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186
J. F. Chacon
These physiologic changes associated with robotic and laparoscopic surgery can
be further worsened depending on the patient position. In Trendelenburg position,
airway pressures will be further increased and lung volumes further decreased. With
the addition of CO2 that is absorbed by the body during insufation, blood CO2
levels rise and it can be difcult to increase the minute ventilation to adequately
ventilate the patient. The benet of autotransfusion will be minimized secondary to
insufation pressures that decrease venous return. In reverse Trendelenburg, venous
return is further reduced, often leading to hypotension. Although airway pressures
are improved slightly compared to supine or Trendelenburg, they still are elevated
compared to non-Robotic or open surgery. As such, mechanical ventilation can still
prove to be challenging [7].
17.3 Summary
In summary, positioning is a team-based exercise that proper knowledge, vigilance,
and execution can lead to improved patient outcomes, increased operating room
efciency, and decreased risk of injury. Although the positions described above are
not all encompassing, these are the positions most frequently encountered in bariatric surgery. Additionally, it is important to recall the physiologic changes associated
with the specic positions and the type of surgery that is being performed.
References
1. Madni TD, Imran JB, Clark AT, Cunningham HB, Taveras L, Arnoldo BD, Phelan HA, Wolf
SE.Prospective evaluation of operating room inefciency. J Burn Care Res. 2018;39(6):977–81.
https://doi.org/10.1093/jbcr/iry016.
2. Stewart JD, Shantz SH.Perioperative ulnar neuropathies: a medicolegal review. Can J Neurol
Sci. 2003;30:15–9.
3. Practice advisory for the prevention of perioperative peripheral neuropathies 2018: an updated
report by the American Society of Anesthesiologists Task Force on Prevention of Perioperative
Peripheral Neuropathies. Anesthesiology. 2018;128:11–26.
4. Warner MA.Supine positions. 3rd ed. Philadelphia: Saunders; 1997.
5. Coppieters MW, Van de Velde M, Stappaerts KH.Positioning in anesthesiology: toward a better
understanding of stretch-induced perioperative neuropathies. Anesthesiology. 2002;97:75–81.
6. Zeuzem-Lampert C, Groene P, Brummer V, Hofmann-Kiefer K.Kardiorespiratorische Effekte
perioperativer Positionierungsmaßnahmen [Cardiorespiratory effects of perioperative posi-
tioning techniques]. Anaesthesist. 2019;68(12):805–13. German. https://doi.org/10.1007/
s00101- 019- 00674- 9.
7. Hsu RL, Kaye AD, Urman RD.Anesthetic challenges in robotic-assisted urologic surgery. Rev
Urol. 2013;15:178–84.
Chapter 18
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Intraoperative Monitoring oftheMorbidly
Obese Patient
JoshuaF.Chacon
18.1 Pulse Oximetry
Pulse oximetry utilizes light absorption to quantitate the amount of oxygen-bound
hemoglobin. Pulse oximetry uses 2 small light emitting diodes (LEDs), red (660nm)
and infrared (940 nm). Oxygenated hemoglobin absorbs more infrared light and
allows red light to pass through. Deoxygenated hemoglobin absorbs red light allowing infrared to pass through. The LEDs re approximately 30 times per second, and
a receiver measures the amount of light that passes through. This ratio provides a
measurement of blood oxygenation.
Multiple physiologic and pathophysiologic conditions that often accompany
morbid obesity necessitate accurate pulse oximetry [2]. The morbidly obese patient
is more likely to have obstructive sleep apnea, in addition to an increased basal
oxygen consumption and potential respiratory disease. As such, these patients are
highly likely to desaturate and become hypoxemic more quickly. These patients
also have a lower resting oxygen saturation at baseline, making appropriate preoxygenation prior to the induction of general anesthesia signicantly more important.
18.2 Electrocardiogram
Given the risk of hypoxemia and the high likelihood of coexistent cardiac disease or
dysfunction, the morbidly obese patient population is undoubtedly at higher than
average risk for intraoperative myocardial ischemia. Given these concerns it is especially important to monitor the EKG correctly [3, 4].
J. F. Chacon (*)
TeamHealth Anesthesia, Palm Beach Gardens, FL, USA
© The Author(s), under exclusive license to Springer Nature
Switzerland AG 2023
A. Teixeira et al. (eds.), Duodenal Switch and Its Derivatives in Bariatric and
Metabolic Surgery, https://doi.org/10.1007/978-3-031-25828-2_18
187
188
J. F. Chacon
When discussing lead placement it is important to identify which leads to monitor and why to select them. When looking for ischemic changes, lead V5 alone will
detect 75% of ischemic episodes in men 40–60years of age [4]. Adding lead V4
increases this to 90%, and the combination of leads II, V4, and V5 add up to a 96%
detection rate.
Meanwhile lead II, when correctly placed, is most appropriate for accurate evaluation of p-waves. For those reasons, when utilizing a 5-lead setup, leads V5 and II
are the most commonly monitored, allowing for ischemic and electrophysiologic
problems to be identied in a timely fashion.
18.3 Blood Pressure
Accurate and consistent blood pressure monitoring is essential in safe anesthetic
care. In the morbidly obese patient obtaining these measurements can be challenging.
Noninvasive blood pressure monitoring utilizes the oscillometric technique.
When the cuff is correctly placed on the patient’s arm, it is inated with air until
arterial ow past the cuff ceases. Then the pressure in the cuff is gradually released.
Sensors in the cuff detect the oscillations of intraarterial ow. As the cuff pressure
declines, the oscillations increase in amplitude to a maximum, which represents the
mean arterial pressure (MAP). This MAP value is the only pressure actually measured, at the point of maximal amplitude.
The system can then use an algorithm using the measured MAP value to calculate a systolic and diastolic pressure. Each manufacturer has its own method, meaning there may be considerable variation between systems. A study in lean and obese
patients found inaccuracies regardless of body weight or arm circumference [5].
Non-invasive blood pressure (NIBP) readings are further complicated in the
obese patient. Finding an appropriately sized blood pressure cuff can prove a difcult endeavor. Often, even if the cuff can appropriately t the circumference of the
arm, the conical shape of the morbidly obese arm makes reading inconsistent [6].
Undersized cuffs typically underestimate the blood pressure for the morbidly obese
patient. Various studies have evaluated the accuracy of blood pressure cuffs on both
the forearms or on the legs. A study from 2002 showed that NIBP measurement
with a cuff placed at the wrist routinely measured the blood pressure higher than
upper arm values. That study concluded that compensation can be performed by
subtracting 10mmHg from the measured values or simply by elevating the wrist
about 15cm and taking the BP at face value.
Aragahi et al. enrolled a group whose mean BMI was approximately 32. They
found that both oscillometric and traditional auscultatory methods were unreliable
compared to intraarterial measurement. Noninvasive blood pressure measurements
consistently underestimated systolic pressure and overestimated diastolic pressure.
Multiple studies have proposed equations to calculate an accurate blood pressure value
by forearm cuff, however there is little to no agreement between various researchers
[7]. Forearm pressures appear to be consistent, but not equivalent to more accurate
18 Intraoperative Monitoring oftheMorbidly Obese Patient
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189
pressure measurements. Leg blood pressure cuff readings appear even more unreliable. Overall, there is no consistent correlation. NIBP readings in the morbidly obese
patient show signicantly different values (some increased and some decreased) that
are neither consistent nor equivalent to more accurate blood pressure measurements.
18.4 Arterial Line
Arterial line placement is often considered the gold standard for accurate blood
pressure measurement. While arterial measurement may pose a more accurate
value, there are both risks to placement and challenges with the morbidly obese
patient.
Body habitus and the amount of subcutaneous tissue in the morbidly obese
patient may make arterial line placement technically challenging. If and when
access is obtained, often a longer catheter must be inserted in order to provide
secure placement in the artery. Risks of arterial line placement include temporary
vascular occlusion, thrombosis, ischemia, hematoma, localized infection, and even
sepsis. Rare complications include severe nerve or artery damage, and critical ischemia requiring surgical intervention.
In general, arterial line placement is reserved for patients with signicant cardiovascular comorbidities and is rarely used for routine monitoring of the bariatric
patient.
18.5 End Tidal Carbon Dioxide Monitoring
Use of the modern gas analyzer has multiple advantages for the morbidly obese
patient. Prior to induction of general anesthesia, preoxygenation can be quantitatively measured by end tidal oxygen values. Optimum preoxygenation can provide
a larger margin for safety during any apnea that occurs prior to intubation [8].
Morbidly obese patients are more likely to have sleep apnea and comorbid pulmonary complications. Appropriate end tidal carbon dioxide monitoring can help
avoid signicant hypercapnia that often occurs in the morbidly obese patient.
18.6 Temperature
Maintaining body temperature during surgery has well documented benets in
terms of healing, coagulation, recovery, and avoiding infection. In the morbidly
obese patient avoiding hypothermia prevents increasing metabolic demands on the
body, which can be crucially important given the likelihood of comorbid cardiovascular disease. Forced air warmers, blankets, bed warmers, and intravenous uid
warmers may all aid in maintaining body temperature.
190
J. F. Chacon
18.7 Additional Monitors
18.7.1 Noninvasive Cardiac Output Monitors
Many companies have produced noninvasive means of measuring cardiac output
(CO). However, the efcacy in using these devices in the morbidly obese patient
population is questionable and has not been formally validated. Studies comparing
CO calculated by either noninvasive means versus thermodilution via pulmonary
artery (PA) catheter have shown poor correlation [9]. Tejedor etal. in the Journal of
Critical Care did a case series that showed noninvasive means reported higher values compared to PA catheter, more than half of patients studied showed greater than
20% variation above PA catheter value.
PA catheter placement, while the gold standard for CO measurement, is not without signicant risk including bleeding, infection, cardiac arrhythmia, PA rupture,
blood clots, stroke, and even death. While a gold standard for cardiac output monitoring, it may be unnecessary for most morbidly obese patients, unless signicant
comorbid conditions are also present.
18.7.2 Processed Electroencephalogram
Processed electroencephalogram (EEG) is becoming more common in the operating room, and may play an important role in the morbidly obese patient. Several
studies have shown that morbidly obese patients undergoing general anesthesia
while utilizing processed EEG monitoring were given lower doses of induction
agents and could be safely maintained on lower end tidal concentrations of anesthetic gas. This translated to quicker wake ups and less time spent in the recovery
room [10].
As with any anesthetic, appropriate monitoring is essential in providing safe and
effective anesthesia care. The morbidly obese patient population has comorbidities
that make monitoring both more important and more challenging. By identifying
the potential difculties that may arise, correct monitors can be identied without
causing unnecessary risk to the patient. Keeping this in mind will help providers
utilize the appropriate monitors to the best of their abilities.
References
1. World Health Organization. Obesity and overweight 2020. 2021. https://www.who.int/
news- room/fact- sheets/detail/obesity- and- overweight.
2. Tsai A, Schumann R.Morbid obesity and perioperative complications. Curr Opin Anaesthesiol.
2016;29(1):103–8. https://doi.org/10.1097/ACO.0000000000000279.
18 Intraoperative Monitoring oftheMorbidly Obese Patient
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191
3. O’Brien PE, Hindle A, Brennan L, Skinner S, Burton P, Smith A, etal. Long-term outcomes
after bariatric surgery: a systematic review and meta-analysis of weight loss at 10 or more years
for all bariatric procedures and a single-centre review of 20-year outcomes after adjustable
gastric banding. Obes Surg. 2019;29(1):3–14. https://doi.org/10.1007/s11695- 018- 3525- 0.
4. Association of Anaesthetists of Great Britain and Ireland Peri-operative management of
the obese surgical patient 2015. Anaesthesia. 2015;70:859–76. https://doi.org/10.1111/
anae.13101.
5. Thorell A, MacCormick AD, Awad S, Reynolds N, Roulin D, Demartines N, etal. Guidelines for
perioperative care in bariatric surgery: enhanced recovery after surgery (ERAS) society recommendations. World J Surg. 2016;40(9):2065–83. https://doi.org/10.1007/s00268- 016- 3492- 3.
6. Petrini F, Di Giacinto I, Cataldo R, Esposito C, Pavoni V, Donato P, etal. Perioperative and
periprocedural airway management and respiratory safety for the obese patient: 2016 SIAARTI
Consensus. Minerva Anestesiol. 2016;82(12):1314–35.
7. Mechanick JI, Apovian C, Brethauer S, Timothy Garvey W, Joffe AM, Kim J, etal. Clinical
practice guidelines for the perioperative nutrition, metabolic, and nonsurgical support of
patients undergoing bariatric procedures—2019 update: cosponsored by American association
of clinical endocrinologists/American college of endocrinology, the obesity society, American
society for metabolic and bariatric surgery, obesity medicine Association, and American society of anesthesiologists. Obesity. 2020;28(4):O1–O58. https://doi.org/10.1002/oby.22719.
8. Association of Anaesthetists of Great Britain and Ireland Recommendations for standards of
monitoring during anaesthesia and recovery 2015. Anaesthesia. 2016;71(1):85–93. https://doi.
org/10.1111/anae.13316.
9. Schumann R, Meidert AS, Bonney I, Koutentis C, Wesselink W, Kouz K, etal. Intraoperative
blood pressure monitoring in obese patients. Anesthesiology. 2020;134(2):179–88. https://doi.
org/10.1097/ALN.0000000000003636.
10. Rogge DE, Nicklas JY, Haas SA, Reuter DA, Saugel B.Continuous noninvasive arterial pressure monitoring using the vascular unloading technique (CNAP System) in obese patients
during laparoscopic bariatric operations. Anesth Analg. 2018;126(2):454–63. https://doi.
org/10.1213/ANE.0000000000002660.
Chapter 19
Method ofAnesthesia: Gas Selection
andAdjunct Medications
AmirSamir
19.1 Denition ofMorbid Obesity
With patients involved in bariatric surgery being morbidly obese with a BMI over
40, it is important to take into consideration the dosing of both inhalational as well
as intravenous anesthetics during such operations.
19.2 Why It Is Important toDose Inhalational Agents
andOther Drugs Differently inObese Patients
Obesity is associated with increased cardiac output and blood volume which in turn
affect the rate of clearance and elimination of anesthetic drugs [1]. The increase in
body weight and fat content in such patients leads to an increase of the volume of
the distribution of lipophilic drugs [2]. Also drug clearance is higher in obese
patients due to the enhancement of their renal and hepatic metabolism (Table19.1).
A. Samir (*)
Orlando Regional Medical Center, Orlando, FL, USA
© The Author(s), under exclusive license to Springer Nature
Switzerland AG 2023
A. Teixeira et al. (eds.), Duodenal Switch and Its Derivatives in Bariatric and
Metabolic Surgery, https://doi.org/10.1007/978-3-031-25828-2_19
193
194
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Table 19.1 Factors affecting pharmacokinetics in obesity [3]
Volume fat mass
Increased fat mass
Increased lean body mass
Increased total body water
Increased blood volume
Increased cardiac output
Organomegaly
Protein binding
Possible increased lipoproteins (e.g., cholesterol or triglycerides)
Altered alpha1-acid glycoprotein
Drug metabolism
Increased activity of some CYP P450 enzymes
Increased phase II drug metabolism via glucuronidation and sulfation
Excretion
Increased renal blood ow
Increased GFR
Increased renal tubular secretion and reabsorption
Individual organ system comorbid conditions
Pharmacokinetics for many drugs have not been well studied in obese patients, and depend on the
degree of lipophilicity or hydrophilicity, protein binding, and mechanisms of metabolism and
excretion. The increased value of distribution can prolong the half-life of elimination, particularly
for lipophilic drugs and prolonged infusion, despite increased drug clearance. Alterations in body
composition and physiologic parameters vary with the degree of obesity, and may be affected by
comorbidities that are commonly associated with obesity (e.g., diabetes mellitus, hypertension,
cardiovascular disease, fatty liver disease) or other etiologies
CYP cytochrome; GFR glomerular ltration rate
a
The effects of obesity on various plasma proteins have not been well established, and may very
among patients. Serum albumin is generally unchanged in obesity
b
Effects of obesity on the CYP enzymes is variable among the different enzymes. Obesity increases
activity of CRP2E1, but studies on the effects of obesity on other isozymes are inconsistent
a
b
A. Samir
19.3 Anesthesia Gases Used forBariatric Surgery
1. Carrier gases:
(a) Oxygen and air mix are the most common carrier gas compositions used in
bariatrics. Initially during induction, 100% oxygen is used for preoxygenation as obese patients tend to desaturate faster due to lower functional
residual capacity, decreased chest wall compliance, and increased intrabdominal mass, all of which can lead to rapid desaturation even during short
periods of apnea [4]. Once the airway is secured, the composition should be
changed to a mix of oxygen and air with the goal of keeping the fractional
inhaled oxygen, FiO2, less than 60%. This will avoid any oxygen toxicity
such as pulmonary toxicity and ocular damage [5].
19 Method ofAnesthesia: Gas Selection andAdjunct Medications
195
(b) Nitrous oxide is usually used as a supplemental anesthetic agent to both
lower the fraction of inspired oxygen (FiO2) and potentiate the effect of
inhalational agents by affecting the brain centers in the brain and the spinal
cord and stimulation of GABA receptors. However, NO is relatively contraindicated for bariatric surgery and other laparoscopic procedures. This
mainly due to the fact it has the ability to expand in air containing spaces
that may lead to bowel distention that may interfere with the surgeon performing the bariatric procedure. It may also be associated with the development of neuropathy and pernicious anemia [6].
(c) Air is used mainly to dilute the concentration of oxygen in order to lower the
fraction of inspired oxygen and lower the chances of oxygen toxicity.
2. Volatile gases:
Volatile gases are used for induction and maintenance of anesthesia in the
operating room. They are liquid at room temperature and require special vaporizers in order to change to an inhalational gas form. They provide both amnesia
and immobility to the patient.
• Mechanism of action: They act mainly by depressing the central nervous
system through augmenting the effect of GABA on its receptors. It causes
immobility by acting via action on the spinal cord [7].
• Minimum alveolar concentration: It is the concentration of gas in the alve-
oli at which 50% of patients would show a motor activity in response to surgi-
cal stimulation [8].
• Volatile agents related physiological changes in obese: Increased work of
breathing, increased oxygen consumption, and increased carbon dioxide pro-
duction. Those changes lead to increased oxygen requirement along with
early desaturation.
Table 19.2 covers the factors that affect anesthetic requirements.
(a) Desurane: This is the most commonly used inhalational agent for patients
that are obese and or have obstructive sleep apnea. This is mainly due to the fact
that it has low oil to gas partition coefcient, which leads to decreased uptake
by the adipose tissue and as a result avoiding prolonged emergence from anesthesia [9]. It also has a very low blood to gas partition coefcient that helps with
rapid induction and rapid recovery.
Table 19.2 Factors that
affect anesthetic requirements
Factors that increase
anesthetic requirements
• Chronic ETOH
• Infant (highest MAC at 6
months)
• Red hair
• Hypernatremia
• Hyperthermia
Factors that decrease
anesthetic requirements
• Acute ETOH
• Elderly patients
• Hyponatremia
• Hypothermia
• Anemia (Hgb<5g/dL)
• Hypercarbia
• Hypoxia
• Pregnancy
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A. Samir
Disadvantages:
• Very high pungency and marked airway irritation that may lead to cough,
breath-holding, or laryngospasm. Therefore, it is not suitable for inhalational
induction that may be needed in obese patients with expected difcult airway.
• Sympathetic stimulation leading to tachycardia and hypertension especially
at high concentration.
• Needs a special electric heated vaporizer. This along with high cost of desurane may make it less available in some countries.
(b) Sevourane: It is the second best gas to be used for obese patients. It is favor-
able due to its sweet smelling and low pungency, which makes it suitable for
inhalational induction for difcult airway patients. It also has low blood to gas
partition coefcient, which helps with rapid induction and quick emergence.
Disadvantage:
• High cost due to the higher fresh gas ow required (2L/min) in order to
prevent compound A formation.
• Compound A associated nephropathy.
(c) Isourane: Older inhalational drug that has a low cost, high potency, and little
effect on cerebral autoregulation.
Disadvantages:
• Highly soluble in adipose tissue, which makes it one of the least desired
inhalational agents to be used on obese patients.
• Highly pungency, which makes it very unsuitable to use in case of a needed
inhalational induction.
(d) Halothane: Older agent that is sweet smelling and has low cost. However,
Halothane is no longer available in North America due to its side effects, especially Halothane hepatitis.
Disadvantages:
• Highly soluble in blood and fat tissue, which results in slow induction and
prolonged emergence.
• Hepatic toxicity and halothane hepatitis.
Inhalational agents delivery (Ventilation) (Table 19.3): Different techniques
of ventilation have been used for obese patients with success. Obesity is not associated with increased lung volume and therefore increased tidal volume is not indicated. Atelectasis is very common in bariatric patients and the use of higher positive
end expiratory pressure (PEEP) may be required as well as a high alveolar recruitment maneuver to keep the alveoli patent. This helps with improving the patient’s
oxygenation. However, such high pressure may lead to increase in the intrathoracic
pressure causing lowering of the venous return and in turn lower cardiac output and
blood pressure [10].
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