration rate. This activates the reninangiotensin system and can impair natriuresis.⁴ Perioperative thromboembolic risk (DVT and PE) is higher in patients with class III obesity compared to non-obese patients.⁴ On a positive note, bariatric surgery resolves metabolic syndrome in 95% of patients who achieve the expected weight loss.^4,19

Perioperative Concerns

Blood Pressure Monitoring

Noninvasive blood pressure monitoring (NIBP) is the most commonly used blood pressure monitor for anesthetic cases, and accurate measurement depends on appropriate sizing of the cuff. Undersized cuffs will overestimate blood pressure while an oversized blood pressure cuff will underestimate blood pressure. Even with an appropriately sized cuff, the conical shaped upper extremity commonly seen in obese patients can compromise accuracy of NIBP readings. Practitioners often choose to use the forearm when faced with the conical upper extremity. However, forearm measurements are known to overestimate systolic and diastolic blood pressure in patients with obesity.^4,20,21 Historically, the alternative for blood pressure monitoring was placement of an arterial line. There are newer continuous noninvasive monitors such as ClearSite (Edwards Lifesciences, Irvine, CA) and continuous noninvasive arterial pressure (CNAP, CNSystems Medizintechnik, AG, Graz, Autria) that use finger cuffs. These monitors are consistent with NIBP and invasive blood pressure measurements in many studies for non-obese patients; however, there have not been enough studies including patients with class III obesity.^20–22

Alterations in dosing medications

Dosing of anesthetic medications are typically weight based, though appropriate dosing depends on understanding volume of distribution for loading dose, maintenance and drug clearance.⁴ Highly lipophilic drugs have an increased volume of distribution in obesity, whereas drugs with weak or moderate lipophilicity do not and can be dosed on lean body weight.⁴ Drugs that are equally distributed between adipose and lean tissue should be dosed based on total body weight. Drugs that undergo phase I metabolism (oxidation, reduction, hydrolysis) are not affected by obesity, whereas phase II reactions (glucuronidation, sulfation) are enhanced due to increased GFR and may require a higher maintenance dose.⁴

Positioning Challenges

Intra-operative patient positioning is one challenge that confronts the anesthesia team and that must be carefully and thoughtfully handled if adverse outcomes are to be avoided. First, proper equipment and instruments with sufficient weight specifications must be available and used judiciously. For example, OR tables with a weight rating of 450 kg or higher may be needed.^4,23 They should be equipped with detachable side arm supports and restraints. They must be adjustable along several different axes to facilitate optimum patient physiology and physical safety as well as to maximize surgical exposure. Often, these goals run counter to each other.^24,25 As an example, changes in respiratory and cardiovascular physiology of the obese patient favor the head-up position, sometimes requiring as much as 45 degrees or more. This promotes ventilation, oxygenation, cardiac output, and airway management.^24,25 Most operations, however, require a supine, head-down, lateral, or even prone position all of which complicate patient management.²⁵ Thus, the OR table is critical to a successful operation. In addition to compromised cardiopulmonary status, surgical positioning of the obese patient raises the risk of physical trauma. An obese patient’s risk for peripheral nerve injury is greater than the general population not only due to the challenging positioning, but also due to their more common pre-existing co-morbidities, neuropathies, and longer surgical times.^23–25 Further, pressure ulcers and sheer trauma are more common. In severe cases, rhabdomyolysis and kidney failure have been reported.²⁴ The use of padding and support devices to distribute weight over as large an area as possible will help to mitigate these risks.²⁵

In addition to compromised cardiopulmonary status, surgical positioning of the obese patient raises the risk of physical trauma. An obese patient’s risk for peripheral nerve injury is greater than the general population not only due to the challenging positioning, but also due to their more common pre-existing co-morbidities, neuropathies, and longer surgical times.^23–25 Further, pressure ulcers and sheer trauma are more common. In severe cases, rhabdomyolysis and kidney failure have been reported.²⁴ The use of padding and support devices to distribute weight over as large an area as possible will help to mitigate these risks.²⁵ Finally, team safety must always remain a priority. In addition to proper equipment and instruments, there must be enough well trained OR staff to ensure that lifting and positioning responsibilities can be reasonably and thoughtfully distributed.^23,24 Though surgical positioning of obese patients is technically more challenging, with proper care, consideration and support, obese patients can be safely managed, and surgical outcomes optimized.

Finally, team safety must always remain a priority. In addition to proper equipment and instruments, there must be enough well trained OR staff to ensure that lifting and positioning responsibilities can be reasonably and thoughtfully distributed.^23,24 Though surgical positioning of obese patients is technically more challenging, with proper care, consideration and support, obese patients can be safely managed, and surgical outcomes optimized.

Additional Concerns

Often overlooked is that operative times are longer in patients with class III obesity. This can be due to technical difficulties from increased retracting, requiring longer surgical instruments, and suboptimal ergonomics for the surgeon. Prolonged operative times means longer exposure to anesthesia and/or may eliminate the possibility of regional anesthesia as a primary anesthetic. Multiple specialties have demonstrated that patients with obesity require higher radiation exposure during fluoroscopy, either from increasing power for each shot or from additional shots needed.^26–28

Challenges with Regional and Neuraxial Anesthesia

Regional anesthesia in the obese may be beneficial in helping avoid difficult airway manipulations, decreases aspiration risk, and is associated with earlier mobilization and shortening of hospital length of stay.²⁹ Peripheral nerve blocks in the obese may have difficulties in proper patient positioning, landmark identification, and need for longer needle.³⁰ There may be need for phrenic nerve sparing in upper extremity blockade, since there may be obesity related respiratory physiological changes that demand avoiding diaphragmatic paralysis. Continuous catheter techniques may have problems due to dislocations and possible increased incidence of infection. Ultrasound guidance may help overcome many difficulties in regional techniques, but targets are more deeply situated in obese patients and the ultrasound beam is weakened as it travels a greater distance through tissue layers, artifact may be present, and the resulting imagery may obscure the underlying anatomy.³⁰

Difficulties in central nerve blockade (epidural or spinal) placement may be due to patient positioning, difficult landmark identification, catheter dislocation, and need for longer needle; neuraxial placement is done as a “blind” procedure.²⁹ Increasing weight is associated with the depth of placement of an epidural at all epidural sites (lumbar, lower, and upper thoracic) and with all approaches (midline, paramedian); the sitting position is the optimal position for placement.³¹ After the epidural catheter is inserted and secured, the catheter position can change and actually be withdrawn from epidural space during repositioning to the lateral decubitus or supine positions. Ultrasound has been used to assist epidural needle placement in morbidly obese patients in both pain medicine and obstetric patients.³² Despite these risks and challenges, regional/neuraxial anesthetics are successful and beneficial to these patients for surgical blockade, to labor analgesia, to mode of post-operative and opioid sparing pain control.

Post-Operative Concerns

Important postoperative analgesic goals for the management of obese patients should include maximal comfort, early mobilization, maintenance of respiratory function and avoidance of over sedation.³³ Obese patients, due to a high incidence of OSA, are predisposed to airway obstruction, hypoxia, and hypercarbia.³³ Because opioid induced respiratory depression is a concern, a multimodal approach to pain control should be employed.³⁴ This approach should include techniques which potentially decrease the total opioid dosage requirements. Both central and peripheral nerve blockade with local anesthetics utilizing single shot or continuous infusion catheters to reduce use of opioids. Further, the analgesic regimen should be supplemented with scheduled doses of NSAIDs and acetaminophen. Other medications to consider are the NMDA receptor blocker ketamine and the alpha-2 receptor agonists clonidine and dexmedetomidine. Also, a continuous infusion of intravenous lidocaine for up to 72 hours has been shown to be effective in reducing total narcotic requirements.³⁴

If postoperative narcotics are needed, a patient-controlled option is best. Avoid basal rate dosing. Opioid dosing should be based on lean body weight.³⁴ Dosing schedules for all medications should be adjusted to individual patients and their unique comorbidities. Perioperative enhanced recovery care pathways (ERAS) have been developed and adopted by many healthcare facilities. They help standardize and optimize the approach to postoperative analgesia to decrease opioid requirements, increase patient safety and improve surgical outcomes. ERAS pathways do not only address pharmacologic therapies. They might include other interventions too, such as noninvasive ventilation and oxygenation, early mobilization, elevated head position and sedation scoring. They might also establish thresholds for pulse oximetry and CO² monitoring as well as when to adjust a patient’s level of care.³⁵ ERAS pathways have proven especially useful for reducing untoward outcomes and increasing patient satisfaction and should continue to be used and refined as new data becomes available.

Special Considerations

Bariatric surgery continues to be the gold standard in surgical options for the obese population. As pharmacologic treatments for obesity continue to advance, a new class of weight loss medications has emerged, the Glucagon-like Peptide-1 agonists (GLP-1 agonists), used in the treatment of type 2 diabetes and obesity. GLP-1 agonists have been shown to inhibit gastric emptying resulting in a reduction of appetite and food intake lowering glucose levels. The benefit of weight loss for obese patients results in increased perioperative risks associated with the side effects of these medications. Nausea, vomiting, and diarrhea can lead to volume contraction and may predispose a patient to acute kidney injury or hemodynamic instability perioperatively; close monitoring of volume status must occur.³⁶ More concerning has been the recognition of a significant decrease in gastric emptying associated with these medications, often associated with a “full stomach” despite proper NPO times and parameters. This potentially predisposes patients to an elevated risk of pulmonary aspiration perioperatively.³⁷

Given these concerns, in June 2023 the American Society of Anesthesiologists (ASA) Task Force on Preoperative Fasting released a press release suggesting the following guidelines for elective procedures:³⁸

Day(s) Prior to the Procedure

1. For patients on daily dosing consider holding GLP-1 agonists on the day of the procedure/surgery. For patients on weekly dosing consider holding GLP-1 agonists a week prior to the procedure/surgery.

2. This suggestion is irrespective of the indication (type 2 diabetes mellitus or weight loss), dose, or the type of procedure/surgery.

3. If GLP-1 agonists prescribed for diabetes management are held for longer than the dosing schedule, consider consulting an endocrinologist for bridging the antidiabetic therapy to avoid hyperglycemia.

Day of the Procedure

1. If gastrointestinal (GI) symptoms such as severe nausea/vomiting/retching, abdominal bloating, or abdominal pain are present, consider delaying elective procedure, and discuss the concerns of potential risk of regurgitation and pulmonary aspiration of gastric contents with the proceduralist/surgeon and the patient.

2. If the patient has no GI symptoms, and the GLP-1 agonists have been held as advised, proceed as usual.

3. If the patient has no GI symptoms, but the GLP-1 agonists were not held as advised, proceed with ‘full stomach’ precautions, or consider evaluating gastric volume by ultrasound, if possible and if proficient with the technique. If the stomach is empty, proceed as usual. If the stomach is full or if gastric ultrasound inconclusive or not possible, consider delaying the procedure or treating the patient as ‘full stomach’ and manage accordingly. Discuss the concerns of potential risk of regurgitation and pulmonary aspiration of gastric contents with the proceduralist/surgeon and the patient.

4. There is no evidence to suggest the optimal duration of fasting for patients on GLP-1 agonists. Therefore, until we have adequate evidence, we suggest following the current ASA fasting guidelines.^39,40

The previously stated guidelines are a work in progress and will evolve as our knowledge about and anesthetic/surgical interactions with these medications grows.

Conclusions

The anesthetic management of morbidly obese patients remains a challenging problem in anesthesia across the spectrum of the perioperative period. This discussion has given a generalized examination of these challenges, but the details remain numerous in the anesthesiologist’s daily practice. With the increasing prevalence of obesity, each patient may present unique challenges with additional comorbidities combined with morbid obesity. We as anesthesiologists strive to remain knowledgeable and vigilant in these challenges to provide the best care to this increasing population and educate our fellow physicians about the perioperative implications.