Gastric bypass under general anesthesia

Gastric bypass under general anesthesia

Learning objectives:

·           anesthesia for the morbidly obese patient

·           obstructive sleep apnea

·           fiberoptic intubation

·           epidural anesthesia for post-operative pain.

A 40-year-old morbidly obese woman comes for a gastric bypass operation.

This intra-abdominal procedure involves restriction of the stomach to give the patient a sense of fullness even with limited oral intake. It may be performed by laparotomy or laparoscopy, and rarely involves significant blood loss.

History: She is morbidly obese despite multiple diets, one of which involved the drug Fen-Phen.

Use of Fen-Phen (fenfluramine–phentermine), a popular diet pill in the 1990s, has been blamed for the development of heart valve abnormalities and pulmonary hypertension.

Review of systems: Chronic hypertension; obstructive sleep apnea (OSA) requiring a CPAP mask at night; adult-onset diabetes mellitus; reflux; chronic low back pain.

We associate all of these finding and symptoms with morbid obesity. The OSA worries us in particular because of its association with pulmonary hypertension and difficult airway management.

Medications: Calcium-channel blocker, diuretic, oral hypoglycemic, H₂ blocker.

We will ask the patient to take her calcium-channel and H₂ blockers the morning of surgery, but neither the diuretic (she will already be dehydrated from her n.p.o. period) nor the oral hypoglycemic (without food intake, her blood sugar could fall dangerously low).

Physical examination: Morbidly obese Caucasian woman in no distress; weight 220 kg; height 5 (150 cm)

BP 150/90 mmHg; HR 90 beats/min; respiratory rate 18 breaths/min

Airway: Mallampati IV; 3fb mouth opening; 4fb thyromental distance; full neck extension

CV: S1, S2 no murmur

Respiratory: Lungs clear to auscultation.

Obesity is an independent risk factor for difficult tracheal intubation. Though the majority of obese patients are easily intubated via direct laryngoscopy, presence of additional risk factors suggests the need for an awake intubation. Intravenous access may be difficult and various possible sites should be examined.

Pre-operative studies: Hgb 12 g/dL; Hct 36%; Plt 250 000/µL;

Na 140 mEq/L; K 4.2 mEq/L; BUN 23 mg/dL; Cr 1.3 mg/dL; glucose 105 mg/dL (5.2 mmol/L)

ABG : pH 7.40; pCO₂ 40 mmHg; pO₂ 95 mmHg; bicarbonate 28 mEq/L

ECG : normal sinus rhythm at 90 beats/min, ST segments at baseline

Echocardiogram: normal valves and pulmonary artery pressures

Infrequently, laparoscopic gastric bypass can result in significant blood loss, hence we like to know the starting hematocrit. Also, should that hematocrit be abnormally high, we will look even more closely at her pulmonary function (chronic hypoxemia-induced polycythemia?). We check her fasting blood glucose level because of her diabetes, and electrolytes because of her use of a diuretic. BUN and creatinine values can reveal renal insufficiency from diabetes and/or hypertension. Other studies further evaluate the impact of her OSA including the arterial blood gas (ABG), which shows no CO₂ retention, and the ECG, which shows no evidence of right ventricular hypertrophy.

Preparation for anesthesia. For post-operative pain management, we offer epidural anesthe-sia, placed awake with sedation carefully titrated to effect. She needs general endotracheal anesthesia. Because we worry about intubation of her airway, we plan an awake fiberoptic intubation and therefore give her glycopyrrolate. We also order metoclopramide and bicitra.

Use of an epidural catheter for post-operative pain control in this setting (morbid obesity, incision near the diaphragm) can reduce the need for opiates and their antitussive effects and thus the threat of pulmonary complications. Sedation must be titrated to effect without compromising the patient’s ventilation.

We facilitate fiberoptic visualization with an anti-sialogogue (glycopyrrolate) to dry secretions. Were she not already taking an H₂ blocker, we might add that to her preoperative medications, intended to reduce the risk of aspiration of gastric contents and its sequelae.

Induction of anesthesia. We place a thoracic epidural catheter under moderate sedation, encountering some (not unexpected) technical difficulty. Once placed and tested we move to the operating room.

After topical pharyngeal lidocaine, we perform superior laryngeal and transtracheal blocks.

We smoothly advance a fiberoptic scope into her trachea and advance the endotracheal tube

without so much as a tiny gag. Once we detect end-tidal CO₂ on the capnograph, we induce general anesthesia with a small dose of thiopental and turn on the isoflurane vaporizer.

Obese patients challenge even the expert at placing epidural catheters. Persis-tence, a cooperative patient, and experience eventually win out. One useful trick: repeatedly ask the patient whether she feels the needle to the right or left of mid-line – sometimes finding the midline itself can be tricky.

Obese patients desaturate rapidly with apnea because of both a reduced func-tional residual capacity (FRC) and increased oxygen consumption. We have cause for concern, given the likely difficulty with mask ventilation (decreased chest wall compliance), potentially difficult tracheal intubation, and even a problem identi-fying tracheal rings should a surgical airway become necessary (heaven forbid!).

Maintenance of anesthesia. We maintain anesthesia with isoflurane in 50% inspired oxygen in air, titrating the volatile agent to maintain hemodynamic stability and a BIS (bispectral index) between 40 and 60. After the surgeon inflates her abdomen with carbon dioxide, she requires high peak inspiratory pressures (40–50 cm H₂O) to achieve an adequate tidal volume. Local anesthetics administered through the epidural catheter provide relaxation of her abdominal muscles, making the operation a little easier for the surgeon. A solid epidural block to the level of T5 also minimizes the need for volatile anesthetic agents. We re-dose the epidural with 2% lidocaine with 1:200 000 epinephrine every 60–90 minutes depending on the clinical situation.

After a lengthy operation, morbidly obese patients have a slow emergence from volatile anesthetics, which are highly soluble in the poorly perfused fat, forming a depot of anesthetics. We do not use nitrous oxide, which might expand gas in bowel and thus add difficulties for the surgeon. Obese patients may also have increased CNS sensitivity to medications, particularly opioids. The reliance on regional anesthesia reduces the need for both volatile agents and narcotics. We allow the epidural anesthetic to wane into analgesia (minimal or no motor block) before the procedure ends so she will be able to maintain her airway, breathe deeply and cough effectively upon extubation of her trachea.

Emergence from anesthesia. Following conclusion of the operation the patient awakens. She is strong, following commands, has a gag reflex and has a good respiratory pattern. We extubate her trachea and transport her to the PACU with the epidural infusion running for postoperative pain relief. We report to the PACU physician, including plans for post-operative pain management.

All patients should meet extubation criteria before the endotracheal tube is removed: fully awake, following commands, able to protect the airway, breath-ing spontaneously. Here we are particularly concerned because this patient was difficult to intubate. Therefore, we delay extubation several minutes (or longer), or use a “tube exchanger” – a long stylet that we place down the endotracheal tube, then leave in the trachea after extubation providing a conduit for reintubation should the need arise.

Post-anesthesia care. We manage her pain with the epidural infusion. Should she require additional analgesics, we should be extremely cautious with those that can depress venti-lation.

Common PACU complications include desaturation, hypertension due to pain, and hypotension due to inadequate fluid replacement and/or epidural-induced sympathectomy. Trouble arises should a synergistic effect of weakened mus-cle power from the epidural block compound respiratory depression from narcotics.

PACU event – Desaturation. After about 30 minutes the nurse calls the PACU physician because the patient’s SpO₂ has fallen below 90% despite 4 L/min oxygen via nasal cannula. She is arousable, after which her saturation improves temporarily, but declines again as she falls back asleep.

We consider the many etiologies of hypoxemia, and investigate the likelihood of each:

·           Narcosis ? She has received no intravenous opioids, and the concentration inthe epidural infusion is unlikely to cause significant respiratory depression.

·           High epidural block with muscle weakness ? Her upper and lower extremities arestrong – ruling out this diagnosis.

·           Residual neuromuscular blockade ? She did not receive any non-depolarizingmuscle relaxants intraoperatively, relying instead on the epidural for relaxation.

·           Atelectasis ? Probably part of the problem, but would not explain desaturationonly while asleep.

·           Obstructive sleep apnea ? This rises to the top of the list when we watch thepatient breathe. She snores loudly and, though difficult to see but readily felt by placing a gentle hand over her larynx, a tracheal tug is evident with each inspiration. With some breaths she fails to move any air at all.

This patient requires CPAP to sleep at home. Lingering anesthetic effects and decreased afferent sensory input from the epidural anesthetic reduce stimula-tion to breathe, which might conspire with her upper airway pathology and thus worsen a sleep apnea. She requires CPAP. We keep the patient awake until a Res-piratory Therapist brings the necessary equipment.

Discharge. After we confirm the epidural block is behaving as expected (return of muscle function but excellent analgesia), and are confident with her ventilation, we discharge the patient to the floor with continuous pulse oximetry. We alert the surgical service of her dependence on CPAP and the need for respiratory monitoring on the ward. We also inform the acute pain service (APS) of her location so they can manage her epidural medications for the next 2–3 days, until the pain level subsides and she is able to take oral medications.

In the post-operative orders we restrict additional opioids or sedatives except as prescribed by the Acute Pain Service. These members of the anesthesia care team will be available on call as needed and will see the patient on rounds at least twice daily to adjust dosing regimens and ensure safety.