Disclaimer: This article is intended solely for informational and educational purposes only. It does not constitute medical advice.
Mechanical ventilation in patients with obesity requires special consideration because excess adipose tissue alters the mechanics of breathing. Increased abdominal mass raises pressure below the diaphragm and pushes it upward into the thoracic cavity, while adipose tissue surrounding the chest increases the chest wall’s resistance to expansion (1). Together, these effects reduce functional residual capacity (FRC), the volume of air remaining in the lungs after a normal expiration. FRC helps maintain the patency of small airways and alveoli between breaths. When FRC decreases, dependent airways may close during expiration, leaving portions of the lung poorly ventilated. During general anesthesia, loss of respiratory muscle tone allows the diaphragm to move further toward the chest, producing an additional reduction in lung volume. The supine position also increases pressure from the abdominal contents against the diaphragm and can further promote airway closure (2).
Persistent airway closure can lead to atelectasis—the collapse of portions of the lung. Blood may continue to flow through these collapsed regions even though little or no fresh gas reaches the alveoli. This produces ventilation-perfusion mismatch, in which pulmonary blood flow and ventilation are poorly matched, and can reduce arterial oxygenation. The reduction in FRC also decreases the oxygen reserve within the lungs, causing patients with obesity to develop hypoxemia more rapidly during periods without effective ventilation (1).
Tidal volume (VT), the volume of gas delivered to the lungs with each mechanical breath, is an important consideration during intraoperative ventilation. In patients with obesity, VT should be calculated according to predicted or ideal body weight rather than actual body weight. Lung size is primarily related to height and sex and does not increase in proportion to excess body weight. Therefore, using actual body weight to calculate VT may deliver volumes that are too large for the patient's lungs, resulting in excessive alveolar distention and mechanical stress. A VT of approximately 6–8 mL/kg predicted body weight is generally recommended (1).
Positive end-expiratory pressure (PEEP) is pressure maintained within the airways at the end of expiration during mechanical ventilation. Without sufficient pressure, alveoli and small airways that are already susceptible to closure may collapse as the patient exhales. PEEP helps maintain these structures in an open state and can therefore improve lung volume and oxygenation in patients with obesity. However, excessive PEEP increases pressure within the chest and may reduce venous return to the heart, resulting in hypotension. In the PROBESE randomized clinical trial, PEEP of 12 cm H2O combined with recruitment maneuvers did not significantly reduce postoperative pulmonary complications compared with PEEP of 4 cm H2O without recruitment maneuvers. The higher-PEEP strategy was also associated with more hypotension and bradycardia (3).
A recruitment maneuver is a brief, controlled increase in airway pressure intended to expand areas of the lung that have collapsed during anesthesia. This may be performed by temporarily increasing inspiratory pressure or PEEP, allowing sufficient pressure to reach collapsed alveoli and reopen them. After recruitment, PEEP can be maintained to reduce subsequent alveolar collapse (4).
The respiratory effects of obesity become more pronounced during general anesthesia as lung volume decreases and the risk of airway closure and atelectasis increases. Mechanical ventilation for patients with obesity should account for these changes while avoiding excessive tidal volumes and airway pressures. PEEP and recruitment maneuvers can help maintain lung expansion and oxygenation, but their use should be guided by the patient’s respiratory and hemodynamic response.