Obesity’s Effects on Respiratory Mechanics in the Perioperative Period

| May 1, 2015

This column is written by medical students and is dedicated to reviewing the science behind obesity and bariatric surgery.

Column Editor: Daniel B. Jones, MD, MS, FACS
Professor of Surgery, Harvard Medical School, Vice Chair, Beth Israel Deaconess Medical Center, Boston, Massachusetts

This month: Obesity’s Effects on Respiratory Mechanics in the Perioperative Period

by Geneviève Chartrand
Medical Student, Weill Cornell Medical College,
New York, New York

The author discusses the differences and potential consequences of altered pulmonary physiology in patients with morbid obesity in the preoperative, intraopertive, and postoperative periods. Considerations for the bariatric surgical team include obstructive sleep apnea, patient positioning, airway monitoring and continuous positive airway pressure, and acute airway management. The author concludes that surgeon attentiveness to the unique physiology of patients with obesity will lead to avoidance or early identification of complications.

Bariatric Times. 2015;12(5):20–22.

Changes in respiratory mechanics in the patient with obesity have been well documented in the literature, especially since the introduction of laparoscopic surgery. There is a curious paradox that the changes in pulmonary mechanics after abdominal insufflation may be slightly less in the patient with obesity compared to the patient without obesity.[1] Nonetheless, in an effort to continue to improve quality of care and safety for our bariatric patients, it is critical that we are aware of the differences and potential consequences of altered pulmonary physiology in patients with morbid obesity and not relegate the consideration of these concerns solely to the anesthesiology team.

Preoperative Evaluation
The bariatric surgery patient is unique in many ways. Large body mass, fat distribution, and associated comorbidities are all part of perioperative risk assessment.

There are several possible mechanisms by which obesity affects the respiratory system. One of the major concerns is an increased demand for ventilation, which elevates the work of breathing. This is compounded by respiratory muscle inefficiency (extrinsic compression of the chest wall, the stretching of the diaphragm by intra-abdominal fat, and limited full excursion of the diaphragm) and diminished respiratory compliance. Individuals with central (android) patterns of fat distribution are more likely to have difficulty with respiration than those with peripheral obesity.[2]

Even in the absence of intrinsic lung disease, the patient with morbid obesity will demonstrate the increased FEV1/FVC (forced expiratory volume in the first second/forced vital capacity) ratio seen in patients with restrictive disease combined with the change in flow-volume curves seen in obstructive diseases. Total lung capacity is decreased, but the most marked change is the decrease of functional residual volume, thus leaving these patients with lower lung volumes and early airway closing volume with expiration. This is associated with the closure of peripheral lung units; ventilation to perfusion mismatch effectively creates a pulmonary shunt during which the patient may become hypercapnic and hypoxemic, especially in the supine position.[3] This is especially consequential when combined with the effects of excess weight on the upper pharynx. The pharynx may collapse during rapid eye movement (REM) sleep or with the combination of anesthesia and narcotics, and obstruction during sleep can lead to prolonged hypoxemia.[4]

Obstructive sleep apnea. Respiratory changes were noted to have significant consequences in the postoperative bariatric patient. Sleep apnea is frequently underdiagnosed in patients with morbid obesity. There are certain clinical predictors of sleep apnea, including the frequently used STOP-BANG acronym (Table 1). Mallampati grade (MMP), tonsil size, and body mass index (BMI) are associated with sleep apnea. Measured thyroid-mental distance (TMD) and hyoid-mental distance (HMD) do not appear to be predictive of OSA.[5] Obesity hypoventilation syndrome (OHS) is a more severe manifestation of OSA. The hallmark of OHS is hypercarbia and hypoventilation while awake, without another identifiable cause. OHS is more prevalent as BMI increases and, if left untreated, mortality among those with OHS is high.[6]

Intraoperative Considerations
Pneumoperitoneum. The effects of exposure to pneumoperitoneum have been documented and studied since the introduction of laparoscopic procedures, such as cholecystectomy in the 1990s. It is known that pneumoperitoneum using carbon dioxide will increase the systemic absorption of CO2, which in turn may result in hypercarbia and eventual acidosis. Insufflation pressures of 15–20mmHg are used to achieve adequate visualization but will increase the intra-abdominal pressure, thus causing physiologic and hemodynamic changes.[7]

Positioning. The respiratory system’s intimate relationship to the abdominal cavity via the diaphragm is no surprise because changes in volume or pressure in the thorax provoke reciprocal changes in the abdomen.

The surgeon must remain cognizant of the effects of patient positioning and especially the length of time in each position, as it will have an effect on the respiratory system.
It has been shown that while the pneumoperitoneum alone has little effect on the partial pressure of oxygen, the addition of Trendlenberg position to pneumoperitoneum will significantly impair the partial pressure and alveolar-arterial differences in oxygen tension in the patient with obesity.[7,8]

Postoperative Care
Monitoring and continuous positive airway pressure considerations. The prevalence of upper airway obstruction in the immediate postoperative period has been found to be frequent among the bariatric surgery population.[9]

Investigations have revealed significant hypoxemia with patient-controlled analgesia (parenteral narcotics) in the immediate postoperative period.[10,11] Postoperative pulmonary complications can include atelectasis, respiratory failure, and pneumonia. In all subjects, it has been demonstrated that postoperative CPAP therapy is an effective tool for reducing pulmonary complications.

CPAP or bilevel positive airway pressure (BiPAP) following the operation involves both the assessment of respiratory status and consideration for the protection of a newly created gastric sleeve or pouch in bariatric surgery.

A discussion should occur between the anesthesia and surgery team prior to initiation of CPAP therapy, as gastric insufflation may be a possible result. It should be noted that while CPAP is necessary for maintaining airway patency, the risk of gastric distention might increase the risk of emesis and aspiration. In the immediate postoperative period there is the theoretical risk of “stressing” a suture line or anastomosis. Though several trials have shown it to be safe immediately following bariatric surgery, simple maneuvers, such as setting adjustments or favoring nasal prongs or BiPAP to adjust pressure per breath, could avoid patient discomfort from pouch/sleeve distention. Increased intra-thoracic pressure can effect venous return in some patients, leading to decreased blood pressure readings and a need for adequate volume resuscitation and monitoring postoperatively.[10,12]

Acute Airway Management
While acute airway management is certainly more in the realm of the anesthesiologist, acute respiratory failure may occur at any time in the postoperative course. A laryngeal mask airway (LMA) may be very useful in managing difficult airway situations. The American Society of Anesthesiologists (ASA) algorithm for difficult airways includes an early attempt at insertion of the LMA if face mask ventilation is not adequate.[13]

The consideration of altered respiratory mechanics in the bariatric patient begins in the office and continues throughout the perioperative period. Surgeon attentiveness to the unique physiology of patients with obesity will lead to avoidance or early identification of complications.

1.    Dumont L, Mattys M, Mardirosoff C, et al. Changes in pulmonary mechanics during laparoscopic gastroplasty in morbidly obese patients Acta Anaesthesiol Scand. 1997;41(3):408–413.
2.    de Melo Barelar J, Aliveri A, Lourdes Lins de Barros T, et al.  Chest Wall Regional Volumes in Obese Women. Respiratory Physiology & Neurobiology. 2013;189 (1): 167-173
3.    Parameswaran K , Todd DC, and Soth M. Altered respiratory physiology in obesity. Can Respir J. 2006;13(4): 203–210.
4.    Chung F. Screening for obstructive sleep apnea syndrome in the pre-operative patients. The Open Anesthesiology Journal. 2011;5:7–11.
5.    Friedman M, Tanyeri H, La Rosa M, et al. Clinical predictors of obstructive sleep apnea. Laryngoscope. 1999;109(12):1901–1907.
6.    Piper A, Yee B. Clinical manifestations and diagnosis of obesity hypoventilation syndrome.  UpToDate, Wolters Kluwer Inc.  http://www.uptodate.com/contents/clinical-manifestations-and-diagnosis-of-obesity-hypoventilation-syndrome. Accessed 3/29/15.
7.    Nguyen N, Wolfe B. The physiologic effects of pneumoperitoneum in the morbidly obese. Ann Surg. 2005;(241):219–226.
8.    Demiroluk S, Salihoglu Z, Zengin K, et al. The effects of pneumoperitoneum on respiratory mechanics during bariatric surgery. Obes Surg. 2002;12:376–379.
9.    Ahmad S, Nagle A, McCarthy RJ et al. Postoperative Hypoxemia in morbidly obese patients with and without obstructive sleep apnea undergoing laparoscopic bariatric surgery.  Anesth Analg. 2008: 107(1): 138- 43
10.    Ramirez A, Lalor P, Szomstein S,et al. Continuous positive airway pressure in immediate post-operative period after laparoscopic Roux en Y gastric bypass: is it safe? Surg Obes Relat Dis. 2009;5(5):544–546.
11.    Gallagher S, Haines K, Osterlund L, et al. Postoperative hypoxemia: common, undetected and unsuspected after bariatric surgery. J Surg Res. 2010;159(2):622–626.
12.    Smetana G. Postoperative Pulmonary complications: An update on risk assessment and reduction. Cleve Clin J Med. 2009;76 Suppl 4:S60–65.
13.    Apfelbaum JL1, Hagberg CA, Caplan RA, et al. Practice guidelines for management of the difficult airway: an updated report by the American Society of Anesthesiologists Task Force on Management of the Difficult Airway. Anesthesiology. 2013;118(2):251–270.

FUNDING: No funding was provided.

FINANCIAL DISCLOSURES: The author reports no conflicts of interest relevant to the content of this article.

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Category: Past Articles, The Medical Student Notebook

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