Anesthetic Management of Morbid Obese Patient
Preoperative:
A) Evaluation:
Read more ☛ Preoperative Anesthetic Evaluation
B) Preparation:
1-Equipments:
➧ New model operating room tables can accommodate up to 270 kg (600 pounds) of weight.
➧ Older model operating room tables could accommodate up to 135-160 kg (300 - 350 pounds) of weight.
➧ In cases of extreme morbid obesity, big boy hydraulic beds are obtained and used in the operating room.
➧ Extra-large cuffs can be used on the upper/lower extremity.
➧ Bed warming devices, fluid warmers, and warm airflow blankets should be employed to prevent hypothermia, which can occur rapidly when large areas of the body surface are exposed.
2-Monitoring:
➧ Selection of ECG leads when possible for detection of myocardial ischemia and pathology (leads II and V5).
➧ Placement of an arterial line for the monitoring hemodynamic status is advocated for all but minor procedures.
➧ Central venous and pulmonary artery catheters should be considered in patients undergoing lengthy operations or those with the serious cardio-pulmonary disease.
3-Aspiration prophylaxis:
➧ Obese patients have great volumes and more gastric fluid than people of normal weight.
➧ Gastro-esophageal reflux and hiatus hernia are more prevalent in the obese, predisposing them to esophagitis and pulmonary aspiration.
➧ Conditions that cause delayed gastric emptying, such as DM and traumatic injury, further increase the risk of aspiration.
➧ An obese patient is considered to have a “full stomach,” even if the fasting period is followed.
➧ Pre-induction administration of histamine-2 and dopamine receptor antagonist coupled with oral administration of non-particulate antacids decreases morbidity resulting from pulmonary aspiration and Mendelson’s syndrome.
➧ Head up the position of the patient, with the application of the Sellick's maneuver during rapid-sequence induction, limits the volume of vomitus that enters the trachea if regurgitation occurs.
➧ Nasogastric/Orogastric suctioning before emergence further reduces the amount of fluid available for aspiration.
4-Airway equipment:
➧ Appropriate laryngoscopy blades, handles, endotracheal tubes, masks, oral and nasopharyngeal airways, and stylets should be ready.
➧ Laryngeal mask airways (LMAs), fiberoptic and bronchoscopic devices, Eschman introducers, a jet ventilator (or Venturi apparatus), an emergency tracheotomy, and cricothyrotomy kits must be available if ventilation by mask or endotracheal tube is unsuccessful.
➧ A difficult airway cart with all available equipment should be placed in the operating room.
Intraoperative:
1-Effects of General Anesthesia on Respiration:
➧ General anesthesia depresses respiration in normal subjects so any preexisting pulmonary dysfunction is exaggerated by anesthesia.
➧ General anesthesia reduces FRC by 50% in obese patients, as compared with a 20% reduction in non-obese patients.
➧ FRC can be increased by ventilating with large tidal volumes (15-20 ml/kg), although this has been shown to improve arterial oxygen tension only minimally. Current ventilation recommendations include using tidal volumes of 10-12 ml/kg to avoid barotrauma.
➧ Positive end-expiratory pressure (PEEP) achieves an improvement in both FRC and arterial oxygen tension but at the expense of cardiac output and oxygen delivery.
➧ During laparoscopic surgeries, the respiratory rate should be 12-14 breaths/min.
➧ Prolonged (longer than 2-3 hours) and extensive procedures (involving the abdomen, thorax, and spine) negatively influence respiratory function.
➧ Sub-diaphragmatic packing, cephalad displacement of organs, and surgical retraction cause decreased alveolar ventilation, atelectasis, and pulmonary congestion.
➧ Recumbent/Trendelenburg positioning further reduces diaphragmatic excursion, which is already impaired by the weight of the panniculus, it also causes elevated filling pressures which then increase right ventricular preload.
➧ Myocardial oxygen consumption, cardiac output, pulmonary artery occluding pressures, peak inspiratory pressures, and venous admixtures are increased above upright-sitting values.
➧ In non-obese patients, cardiac output increases in response to supine posturing to maintain hemodynamic stability. By increasing left ventricular output the centrally located circulating volume is propelled forward, thereby minimizing pulmonary congestion and hypoxia.
➧ In morbidly obese patients, positive pressure, ventilation (which impedes venous return), and an inability to increase cardiac output may result in cardiopulmonary decompensation. This is exhibited intraoperatively by hypoxia, rales, ventricular ectopy, congestive heart failure, and hypotension. Bag ventilation by hand may be useful in reducing hypotension resulting from positive pressure.
➧ Adequate ventilator settings inflate the morbidly obese thorax to minimize hypoxia. Pressure or volume-controlled ventilators can be used to maintain adequate oxygenation and normocapnia.
➧ Avoidance of prolonged prone, Trendelenburg or supine positioning also decreases ventilation-perfusion mismatch.
➧ Optimization of oxygenation by using no less than 50% inspired oxygen is recommended.
➧ PEEP can reduce venous admixture and support adequate arterial oxygenation, but PEEP can impair arterial oxygenation in some patients when it is superimposed on large tidal volumes. Recommended PEEP should not exceed 15 cmH₂O.
➧ Intraoperative events, such as hemorrhage or hypotension, further impair ventilatory homeostasis and result in hypoxemia that extends into the postoperative period.
➧ A vertical abdominal incision, compared with a horizontal (transverse incision also prolongs postoperative hypoxia).
➧ Pain causes further reductions in diaphragmatic excursion and vital capacity, leading to atelectasis and ventilation-perfusion mismatch.
➧ 24 h. postoperative admission to a monitored bed is prudent for morbidly obese patients who exhibit higher morbidity and mortality apart from anesthesia and surgery.
2-Choice of Anesthetic Technique:
➧ The selection of the anesthetic technique is dependent on the patient, coexisting history, planned surgical procedure, anesthetist skills and preference, and patient preference.
➧ Anesthetic management of obese patients can include local or monitored anesthesia; general (narcotic, inhalation) anesthesia; regional blocks; or a combination of techniques.
➧ The use of a short-acting water-soluble anesthetic facilitates a smooth anesthetic induction, maintenance, and emergence.
➧ The objective for maintenance of anesthesia in the obese include:
-Strict maintenance of airway
-Optimum oxygenation
-Adequate skeletal muscle relaxation
-Avoidance of the residual effects of muscle relaxants
-Provision of appropriate intraoperative and postoperative tidal volume
-Effective postoperative analgesia
3-Airway Management:
➧ To facilitate airway management, an obese patient should be positioned with the head elevated (reverse Trendelenburg position) on the operating room table.
➧ This position promotes patient comfort, reduces gastric reflux, provides easier mask ventilation, improves respiratory mechanics, and helps maintain functional residual capacity (FRC).
➧ Reduced FRC in obese patients contributes to the rapid desaturation that occurs with induction of general anesthesia, to attenuate the desaturation and maximize oxygen content in the lungs, patients are pre-oxygenated with 100% oxygen for at least 3-5 min.
➧ The patient's head and neck should be carefully moved into the “sniffing position” by using pillows, doughnuts, or foam head supports.
➧ Without proper support and alignment of the oropharynx and trachea, ventilation may be obstructed and visualization of the laryngeal structures may be obscured.
➧ Some anesthetists make an “Awake Look” to visualize the difficulty of the airway. Careful administration of sedative drugs and application of topical anesthesia to the oropharyngeal structures, possibly including transtracheal and superior laryngeal nerve blocks. Nasal oxygen is used as a supplement during awake laryngoscopy.
➧ If the epiglottic and laryngeal anatomy is easily visible, successful asleep intubation can be done, if not, an intubating LMA or awake fiberoptic intubation should be used.
➧ The endotracheal tube must be safely secured to prevent movement during positioning and surgery.
➧ The anesthetist and another skilled anesthesia provider must also be in attendance during induction of anesthesia because muscle hypotonia in the floor of the mouth, followed by the rapid occurrence of soft tissue obstruction and hypoxia, requires one person to support the mask and airway while another person bag ventilates the patient.
➧ In the case of inability to Intubate/ventilate (CICV), the American Society of Anesthesiologists (ASA) difficult airway algorithm should be followed.
4-Volume Replacement:
➧ In obese patients, the estimated blood volume is actually diminished.
➧ Fat, which contains only 8-10% water, contributes less fluid to total body water than equivalent amounts of muscle.
➧ The percentage of total body water is 60-65% in non-obese adults while in morbid obese it is reduced to 40%; therefore calculation of estimated blood volume should be 45-55 ml/kg of actual body weight rather than 70 ml/kg in non-obese adults.
➧ Accurate volume replacement and avoidance of rapid rehydration lessen cardiopulmonary compromise.
➧ Fluid management is guided by blood pressure, heart rate, and urine output measurements. Dilutional coagulopathy, factor VIII inhibition, and decreased platelet aggregation can result from excessive administration.
➧ Volume expanders should not be administered at greater than 20ml/kg of Ideal Body Weight (IBW).
➧ Blood loss replacement of a 3:1 ratio (3 ml of crystalloid to 1 ml of blood loss) is applicable in morbidly obese patients.
➧ Blood products after careful identification should be replaced according to the patient’s laboratory values and hemodynamic or surgical needs.
5-Intraoperative Positioning:
➧ Surgical positioning of morbidly obese patients necessitates extra precautions for the prevention of nerve, integumentary, and cardiorespiratory compromise.
➧ Many peripheral nerves are subjected to possible ischemia or necrosis, the lunar, brachial plexus, radial, personal, and sphenoid nerves are the most vulnerable to injury in an anesthetized patient. In morbidly obese patients the incidence may be increased because of excessive weight on the anatomic structures.
➧ Care is necessary when one is positioning obese extremities in slings, draping them on Mayo stands, or securing them in lithotomy stirrups.
➧ Prolonged hyperextension, external rotation, or abduction can cause postoperative muscle pain, nerve palsies, or paralysis; therefore less flexion or abduction, and rotation of hips, legs, and arms may be necessary.
➧ Frequently repeated inspections of extremities for color and temperature can help diminish the incidence of positioning-related injuries.
➧ Lower back pain can be aggravated by both spinal and general anesthesia because of ligamentous relaxation that results in loss of lumbar curvature. Surgical towels placed under the lumbar spine before induction will enhance lordosis and reduce postoperative discomfort.
6-Integumentary Concerns:
➧ Decubitus, skin infection and wound dehiscence are exceedingly common in morbidly obese patients, therefore ensure all pressure points are padded and in a proper position.
7-Extubation:
➧ The risk of airway obstruction after extubation is increased in obese patients.
➧ A decision to extubate depends on the evaluation of the ease of mask ventilation and tracheal intubation, the length, and type of surgery, and the presence of preexisting medical conditions, including OSA.
➧ Criteria for extubation consist of:
-Awake state, tidal volume, and respiratory rate at preoperative levels
-Ability to sustain head lift or leg lift for at least 5 seconds
-Constant hand grip
-Effective cough
-Adequate vital capacity of at least 15 ml/kg and inspiratory force of at least -25 to -30 cm H₂0.
➧ Patients must be placed with their heads up or in a sitting position.
➧ If doubt exists regarding the ability of the patient to breathe adequately, the endotracheal tube is left in place.
➧ Extubation over an airway exchange catheter or a fiberoptic bronchoscope may be performed.
8-Regional Anesthesia:
➧ Regional anesthesia can be used as the primary anesthetic in selected cases or as an accompaniment to postoperative pain and mobility management.
➧ Difficulties are frequently encountered with morbidly obese patients as anatomical landmarks are not easily visualized or palpable.
➧ Brachial plexus anesthesia can be hampered by adipose tissue in the axillary region due to an inability to position the arm or undetectable pulses.
➧ Redundant roles of adipose tissue, unsatisfactory ventilation, and the inability of the patient to sustain optimal positioning make neuraxial anesthesia more challenging.
➧ For subarachnoid or epidural anesthesia it is recommended that the patient is sitting upright so that landmarks such as C7 or L3-L4 can be more easily identified and longer needles should be used.
➧ Generous infiltration with local anesthetic will provide greater patient comfort during needle insertion due to the repeated insertions and repositioning.
➧ Another consideration regarding subarachnoid or epidural anesthesia in morbid obese pregnant or surgical patients is the lack of predictability of the spread of local anesthetic. Undesirable cephalad spread of local anesthetic can be obviated by reducing the volume and increasing the patient’s upright sitting time.
Postoperative:
1-Pain Management:
➧ Postoperative pain management is facilitated by the use of oral analgesics, non-steroidal anti-inflammatory drugs, narcotics, patient-controlled analgesia, local infiltration of the surgical site, and epidural anesthesia
➧ Obese patients are more sensitive to the respiratory depressant effects of opioid analgesics; therefore caution and close monitoring are warranted. Supplemental oxygen and pulse oximetry monitoring are mandated.
2-Postoperative Complications:
➧ Morbidity and mortality rates are higher in obese patients than in non-obese patients.
➧ Ventilation abnormalities are exacerbated in obese patients with OSA and OHS and may last for several days.
➧ The maximum decrease in partial pressure of arterial oxygen occurs 2-3 days postoperatively.
➧ The risk of thromboembolism wound infections and atelectasis is amplified in patients with increased BMI.
➧ Thromboembolism is facilitated by immobility (Venous stasis, increased blood viscosity (polycythemia, hypovolemia) increased abdominal pressure, abnormalities in serum procoagulants, and anticoagulants.
➧ Antiembolic stockings and correctly fitting pneumatic compression boots can lessen the occurrence of deep vein thrombosis in the early postoperative period.
➧ Early ambulation and maintenance of vascular volume further reduce the likelihood of clots formation.
➧ Wound infections and pulmonary embolism are 50% higher in obese patients than in non-obese patients.
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