Morbid obesity: Hypoxemia physiology

The predominant cause of worsened oxygenation in obesity is a reduction in functional residual capacity (FRC) andincreased airway closure which essentially produces a right-to-left shunt. High FiO₂ can contribute to this pathology by promoting atelectasis and further worsening shunting.BMI is correlated to the degree of both atelectasis and shunting. Anesthesia providers should endeavor to prevent this right-to-left shunting byemploying CPAP during induction

Note that 100% FiO₂maynot be the best management decision – it promotes atelectasis and if the right-to-left shunt is larger than 30%, willnotimprove PaO₂

Obesity and Hypoxemia

• Cause:right-to-left shuntcaused by airway closure (FRC reduced) and atelectasis
• Management: CPAP may prevent R-L shunting, consider avoiding 100% O₂