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A 47-year-old male presents for right ulnar open reduction. His medical history includes morbid obesity (BMI 58), obstructive sleep apnea (OSA), hypertension, and diabetes. He uses nasal continuous positive airway pressure (CPAP) 10 cmH2O in the evening.
Objectives
1. Explore the subclassifications of obesity.
2. Discuss the physiologic changes in the morbidly obese patient.
3. Review concerns that exist with the super morbidly obese patient.
4. Describe potential anesthetic options for morbidly obese patients.
5. Present potential anesthetic procedural difficulties associated with morbid obesity.
6. Elaborate on the concerns in regards to the patient’s history of OSA.
7. Discuss ways to minimize postoperative opioid consumption.
1. Explore the subclassifications of obesity
The most common quantification of adult weight is the body mass index (BMI), defined as weight in kilograms divided by the height in meters squared. Obesity is defined by BMI >30 kg/m2 and morbidly obese by BMI >40 kg/m2 [1] (Table 13.1).
World Health Organization International Classification of adult overweight and obesity according to BMI [1].
| Classification | BMI (kg/m2) |
|---|---|
| Normal range | 18.5 to 24.99 |
| Overweight | >25 |
| Pre-obese | 25 to 29.99 |
| Obese | >30 |
| Obese class I | 30 to 34.99 |
| Obese class II | 35 to 39.99 |
| Obese class III (morbidly obese) | >40 |
The medical literature gives further subcategories [2]:
Super obese: 50 to 59.9 kg/m2
Super-super obese: 60 to 69.9 kg/m2
Hyper-obese: >70 kg/m2
Obesity can also be classified by fat distribution, which provides information about perioperative risk. Patients are categorized as either having peripheral or central obesity, based on their waist-to-hip ratio. A ratio greater than 1.0 in males and greater than 0.85 in females suggests central obesity and an increased risk [2].
2. Discuss the physiologic changes in the morbidly obese patient
Respiratory
Morbidly obese subjects often have restrictive pulmonary disease. This is characterized by decreased pulmonary volumes including decreased functional residual capacity (FRC), expiratory reserve volume (ERV), vital capacity, total lung capacity, and residual volume. Notably, FRC and ERV decrease exponentially with increasing BMI [3]. The additional alveolar collapse leads to decreased ventilation of the lung bases, greater ventilation–perfusion mismatch and increased right-to-left shunting [4]. These result in decreased arterial oxygenation and increased alveolar-to-arterial oxygen partial pressure differences.
The combination of reduced respiratory compliance and increased respiratory resistance increases the work of breathing. Total respiratory resistance is increased due to reduction in airway caliber and increased airway resistance. This is attributable to the reduction in lung volumes rather than to airway obstruction [5]. Increased pulmonary blood volume and closure of dependent airways further reduces respiratory compliance [6]. Consequently, morbidly obese patients utilize a significant percentage of total body oxygen for respiration, have increased metabolic activity, and increased carbon dioxide production [4].
Cardiovascular
Obesity is associated with cardiovascular changes that predispose to hypertension, ischemic cardiac events, and heart failure. Systemic blood flow and cardiac output increase with total fat mass to maintain perfusion to excess adipose [7]. Total peripheral vascular resistance decreases to compensate for increases in blood volume, stroke volume, and cardiac output. However, hyperinsulinemia, activation of the sympathetic system, and sodium retention may further contribute to the increases in blood pressure [4].
Over time, increased left ventricular filling pressure and volume cause left ventricular dilatation. This results in left ventricular eccentric hypertrophy and diastolic dysfunction [8]. The right ventricle may undergo similar changes, especially in the presence of OSA or pulmonary hypertension [7]. Ventricular hypertrophy and dysfunction worsen with chronic obesity but can be reversed with weight loss [4].
Other changes
Obesity, dyslipidemia, hypertension, and insulin resistance are components of the metabolic syndrome, which is associated with an increased risk for cardiovascular and cerebrovascular disease [9]. Central fat distribution has additionally been associated with insulin resistance, impaired glucose tolerance, chronic inflammation, increased plasma triglycerides with low HDL, increased cancer incidence, endothelial dysfunction, and hypercoagulability.
3. Review concerns that exist with the super morbidly obese patient
Super obesity (50 to 59.9 kg/m2) is associated with additional comorbidities, longer operating times, and more intraoperative surgical complications as compared to morbid obesity; however, respiratory and hemodynamic changes are similar and differences in outcome have not been noted during bariatric surgery [10–11].
Airway
Similar rates of successful tracheal intubation by direct laryngoscopy have been demonstrated in the ramped position in the morbid and super obese populations [10]. However, the potential for a difficult airway should always be anticipated. The risk of aspiration is increased due to the increased gastric volume, gastric acidity, and incidence of hiatus hernia.
Respiratory
Lung volumes and gas exchange worsen with increasing BMI in the supine position [3]. General anesthesia compounds this effect, leading to rapid desaturation after induction. Utilization of positive end expiratory pressure (PEEP), head elevation (>25 degrees), and recruitment maneuvers assist to counteract atelectasis and reduce desaturation times [13]. Postoperatively, acute respiratory dysfunction is common and CPAP should be considered immediately after extubation. Dysfunction can be compounded by opiates, sedating medications, and presence of OSA, which occurs in up to 77% of the super morbid group [14].
Cardiovascular
Cardiovascular issues are common, as discussed previously, making accurate blood pressure monitoring essential. Non-invasive monitoring may be challenging even with the appropriate-sized cuff; arterial line insertion may be required.
Diabetes mellitus
BMI >35 kg/m2 is associated with an increased risk of developing diabetes due to insulin resistance and reduced insulin production [15]. Optimal medication titration and perioperative glucose monitoring is imperative.
Medication dosing
Drug titration in super-obese patients is challenging and careful consideration is necessary to avoid overdose or subtherapeutic doses. Lean body weight is the optimal weight to use for dosing most intravenous opioids and anesthetics [16].
Positioning
Positioning is critical as this population is at increased risk of perioperative nerve, joint, and soft tissue injury [12]. Transferring and patient positioning may also pose a risk to OR personnel. Specialized beds to accommodate the extreme weight must be provided.
Thromboembolic disease
Hypercoagulability and venous embolism risk are increased with super obesity as compared to morbid obesity [17]. Early postoperative mobilization and thrombosis prophylaxis are extremely important.
Infection
Patients with obesity are more likely to develop postoperative infectious complications. This may relate to the combined effect of immune dysfunction with altered tissue perfusion and the effects of comorbidities such as diabetes [13].
4. Describe potential anesthetic options for morbidly obese patients
Although no anesthetic technique has been found to be superior to another, general anesthesia is often avoided to minimize airway and drug-related respiratory problems. Opioids should be administered with caution to decrease the risk of respiratory depression, particularly in patients with OSA, and a multimodal analgesic approach that avoids sedating analgesics should be employed.
Regional anesthesia should be utilized when possible. Intraoperative, regional techniques minimize airway interventions and decrease cardiopulmonary depression. Even when general anesthesia is required, the addition of regional anesthesia for postoperative analgesia provides superior pain relief while reducing systemic opioid requirements and the incidence of opioid-induced side effects [18–19].
For the presented case, a brachial plexus block is appropriate. Similar success rates for distal arm anesthesia have been demonstrated between ultrasound-guided supraclavicular (SCB), infraclavicular (ICB), and axillary (AXB) brachial plexus blocks [20]. The chosen approach is often based on practitioner preferences, potential complications, and patient characteristics. One must remain cautious with the block technique chosen and weigh the risk of phrenic nerve blockade in the obese patient. Supraclavicular block remains relatively contraindicated in patients with severe respiratory disease or pre-existing contralateral hemidiaphragmatic paresis. Perlas et al. [21] in a series of 510 ultrasound-guided SCBs, found a 1% incidence of symptomatic hemidiaphragmatic paresis, but the incidence of asymptomatic paresis is likely higher. Infraclavicular block is a much deeper approach and may prove difficult in a super-obese patient. Consequently, AXB would be a safe and relatively superficial approach for this patient.
Patients with diabetes may have increased risk of nerve injury caused by regional anesthesia. The use of the lowest possible local anesthetic dose is recommended [22]. Ultrasound-guided techniques may help to achieve this aim.
5. Present potential anesthetic procedural difficulties associated with morbid obesity
Airway management
Intubation in patients with morbid obesity is not necessarily difficult; however, facemask ventilation and maintenance of adequate oxygenation is more likely to be an issue [2, 23].
The optimal management of the airway in the obese patient is to anticipate difficulty. A retrospective analysis by Cook et al. involving 2.9 million patients linked obesity to 40% of airway-related adverse events [24]. Time for airway rescue is limited due to lower pulmonary reserves and increased basal oxygen consumption. Pre-oxygenation and ramped or head-elevated laryngoscopy position is mandatory [23].
Airway changes depend on the distribution of body fat. Peripheral obesity spares the airway. Conversely, central obesity increases parapharyngeal and superficial neck adipose deposition narrowing the airway, predisposing to collapse, and increasing the incidence of OSA [23]. Consequently, central obesity has a strong association with difficult airway. A short neck, limited neck extension, neck circumference >40 cm and the presence of moderate- to high-grade OSA are associated with difficult or impossible mask ventilation and difficult intubation [23, 25].
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