Challenges in Pulmonary Risk Assessment and Perioperative Management in Bariatric Surgery Patients
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- Kaw, R., Aboussouan, L., Auckley, D. et al. OBES SURG (2008) 18: 134. doi:10.1007/s11695-007-9282-0
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Bariatric surgery has serious associated medical comorbidity and procedure-related risks and is, thus, considered an intermediate-to-high-risk non-cardiac surgery. Altered respiratory mechanics, obstructive sleep apnea (OSA), and less often, pulmonary hypertension and postoperative pulmonary embolism are the major contributors to poor pulmonary outcomes in obese patients. Attention to posture and positioning is critical in patients with OSA. Suspected OSA patients requiring intravenous narcotics should be kept in a monitored setting with frequent assessments and naloxone kept at the bedside. Use of reverse Tredelenburg position, preinduction, maintenance of positive end-expiratory pressure, and use of continuous positive airway pressure can help improve oxygenation in the perioperative period.
KeywordsPerioperative managementPulmonary complicationsObstructive sleep apneaPulmonary hypertensionBariatric surgery
Obesity increases the likelihood of perioperative hypoxia , and respiratory dysfunction is an important cause of postoperative morbidity and mortality after abdominal surgery. Complications from impaired pulmonary mechanics facilitate development of pneumonia  and respiratory failure  and augment the risk of obstructive sleep apnea (OSA), pulmonary hypertension (PH), and pulmonary embolism (PE) after surgery [3, 4].
Effects of Obesity on Pulmonary Function in Surgical Patients
Increases in body mass index (BMI) lead to exponential decreases in expiratory reserve volume (ERV), forced expiratory volume (FEV1), functional residual capacity (FRC), forced vital capacity, and maximum voluntary ventilation . Weight gain does not affect residual volume. The reduction thereby in FRC is primarily related to a decrease in the ERV , particularly in the supine position. When the ERV falls below the closing volume, gas trapping with ventilation–perfusion mismatching results causing hypoxemia.
Mechanically ventilated and paralyzed morbidly obese patients also have reduced respiratory system compliance . In one study, anesthetized morbidly obese patients in the supine position had 29% lower pulmonary compliance compared to nonobese patients, and, unfortunately, neither a doubling of tidal volume nor a doubling of respiratory frequency reduced the alveolar–arterial gradient . Hypoxemia during mechanical ventilation in obese patients is mediated, at least in part, through unopposed increases in intra-abdominal pressure (especially during laparoscopic surgery) that reduce lung volumes, resulting in ventilation–perfusion mismatch .
OSA in Obesity and Effects on Bariatric Surgical Outcomes
The incidence of OSA in patients being evaluated for bariatric surgery has not been previously defined. However, the prevalence of OSA is better characterized and is higher than what is found in the general population, with estimates running from 35 to 91% when formal testing for OSA with polysomnography is performed on all patients [10–12]. The Epworth Sleepiness Scale (ESS) and overnight pulse oximetry are poor predictors of OSA. One study of obese patients without nighttime or daytime symptoms of OSA showed that male gender, the waist hip ratio, and neck circumference were independent predictors of OSA . BMI alone is not a good predictor of the presence of OSA, and many authors now recommend routine evaluation of OSA by polysomnography before bariatric surgery.
Up to one third of patients with OSA undergoing hip and knee replacements developed substantial respiratory or cardiac complications including arrhythmias, myocardial ischemia, unplanned ICU transfers, and/or reintubation mostly within the first 72 h after surgery . Looking specifically at OSA patients undergoing bariatric surgery, a review of more than 3,000 bariatric surgery cases from a single institution found OSA to be a positive predictive factor for anastomotic leaks . In patients undergoing bariatric surgery, the presence of OSA can double the odds of a length of stay longer than 3 days (OR 2.25)  and is associated with a higher cost for postoperative care .
Pulmonary Hypertension in Obesity and the Effects of Bariatric Surgical Outcomes
Several studies have reported no association between BMI and PH [18, 19]. Contrarily, Bady et al.  found that mean pulmonary arterial pressure correlated with BMI and a lower PaO2. In addition, BMI was significantly higher in patients with PH compared to those without PH (37.4 vs 30.3). The issue is likely complicated by the fact that many obese patients have concomitant cardiopulmonary risk including OSA, increased risk of thromboembolic disease, and use of anorexic agents that may confound the association between PH and OSA . One study shows that PH decreased significantly only in the group of patients in whom OSA resolved after bariatric surgery, whereas it remained elevated in those without postoperative resolution of OSA despite similar weight loss between those two groups . Most studies, however, suggest that obstructive pulmonary impairment, OSA, hypoxemia, and hypercarbia are independent determinants for the development of daytime PH. Some controversy persists regarding the role of BMI as a predictor of PH, possibly because of differences in the severity of obesity and the definition of PH.
Pulmonary Embolism After Bariatric Surgery
A large 10-year surgical necropsy series from University of Michigan suggested that obesity is one of the major risk factors for the development of PE . Although the reported incidence of PE after gastric bypass is low (0.6–0.8%) [3, 24], it accounts for most of the mortality associated with the procedure . In another study, 12 fatal emboli occurred in 5,554 bariatric operations (0.21%) . This study identifies venous stasis, BMI ≥ 60, truncal obesity, and obesity hypoventilation/OSA as risk factors for the complication and recommends prophylactic inferior vena cava (IVC) filters placement in bariatric surgery candidates with those risks factors. In a more recent study, prophylactic IVC filters in patients with BMIs > 55 kg/m2 resulted in marked reduction in both fatal and nonfatal PE .
Preoperative Evaluation of Obese Patients
Obesity has long been associated with difficult airway management. A thorough upper airway examination of the obese patient should include a measurement of the neck circumference, mouth opening, and neck range of motion. A neck circumference of approximately 44 cm slightly increases the difficulty in intubation as opposed to a 35% risk of difficult intubation at a neck circumference of 60 cm . Failed intubation can occur in as many as 5% of attempted surgeries in patients with OSA [27, 28].
Limited mandibular protrusion and the degree of difficulty in visualizing the faucial pillars, the soft palate, and the base of the uvula predicts difficulty with intubation and should increase the suspicion of OSA [29, 30]. A study of obese patients who had OSA-related symptoms found that neck circumference greater than or equal to 43 cm was the best clinical predictor of apnea/hypopnea index . Airway obstruction out of proportion to the apparent degree of sedation and a pronounced tendency for upper airway obstruction during or upon recovery from anesthesia can be the first suggestion of undiagnosed sleep apnea intraoperatively .
Some of the obese Pickwickian patients suffer from both OSA and obesity-hypoventilation syndrome (OHS) defined by chronic daytime hypoxemia (PaO2 < 65) and hypoventilation (PaCO2 > 45) in patients without chronic obstructive pulmonary disease (COPD). Patients with OHS can be best screened by room air pulse oximetry. Arterial blood gas is warranted for O2 saturation <96% to document CO2 retention and screen for possible PH and associated right heart failure. In patients with findings suggestive of OSA with hypoxia (OHS), history of thromboembolic disease, and anorectic drug use, preoperative echocardiogram seems prudent. Despite known and expected abnormalities, spirometry is of no added benefit in obese patients unless COPD is suspected.
Perioperative Interventions in Obese Patients
Patients with severe OSA who undergo major surgery with a significant amount of anesthesia should be monitored closely for potential upper airway compromise over a few days. Extubation should be performed when the patient is fully awake and neuromuscular function has returned as verified by, sustained head lift for > 5 seconds and evidence of adequate vital capacity and peak inspiratory pressure. Continuous positive airway pressure (CPAP) or access to CPAP should be available during this period .
Attention to posture and positioning are very important in patients with OSA. The head of the patient’s bed should be elevated to at least 30° in patients with OSA to increase the stability of upper airway  at all times while in the PACU and or hospital.
Sedation and Analgesia
Benzodiazepines should be avoided for the most part in this population because of their effects on the central nervous system and upper airway musculature. Cautious use of newer non-benzodiazepine sedatives such as zolpidem or ramelteon is advised. Likewise, narcotics should be limited, and alternative forms of analgesia such as nonsteroidal antiflammatory medications, nerve blocks, or local analgesics should be considered. Epidural analgesia, however, may not eliminate this risk. Respiratory arrest has been reported in those with OSA receiving epidural opioids at 2 to 3 days postoperatively .
If narcotics are required for pain control, then patient-controlled analgesia (PCA) with no basal rate and restricted dosing may help to limit dosing . Dosing of the morphine PCA is not dependent on weight or body surface area and should be administered based on ideal body weight . Similarly for lipophilic opioids like fentanyl, dosing weights of 100 to 108 kg (ideal body weight estimates) are recommended in obese individuals weighing 100 to 200 kg to achieve the same therapeutic plasma . Furthermore, intramuscular injections in obese patients are rarely intramuscular and more likely to be in subcutaneous fat where a poor blood supply makes analgesic absorption difficult to determine . Suspected OSA patients requiring intravenous narcotics should be kept in a monitored setting with frequent assessments, and naloxone should be kept at the bedside.
At present, three approaches are suggested to improve ventilation and oxygenation in obese patients: high tidal volume, positive end expiratory pressure, and vital capacity maneuver .
Reverse Tredelenburg Position
RTP is presumed to improve oxygenation in anesthetized obese patients  and enable better exposure of the subdiaphragmatic region, allowing mechanical ventilation with safe levels of airway pressure. It also decreases the push of abdominal contents on the diaphragm, thereby increasing FRC . Dixon et al.  demonstrated that the decrease in oxygen saturation during apnea under anesthesia is slower when the patient is positioned head-up as compared with flat. Additionally, there were improvements in lung function, atelectasis, and shunting in the 25° head-up position, which were lost once positive-pressure ventilation was commenced.
Using Pre-Induction and Maintenance PEEP
The prevention or reduction of atelectasis from the induction and maintenance of general anesthesia improves arterial oxygenation. Pre-oxygenation with 100% fraction of inspired oxygen (FIO2) and 10-cm positive end-expiratory pressure (PEEP) for 5 min before the induction of general anesthesia followed by 10-cm PEEP during mask ventilation and after intubation reduces immediate post-intubation atelectasis as assessed by computed tomography scan and improves immediate post-intubation arterial oxygenation on 100% FIO2 (PAO2 of 457 ± 130 mmHg versus 315 ± 100 mmHg in the control group) . Whether this reduction is maintained and for how long is not known. In obese patients without gastroesophageal reflux, improvement in oxygenation that can be achieved with the pre-induction use of PEEP is significant and will increase the time period before desaturation begins. In contrast, in normal weight patients, neither the PaO2 increased nor the A-a gradient decreased with PEEP. Practitioners should strongly consider taking advantage of the improved arterial oxygenation that pre-induction PEEP offers.
CPAP in Moderately Obese Patients Without Known Sleep Apnea
Prophylactic use of CPAP in the first 24 h after surgery significantly reduces the risk of pulmonary restrictive syndrome that occurs in morbidly obese patients after gastroplasty . CPAP also reduces the risk of acute respiratory distress syndrome after upper abdominal surgery . Sixteen percent (95% confidence interval 2.9–29.3%) absolute risk reduction in the rate of respiratory failure was reported in one study with the use of noninvasive ventilation in severely obese patients when applied during the first 48 h post-extubation . Bilevel positive airway pressure in the first 24 h after gastric bypass surgery has been shown to improve FVC, FEV1, and SpO2. However, there was no benefit in terms of a decreased length of stay or a lower complication rate .
CPAP in Moderately Obese Patients with Known Sleep Apnea
Gupta et al. found that the use of home CPAP in patients with OSA who underwent joint replacements had a possible carryover protection resulting in a significantly lower rate of postoperative complications, although most of them did not receive CPAP in the hospital. This may, in part, be related to upper airway stabilization, a residual effect of CPAP after as little as 4 h of use . Other benefits of CPAP in the postoperative state include reduction in hemodynamic fluctuations accompanying early episodes of respiratory obstruction in patients with OSA . Obese patients with hypercapnia from OHS have also been shown to benefit from CPAP . Many patients will likely need lower pressures auto-titrating PAP machines in the setting of rapid weight loss after bariatric surgery  to treat their OSA as weight loss occurs. Use of CPAP after gastric bypass surgery may prevent the development of respiratory complications such as pneumonia .
Safety of CPAP Use After Bariatric Surgery
Concern has been raised that the use of positive pressure devices after upper intestinal surgery may increase the risk of anastomotic disruption and leakage. However, in a large prospective series of 1,067 bariatric surgical patients of whom 159 were on CPAP postoperatively for known OSA, there were no increase in anastomotic leaks related to the use of CPAP . Two additional smaller studies regarding the prophylactic use of bilevel positive airway pressure after gastric bypass surgery also did not find an increase in the risk of this complication [4, 52].