Several factors contribute to the fact that morbidly obese patients are more prone to rapidly desaturate when apneic. Morbidly obese patients have increased oxygen demand, CO2 production, and alveolar ventilation, all because metabolic rate is proportional to body weight. Minute ventilation increases to maintain a normal CO2 level. Also, excessive adipose tissue over the chest decreases the chest wall compliance, and increased abdominal mass forces the diaphragm upwards and reduces lung volumes. Inspiratory and expiratory reserve volumes are decreased, which leads to a lower functional residual capacity and vital capacity. Residual volume remains normal in these patients. ERV is further reduced in the supine position and even more so by the Trendelenburg position.
The decrease in FRC may be less than the closing capacity resulting in air trapping and areas of shunt. These physiologic changes often lead to hypoxia in obese patients in an awake state, and make them susceptible to rapid desaturation in periods of apnea.
Preoperative anesthetic considerations include avoiding respiratory depressant drugs in patients with signs of preoperative hypoxia. Preoperative evaluation of the morbidly obese patient should include an estimation of cardiopulmonary reserve with an ECG, chest xray, and perhaps an ABG and PFTs. The airway in obese patients may be difficult as they may have a narrowed upper airway and shortened space between mandible and sternal fat tissue. Therefore, an intubation plan should be made ahead of time.
Postoperatively, respiratory failure is a major concern for morbidly obese patients. Morbidly obese patients are particularly susceptible during the postoperative period if sedatives or opioids have been given during the case. Also, when they are placed in the supine position the upper airway is prone to obstruction.
The risk of postoperative hypoxia is increased by preoperative hypoxia and by any surgery involving the thorax or abdomen. Extubation should be postponed until the effects of NMBAs are reversed and the patient is fully awake. The patient should be sitting upright to unburden the diaphragm and improve oxygenation and ventilation. The risk of hypoxia extends into the postoperative period for several days and supplemental oxygen may need to be provided.