Respiratory Physiology in Morbidly Obese Perioperative Patients

By January 6, 2020Uncategorized

Obese patients are becoming more commonplace in the operating room with each passing year. By the year 2030, an estimated 86.3% of adults will be overweight, and another 51% will be obese. Of these, the American Society of Metabolic and Bariatric Surgery estimates that 24 million will be morbidly obese [1]. Obesity is classified at a BMI greater than 30 kg/m2, and morbid obesity, or extreme obesity, is defined as a BMI greater than 40 kg/m2. These patient present unique respiratory problems in the operating room such as increased work of breathing, difficulty with preoxygenation and induction, and complicated emergence and postoperative care.

Morbid obesity is responsible for several derangements in pulmonary physiology. Oxygen demand, carbon dioxide production, and alveolar ventilation are elevated because metabolic rate is proportional to body weight [2]. The excessive adipose tissue over the chest wall compounds this problem by reducing chest wall compliance while leaving pulmonary compliance intact, which increases work of breathing and oxygen demand [3]. Increased abdominal adiposity is also responsible for forcing the diaphragm cephalad, further worsening restrictive lung physiology [2]. For every unit of BMI increased over 30 kg/m2, the residual volume, total lung capacity, and vital capacity drop by 0.5% [4]. Because functional residual capacity may fall below the normal tidal volume of these patients, they will experience a larger amount of atelectasis, which causes ventilation perfusion mismatching. The above changes are significantly exacerbated in the supine and Trendelenburg positions [2].

These pulmonary complications can make for particularly challenging inductions. Many morbidly obese patients have comorbid obstructive sleep apnea (OSA) making them particularly susceptible to rapid desaturation. An apnea hypopnea index greater than 30 implies severe OSA and is a predictor of rapid desaturation. Additionally, following induction of anesthesia, morbidly obese patients are more likely to derecruit gas exchange units, which exacerbates the likelihood of desaturation [5]. Preoxygenation with 100% FiO2 and a CPAP of at least 10cm H20 have been shown to improve PaO2 prior to induction and lengthen apnea time. After induction, CPAP of 10-12 cm H2O can be used to reduce atelectasis throughout the case, but care must be taken to prevent potential hypotension that results from increasing intrathoracic pressure [6].

Prior to extubation, neuromuscular blockade should be fully reversed [7]. Once spontaneous ventilation has resumed, morbidly obese patients can be maintained on pressure support. Extubation can be performed once a patient demonstrates adequate strength via sustained tetanus on nerve stimulator or adequate performance of the 5-second head lift. They should also adequately follow commands prior to extubation. Patients preparing for extubation should be placed in the trunk-up head-up position to improve oxygenation and decrease work of breathing. Immediately following extubation, pressure support or CPAP may be delivered via face mask in a similar fashion to induction preoxygenation [8].

In caring for morbidly obese patients post-operatively, respiratory and ventilatory concerns are at the forefront. The risk of postoperative hypoxia is increased in these patients, especially when there is preoperative hypoxia or with surgery involving the thorax or upper abdomen. An obese patient should remain intubated until there is no doubt that an adequate airway and tidal volume will be maintained, neuromuscular blockers are completely reversed, and the patient is awake [2]. In the post-anesthesia care unit (PACU), morbidly obese patients should have continuous pulse oximetry until they can demonstrate adequate oxygenation while unstimulated. A high level of suspicion should be maintained for hypoventilation, and the threshold for administering an oral airway should be low. If arousal and oral airway are inadequate to support the patient, it is reasonable to use non-invasive ventilation to prevent reintubation. While data is conflicting, there is a possible benefit to using incentive spirometry for the first two hours post-op to improve oxygenation [9]. Additionally, like in preinduction and extubation, there is a benefit to keeping morbidly obese patients upright in the PACU.

The respiratory changes in morbidly obese patients demand careful consideration of their physiology, induction, emergence, and postoperative care. Induction of anesthesia requires particular care for preoxygenation that other patients would not require. Emergence from anesthesia in the morbidly obese requires stricter adherence to extubation criteria. And, finally, post-operative care requires greater vigilance for hypoxia than in typical patients. As the population of obese and morbidly obese patients continues to grow, we will have to become more accustomed to recognizing and treating these respiratory complications in the operating room.

1. Ogden, C.L., et al., Prevalence of obesity among adults: United States, 2011-2012. NCHS data brief, 2013(131): p. 1-8.
2. Morgan, G.E., M.S. Mikhail, and M.J. Murray, Clinical anesthesiology. 4th ed. 2006, New York: Lange Medical Books/McGraw Hill, Medical Pub. Division. xiv, 1105 p.
3. Schumann, R., Pulmonary physiology of the morbidly obese and the effects of anesthesia. International anesthesiology clinics, 2013. 51(3): p. 41-51.
4. Jones, R.L. and M.-M.U. Nzekwu, The effects of body mass index on lung volumes. Chest, 2006. 130(3): p. 827-833.
5. Eikermann, M., et al., Do Patients with Obstructive Sleep Apnea have an Increased Risk of Desaturation During Induction of Anesthesia for Weight Loss Surgery? The open respiratory medicine journal, 2010. 4: p. 58-62.
6. Cressey, D.M., M.C. Berthoud, and C.S. Reilly, Effectiveness of continuous positive airway pressure to enhance pre-oxygenation in morbidly obese women. Anaesthesia, 2001. 56(7): p. 680-684.
7. Pelosi, P. and C. Gregoretti, Perioperative management of obese patients. Best practice & research. Clinical anaesthesiology, 2010. 24(2): p. 211-225.
8. Miller, R.D., Miller’s anesthesia. Eighth edition. ed. 2015, Philadelphia, PA: Elsevier/Saunders. 2 volumes (xxx, 3270, I-122 pages).
9. Zoremba, M., et al., Short-term respiratory physical therapy treatment in the PACU and influence on postoperative lung function in obese adults. Obesity surgery, 2009. 19(10): p. 1346-1354.