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Describe an anesthetic plan for bariatric surgery.
Patient preparation begins well before entering the operating room. Preoperative functional status of the upper airway, lower airway, and cardiovascular system help predict perioperative outcome and should be assessed. Although controversial, some form of aspiration prophy-laxis is commonly administered. Often this is simply a non-particulate antacid by mouth, just prior to entering the operating room. Most patients walk into the operating room and lie down on the operating room table. Intravenous access is frequently difficult, but rarely requires central venous cannulation. Large blood pressure cuffs are useful for many patients, but for others large cuffs slide distally and become unreliable. In such cases, normal-sized cuffs are placed on the forearm. Infrequently, intra-arterial cannulation is required for blood pressure measurements. Standard electrocardiogram electrodes and pulse oximeter probes generally work well on MO patients.
Preoxygenation is achieved employing an anesthesia facemask with an airtight seal. Three vital capacity breaths or 3 minutes of tidal breathing is rarely sufficient. Effective preoxygenation requires varying amounts of time, depend-ing on several factors. A clinically useful endpoint for pre-oxygenation is an expired oxygen concentration of 90% or above.
Although some might argue it is unnecessary, anesthesia is generally begun with a rapid sequence induction, utilizing cricoid pressure. Any one of multiple induction agents can be administered. Determining induction doses based on total body weight may result in overdosing, while selecting induction doses predicated on ideal body weight can result in underdosing. For most agents, dosing is predicated on lean body mass. As a rule of thumb, lean body mass frequently approximates 100 kg in men and 80 kg in women. Actual doses of induction agents are modified based on myocardial reserves. Succinylcholine is an excellent choice for laryngoscopy, although nondepolarizing neuromuscu-lar blockers can be used as well. A Macintosh 3 blade or a Miller 2 blade provides adequate laryngoscopic views in most patients and is a good way to start out. Tracheal tubes of various sizes are prepared with one or more stylets.
Just about any anesthetic agent or adjuvant can be used for bariatric patients. Since the goal is to have them awake and extubated at the end of surgery, short-acting agents are preferable. The duration of action of midazolam and its active metabolite often extends into the emergence phase, thereby delaying awakening, and in combination with opi-oids depresses respiration. For these reasons, midazolam seems to be counterproductive as an anesthetic adjuvant during bariatric surgery. Muscle relaxants are necessary for laparotomy and helpful for laparoscopy. Opioids are useful as anesthetic adjuvants and required for postoperative analgesia. Even laparoscopy patients suffer from significant abdominal wall soreness and benefit from analgesia. Air is used instead of nitrous oxide to avoid bowel distention. Potent inhalation agents are generally used.
MO patients are prone to soft tissue infections, necessi-tating antibiotic prophylaxis. Two grams of cephalosporin or equivalent is commonly administered intravenously prior to incision. The stomach is decompressed before inserting trochars to help prevent gastric damage by these sharp instruments. Additionally, the evacuated stomach takes up less space in the abdomen, thereby providing improved surgical vision and facilitates manipulations of the organ. Forced-air warming blankets help prevent heat loss during laparotomy and laparoscopy.
For lean patients, intraoperative tidal volumes are generally based on weight. To do so for MO patients results in excessive tidal volumes and very high inspiratory pressures. In fact, selection of tidal volume should be pred-icated on patient height. As a general rule of thumb, short patients do well with tidal volumes approximating 500 ml and tall patients do well with tidal volumes approximating 700 ml. Although these starting points are adequate for many MO patients, they should be adjusted depending upon the individual’s requirements. Selecting a tidal volume must account for oxygenation, ventilation, and inspiratory pressures. Some authors advocate larger tidal volumes as prophylaxis against intraoperative decreases in FRC, that often result in hypoxemia. This was challenged by Sprung et al. (2003), who showed that large tidal volumes did not improve oxygenation during laparoscopy. An alternative to large tidal volumes is the use of positive end expiratory pressure (PEEP). PEEP can improve oxygenation, but tends to increase inspiratory pressures, predisposing to pneumothorax, and decreases venous return to the heart, resulting in decreased blood pressures.
Emergence and extubation are critically important. At the completion of surgery, bariatric patients still suffer from the same preoperative problems with which they started, plus others. Residual anesthetic agents depress respiratory drive and diminish upper airway dilator muscle efficiency. Lingering muscle weakness impairs coughing and deep breathing, as well as interfering with upper airway dilator muscle function. Abdominal wall pain from laparotomy or abdominal wall soreness from laparoscopy predispose to splinting and all its associated problems. Consequently, MO patients are at risk for hypoxemia in the postoperative period. To prevent this problem, those who do not meet extubation criteria remain intubated and ventilated postoperatively. Those who satisfy extubation criteria are extubated and given supplemental oxygen to breathe by nasal cannula or facemask. They are nursed in the upright position to help restore FRC and ventila-tion/perfusion matching.
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