**3. Intraoperative management**

#### **3.1. Positioning in morbidly obese patients**

Prevalence of obesity continues to increase rapidly throughout the world [49]. Therefore, all anesthesiologists should be familiar with this issue not only for obesity surgery, but also for other types of surgery [50]. Inappropriate surgical position may lead to serious physiological problems, and even physical injuries [51, 52]. On the other hand, appropriate position may ease procedures, including especially endotracheal intubation, reduce physiological problems, and minimize neural and soft tissue injury [53].

Operation tables having a carrying capacity of near 400 kg should be used for safe anesthetic and surgical procedures in obese patients. If no special operation table is present, two standard ones having a weight-bearing capacity of 200 kg may be adjoined. Patients should be tightly bound to the table, ensuring supporting of areas prone to pressure by gels and pads. These patients may develop renal failure and potentially fatal complications at even supine positions [54, 55]. In a study, Bostanjian et al. [54] described six patients undergoing bariatric surgery, rhabdomyolysis secondary to gluteal muscle necrosis developed after supine position, where the outcomes were fatal in three of cases.

Head-elevated laryngoscopy position, which is described as the position of head and shoulders above the level of the chest, i.e., above an imaginary horizontal line joining sternal notch and external auditory canal, makes laryngoscopy and intubation easy [56]. The position where the head is lifted 25° and reverse Trendelenburg in induction anesthesia were shown to prolong apnea in obese patients without desaturation [57]. Functional residual capacity (FRC) is severely diminished in the supine position after induction of anesthesia. If the reduction in FRC exceeds closure volume, small airways become also closed, ensuing a ventilation perfusion disturbance [58].

Supine position: Switching from the sitting position to the supine position causes an increase in venous load of the heart in some patients. Reduced diaphragmatic movement by abdominal organs leads to increased respiration work, relative hypoxemia, and marked reduction in lung volumes [59]. Lung volume is further decreased in general anesthesia procedures where muscles are completely paralyzed [60]. Compared to normal-weighed patients, FRC and pulmonary compliance are reduced in the supine position in obese patients, eventually increasing ventilation/perfusion mismatch [59]. All these alterations increase as the body mass index (BMI) is elevated. Induction of the anesthesia is recommended to be performed in the lateral decubitus position to overcome these difficulties [61]. Positive end-expiratory pressure (PEEP) may improve lung functions in mechanically ventilated patients [62]. Prolonged supine position should be avoided in patients with reduced cardiac reserve since venous return to the heart is diminished by increased compression onto the inferior vena cava secondary to abdominal pressure and weight. In such cases, operation table or the patient may be turned to the side so as to decrease aortocaval compression [53, 63].

Trendelenburg position: Patient's head is below the horizontal plane in Trendelenburg position, which may increase operative exposure and decrease bleeding in selected cases. It is less tolerated than that in the supine position. In obese patients who already have limited cardiac reserves, blood in lower extremities is added into central and pulmonary circulation by Trendelenburg position, making it hard to tolerate this position [64]. It should be especially avoided in morbidly obese patients. Further diminished residual capacity and pulmonary compliance in this position also lead to atelectasis and hypoxemia. In addition, endotracheal tube may be displaced depending on the position. In brief, this position is often not preferred in obese patients due to all these factors [53, 63].

Head-upward position: Upper torso of morbidly obese patients should be nearly 35–40° in the sitting position or reverse Trendelenburg position in a way where the operation table allows for adequate ventilation. Such position simplifies mask ventilation and conditions of tracheal intubation.

The combined effect of reverse Trendelenburg position and pneumoperitoneum during laparoscopic gastric bypass surgery decreases femoral blood flow and increases venous stasis, thereby increasing risk of pulmonary embolism. Therefore, prolonged applications of this position should be avoided with altering positions occasional breaks during surgery [65].

Prone position: Prone position was shown to increase oxygenation in normal-weighed patients under anesthesia than that in supine position [66]. As long as the chest and pelvis are supported such adequately that allows for abdominal movements, prone position is usually well-tolerated by obese patients. Cardiovascular functions are preserved when appropriate position and supports are provided. Otherwise, cardiac venous return is diminished by compression onto the inferior vena cava and femoral veins, which in turn leads to decreased volume in the left ventricle, causing hypotension. Prone position in obese patients under anesthesia improves pulmonary functions and increase FRC, pulmonary compliance, and oxygenation [53, 67].

Lateral decubitus position: This position is often tolerated well by obese patients. A decrease in the abdominal fat mass' compression on the abdomen diminishes intraabdominal pressure, which eases diaphragmatic movements during mechanical ventilation. However, maintenance of the same position for a long while may lead to vascular congestion and resulting in hypoventilation in underlying lung [68].

Lithotomy position: This position causes increased venous return and cardiac output, and high risk of thromboembolism secondary to venous stasis after prolonged surgery. Another complication of this position may be the development of compartment syndrome when the lower extremities are inappropriately positioned [53, 69].

#### **3.2. Airway management**

monary compliance are reduced in the supine position in obese patients, eventually increasing ventilation/perfusion mismatch [59]. All these alterations increase as the body mass index (BMI) is elevated. Induction of the anesthesia is recommended to be performed in the lateral decubitus position to overcome these difficulties [61]. Positive end-expiratory pressure (PEEP) may improve lung functions in mechanically ventilated patients [62]. Prolonged supine position should be avoided in patients with reduced cardiac reserve since venous return to the heart is diminished by increased compression onto the inferior vena cava secondary to abdominal pressure and weight. In such cases, operation table or the patient may be turned to

Trendelenburg position: Patient's head is below the horizontal plane in Trendelenburg position, which may increase operative exposure and decrease bleeding in selected cases. It is less tolerated than that in the supine position. In obese patients who already have limited cardiac reserves, blood in lower extremities is added into central and pulmonary circulation by Trendelenburg position, making it hard to tolerate this position [64]. It should be especially avoided in morbidly obese patients. Further diminished residual capacity and pulmonary compliance in this position also lead to atelectasis and hypoxemia. In addition, endotracheal tube may be displaced depending on the position. In brief, this position is often not preferred

Head-upward position: Upper torso of morbidly obese patients should be nearly 35–40° in the sitting position or reverse Trendelenburg position in a way where the operation table allows for adequate ventilation. Such position simplifies mask ventilation and conditions of tracheal

The combined effect of reverse Trendelenburg position and pneumoperitoneum during laparoscopic gastric bypass surgery decreases femoral blood flow and increases venous stasis, thereby increasing risk of pulmonary embolism. Therefore, prolonged applications of this position should be avoided with altering positions occasional breaks during surgery [65].

Prone position: Prone position was shown to increase oxygenation in normal-weighed patients under anesthesia than that in supine position [66]. As long as the chest and pelvis are supported such adequately that allows for abdominal movements, prone position is usually well-tolerated by obese patients. Cardiovascular functions are preserved when appropriate position and supports are provided. Otherwise, cardiac venous return is diminished by compression onto the inferior vena cava and femoral veins, which in turn leads to decreased volume in the left ventricle, causing hypotension. Prone position in obese patients under anesthesia improves pulmonary functions and increase FRC, pulmonary compliance, and

Lateral decubitus position: This position is often tolerated well by obese patients. A decrease in the abdominal fat mass' compression on the abdomen diminishes intraabdominal pressure, which eases diaphragmatic movements during mechanical ventilation. However, maintenance of the same position for a long while may lead to vascular congestion and resulting in

the side so as to decrease aortocaval compression [53, 63].

in obese patients due to all these factors [53, 63].

intubation.

84 Current Topics in Anesthesiology

oxygenation [53, 67].

hypoventilation in underlying lung [68].

Obesity leads to many anatomic alterations in airways. Upper thoracic and lower cervical fat pillows result in a limited range of motion in atlantoaxial joints and cervical vertebra. Excessive tissue folds in the pharynx, short and thickened neck, suprasternal, presternal, and posterior cervical fat tissue, and thick submental fat tissue are formed. All these alterations contribute to potentially difficult airway management, which has been reported as 10.3–20.2% in obese patients compared to 1.5–3.2% of general population [70]. Despite all these anatomical and pathological changes, extent of BMI did not appear to influence difficulty of laryngoscopy. This type of difficulty is rather associated with advanced age, male sex, temporomandibular joint pathology, Mallampati class III and IV, history of obstructive sleep apnea, and abnormal upper teeth [71]. Neck diameter has been defined as the best determinant for intubation difficulty in morbidly obese patients. While the probability of problematic intubation was 5% in patients with a neck diameter of 40 cm, this was found to be 35% in patients with a neck diameter of 60 cm [72]. Increased adipose tissue on pharyngeal walls in obese patients complicates mask ventilation and intubation by leading to alterations in upper airway anatomy. The presence of obstructive sleep apnea is an additional pathology that increases the risk for difficult intubation, hence warranting careful consideration in this patient population [73].

The prevalence of aspiration is low in obese patients, though risk of aspiration-related pulmonary complication is known to be increased in this group [74]. Gastroesophageal reflux that may cause aspiration is common in obese patients. Attention should be paid in patients with history of gastric band application especially in terms of aspiration [64].

Intraoperative ideal ventilation strategies are still contradictory in morbidly obese patients, where diminished lung and thorax compliance is particularly important. The increased amount of thoracic fat tissue is associated with decreased FRC that may be increased by elevation of upper torso though this may not provide an increment as effective as in normal-weighed people [75]. While lung volume is not altered, respiratory load, oxygen consumption, and carbon dioxide synthesis are increased following diminution of lung and thorax compliance, which in turn leads to decreased tolerance to respiratory stress. By causing cyclic alveolar collapse, low FRC and unchanged closure volume induce alveolar injury associated with mechanical ventilation [29]. These patients have predisposition for postoperative atelectasis. An association between the extent of atelectasis and the incidence of postoperative ARDS was also demonstrated [76]. An adequate PEEP administration is important to decrease probability of atelectasis during mechanical ventilation. In obese patients, PEEP provides beneficial effects both on PaO<sup>2</sup> and alveolar-arterial oxygen difference, even these benefits were shown to be more prominent as compared to normal-weighed people [15].

Studies showed that applications of PEEP of 10–15 cm H<sup>2</sup> O, lung-protective low tidal volume of 6–8 ml/kg, and pressure limit below 30 cm H<sup>2</sup> O proved to be beneficial to obese patients [77]. Combined use of recruitment maneuvers and PEEP revealed better effects on intraoperative oxygenation and compliance compared with PEEP use alone during obesity surgery or in surgical obese patients. A meta-analysis reported that pressure-controlled ventilation and volume-controlled ventilation did not differ in terms of outcomes [78].

#### **3.3. Induction and maintenance**

All agents used in anesthesia may also be used in obese patients. However, obesity alters pharmacokinetic parameters depending on the lipid solubility and tissue distribution of the administered anesthetic agent. Nonadipose mass is also increased in obese patients. Drug dosages should be adjusted by considering volume of distribution for loading dose and by considering clearance for maintenance dose. Obese people may highly metabolize lipophilic agents compared to underweight people. Pharmacokinetic studies show that weakly or intermediately lipophilic drugs (e.g., vecuronium) are mainly distributed into nonadipose tissues and the dose needs to be calculated according to the ideal body weight. If the clearance is equal to or less than nonobese patients, the ideal body weight should be taken into account for maintenance dose. If the clearance is increased with obesity, then the total body weight should be considered for maintenance dose.

The ideal body weight is calculated as the sum of 49.9 and 0.89 kg for each cm above the height of 152.4 cm in men, and as the sum of 45.4 and 0.89 kg for each cm above the height of 152.4 cm in women. Agents partially distributed into the adipose tissue have variable pharmacokinetic characteristics; they usually have prolonged and unpredictable effects due to altered volume of distribution and clearance rates, respectively [79].

#### **3.4. Induction agents**

#### *3.4.1. Thiopental sodium*

Thiopental sodium, a frequently used agent for the induction of general anesthesia, is rapidly distributed to highly perfused organs such as brain, liver, lung, intestines, kidneys, heart, and pancreas after bolus administration into the plasma. Reduced plasma concentration and consequent loss of its effect after a short while depends on its rapid distribution to peripheral tissues. High lipophilicity of thiopental increases its volume of distribution and elimination half-life in obese patients. Due to uptake by fatty tissues, its plasma levels decrease within 10 min after induction and the agent is eliminated via liver. A clearance rate of the drug increases twofold in obese patients compared to patients with normal weight. It is reported that administration of the drug according to nonfat body weight is more reasonable for the purposes of induction anesthesia. Nevertheless, increased cardiac output leads to more rapid distribution of thiopental from its effective compartment into the plasma, hence causing an accelerated awaking in procedures where it is administered as single-dose bolus [5, 11].

#### *3.4.2. Propofol*

Studies showed that applications of PEEP of 10–15 cm H<sup>2</sup>

patients [77]. Combined use of recruitment maneuvers and PEEP revealed better effects on intraoperative oxygenation and compliance compared with PEEP use alone during obesity surgery or in surgical obese patients. A meta-analysis reported that pressure-controlled venti-

All agents used in anesthesia may also be used in obese patients. However, obesity alters pharmacokinetic parameters depending on the lipid solubility and tissue distribution of the administered anesthetic agent. Nonadipose mass is also increased in obese patients. Drug dosages should be adjusted by considering volume of distribution for loading dose and by considering clearance for maintenance dose. Obese people may highly metabolize lipophilic agents compared to underweight people. Pharmacokinetic studies show that weakly or intermediately lipophilic drugs (e.g., vecuronium) are mainly distributed into nonadipose tissues and the dose needs to be calculated according to the ideal body weight. If the clearance is equal to or less than nonobese patients, the ideal body weight should be taken into account for maintenance dose. If the clearance is increased with obesity, then the total body weight

The ideal body weight is calculated as the sum of 49.9 and 0.89 kg for each cm above the height of 152.4 cm in men, and as the sum of 45.4 and 0.89 kg for each cm above the height of 152.4 cm in women. Agents partially distributed into the adipose tissue have variable pharmacokinetic characteristics; they usually have prolonged and unpredictable effects due to

Thiopental sodium, a frequently used agent for the induction of general anesthesia, is rapidly distributed to highly perfused organs such as brain, liver, lung, intestines, kidneys, heart, and pancreas after bolus administration into the plasma. Reduced plasma concentration and consequent loss of its effect after a short while depends on its rapid distribution to peripheral tissues. High lipophilicity of thiopental increases its volume of distribution and elimination half-life in obese patients. Due to uptake by fatty tissues, its plasma levels decrease within 10 min after induction and the agent is eliminated via liver. A clearance rate of the drug increases twofold in obese patients compared to patients with normal weight. It is reported that administration of the drug according to nonfat body weight is more reasonable for the purposes of induction anesthesia. Nevertheless, increased cardiac output leads to more rapid distribution of thiopental from its effective compartment into the plasma, hence causing an accelerated awaking in procedures where it is administered

altered volume of distribution and clearance rates, respectively [79].

lation and volume-controlled ventilation did not differ in terms of outcomes [78].

ume of 6–8 ml/kg, and pressure limit below 30 cm H<sup>2</sup>

**3.3. Induction and maintenance**

86 Current Topics in Anesthesiology

should be considered for maintenance dose.

**3.4. Induction agents**

*3.4.1. Thiopental sodium*

as single-dose bolus [5, 11].

O, lung-protective low tidal vol-

O proved to be beneficial to obese

Although propofol has a high lipophilicity, dose adjustment should be performed according to the total body weight due to its high clearance [80]. Its high lipophilicity and rapid distribution from plasma into peripheral tissue render it as the currently most commonly used induction agent in morbidly obese patients. It could be used safely as total intravenous anesthetic drug. Its short-acting nature after single-dose bolus administration is explained by its redistribution from the compartment it acts on, into the plasma and peripheral tissues. As in thiopental sodium, cardiac output is also an important marker in achieving peak plasma concentrations of this agent. When administered as continuous infusion in obese patients, both its volume of distribution and clearance increases along with increased total body weight [53, 59].

#### *3.4.3. Etomidate*

Use of etomidate should be considered in patients with hemodynamic instability. Its use is contradictory due to increased incidence of end-organ dysfunction and in-hospital mortality secondary to adrenal suppressive effects in patients administered etomidate for anesthesia induction. Its induction dose should be adjusted by nonfat body weight due to similar pharmacokinetic and pharmacodynamic properties to propofol and thiopental [53, 59].

#### *3.4.4. Opioids*

Opioids were quite commonly used to control sympathetic response to tracheal intubation and surgical stress during induction and maintenance of the anesthesia. These agents effectively block the response to nociceptive stimulation in the perioperative period. Increased cardiac output and alteration in body composition (increased fatty tissue and nonfat body weight) in obese patients may change pharmacokinetic properties of the opioids. Administration of opioids leads to upper airway obstruction, central sleep apnea, obstructive sleep apnea, ataxic respiration, and hypoxemia [53, 59].

Fentanyl, one of the most frequently used opioids in anesthesia, has a significantly higher clearance in obese patients, which exhibits a nonlinear increase with the total body weight [53, 59]. As fentanyl, the onset of action of sufentanyl is 3–5 min. Although it has similar plasma clearance, its volume of distribution and elimination half-life is increased in obese patients compared to normal-weighed patients [53, 59, 81].

Alfentanyl, a derivative of fentanyl, has one-tenth of the potency than that of fentanyl. It is more lipophilic and has lower volume of distribution than that in fentanyl. Increased cardiac output decreases concentration of plasma alfentanyl during early distribution phase [53, 59, 81].

Remifentanyl is an ester opioid, which is rapidly metabolized by tissue and plasma esterases. Its administration by continuous infusion is widely adopted. Effects will terminate within 5–10 min after cessation of the infusion, which should be given adjusted to the ideal body weight. Administration of remifentanyl based on the total body weight in obese patients may cause some adverse effects such as bradycardia, hypotension, and muscle rigidity due to supratherapeutic plasma concentrations [53, 59, 81].

#### **3.5. Inhalation agents**

Release of inhalation agents is increased due to high solubility in lipids and excessive fat tissue. Furthermore, obese patients were reported to have slow recovery from anesthesia because of extended release of the inhalation agent from adipose tissue [81, 82]. In fact, this slow recovery not only originates from accumulation in adipose tissue but also from increased sensitivity in central nervous system secondary to decreased blood flow in adipose tissue. On the other hand, duration of recovery after procedures of 2–4 h was reported to be similar between obese and nonobese patients [83]. Recovery time after desflurane and sevoflurane, which have low lipid solubility, is also rapid in obese patients [84]. Torri et al. [85] compared obese and nonobese patients and reported that alveolar and inspiratory sevoflurane concentrations were not much changed, yet exhaling of sevoflurane from alveoli was slower in obese patients.

#### *3.5.1. Isoflurane*

Isoflurane is more lipophilic than sevoflurane and desflurane, therefore not commonly used in obese patients. Blood flow is decreased as long as the body weight increases. In clinical practice, impact of body mass index on uptake of isoflurane is not clinically relevant [59, 81].

#### *3.5.2. Sevoflurane*

Having low lipophilicity and solubility, sevoflurane is rapidly absorbed and eliminated compared to isoflurane [59, 81].

#### *3.5.3. Desflurane*

Desflurane, due to its limited distribution in adipose tissue and least lipophilicity and solubility among available inhalation agents, is recommended in obese patients. Nevertheless, the effect of BMI on the absorption of desflurane is not significant. Recovery and wakening from desflurane than that from isoflurane occur more rapidly in both obese and nonobese patients [59, 81].

#### **3.6. Neuromuscular blockers**

Neuromuscular blockers are polar and hydrophilic agents, so they have limited distribution in adipose tissue [86]. Except succinylcholine, administrating doses of neuromuscular blockers are usually calculated according to ideal body weight.

#### *3.6.1. Succinylcholine*

This neuromuscular blocker is a short-acting agent with a rapid onset of action. It may be preferred in obese patients in order to provide quick tracheal intubation. Pseudocholinesterase levels and extracellular fluid are elevated in obese patients, which determine the duration of action of succinylcholine and thereby warrant the need for dose adjustment with respect to the total body weight [87].

#### *3.6.2. Vecuronium*

**3.5. Inhalation agents**

88 Current Topics in Anesthesiology

was slower in obese patients.

*3.5.1. Isoflurane*

*3.5.2. Sevoflurane*

*3.5.3. Desflurane*

patients [59, 81].

*3.6.1. Succinylcholine*

the total body weight [87].

pared to isoflurane [59, 81].

**3.6. Neuromuscular blockers**

ers are usually calculated according to ideal body weight.

Release of inhalation agents is increased due to high solubility in lipids and excessive fat tissue. Furthermore, obese patients were reported to have slow recovery from anesthesia because of extended release of the inhalation agent from adipose tissue [81, 82]. In fact, this slow recovery not only originates from accumulation in adipose tissue but also from increased sensitivity in central nervous system secondary to decreased blood flow in adipose tissue. On the other hand, duration of recovery after procedures of 2–4 h was reported to be similar between obese and nonobese patients [83]. Recovery time after desflurane and sevoflurane, which have low lipid solubility, is also rapid in obese patients [84]. Torri et al. [85] compared obese and nonobese patients and reported that alveolar and inspiratory sevoflurane concentrations were not much changed, yet exhaling of sevoflurane from alveoli

Isoflurane is more lipophilic than sevoflurane and desflurane, therefore not commonly used in obese patients. Blood flow is decreased as long as the body weight increases. In clinical practice, impact of body mass index on uptake of isoflurane is not clinically relevant [59, 81].

Having low lipophilicity and solubility, sevoflurane is rapidly absorbed and eliminated com-

Desflurane, due to its limited distribution in adipose tissue and least lipophilicity and solubility among available inhalation agents, is recommended in obese patients. Nevertheless, the effect of BMI on the absorption of desflurane is not significant. Recovery and wakening from desflurane than that from isoflurane occur more rapidly in both obese and nonobese

Neuromuscular blockers are polar and hydrophilic agents, so they have limited distribution in adipose tissue [86]. Except succinylcholine, administrating doses of neuromuscular block-

This neuromuscular blocker is a short-acting agent with a rapid onset of action. It may be preferred in obese patients in order to provide quick tracheal intubation. Pseudocholinesterase levels and extracellular fluid are elevated in obese patients, which determine the duration of action of succinylcholine and thereby warrant the need for dose adjustment with respect to Vecuronium has a nondepolarizing aminosteroid structure and is mainly eliminated via the liver and gallbladder. Since its duration of action may be prolonged when its dose is calculated according to the total body weight, the dose should be adjusted according to the ideal body weight [88].

### *3.6.3. Rocuronium*

Being a weakly lipophilic and quaternary ammonium neuromuscular blocker, it is highly ionized with a limited extracellular distribution. Induction dose of 1.2 mg/kg calculated according to the ideal body weight provides excellent intubation settings within 60 s. Administration dose should be adjusted according to the ideal body weight in order to avoid prolonged drug metabolism [59, 87].

#### **3.7. Reversal of neuromuscular blocking agents**

Obese patients have increased risk due to upper airway collapse and use of neuromuscular blockers. Therefore, neuromuscular block should be completely reversed before tracheal intubation. Doses of agents reversing neuromuscular blockers should be calculated according to the total body weight. Rapid and thorough reversal of neuromuscular block is particularly important for early restoration of lung functions during early postoperative period [89].

#### *3.7.1. Neostigmine*

A delayed time to antagonize neuromuscular block by neostigmine has been reported in obese patients. In the study of Suzuki et al. [90], the time elapsed to make train-of-four ratio as 0.9 increased fourfold than normal to antagonize vecuronium. Block should not be reversed by neostigmine under deep neuromuscular block. Recommended dose for neostigmine is 0.04–0.08 mg/kg, whose total dose should not exceed 5 mg [89, 90].

#### *3.7.2. Sugammadex*

A modified and most potent derivative of cyclodextrin, sugammadex binds to steroidal muscle relaxants with high affinity. Muscle relaxants are encapsulated within lipophilic cavity. Resultant inclusion complex is excreted through kidneys. Affinity of sugammadex to rocuronium is higher than to either pancuronium or vecuronium. It has no effect on acetylcholine, endogenous steroids, or other muscle relaxing agents. It is not recommended for use in severe renal impairment. In intermediate and deep block, dose calculation is inadequate if done according to the ideal body weight; therefore, the dose needs to be adjusted by the total body weight or ideal body weight plus 40% [89, 91].

#### **3.8. Regional anesthesia**

Regional anesthesia may be preferred to avoid potentially difficult airway control or postoperative respiratory complications. Detection of landmarks for central blocks or peripheral nerve blocks is especially very compelling in morbidly obese patients. Seventh cervical vertebra or gluteal fissure may be used to identify midline for central blocks. Distribution of the local anesthetics is hard to estimate due to lipid infiltration into epidural space and increased intraabdominal pressure, in which case 75–80% of normal local anesthetic dose may suffice. Regional block practices are regarded as more difficult in obese patients. In a study of 2020 supraclavicular block applications, success rate in obese patients was 94.3% compared with 97.3% in nonobese patients, which was significantly different [92]. The prospective study by Nielsen et al. [93] with over 9000 regional block procedures showed that the failure rate was 1.62 times higher in obese patients than in nonobese patients. Block procedures may be safely performed under the guidance of ultrasonography in obese patients [93, 94].
