Morbid obesity: Hypoxemia physiology


The predominant cause of worsened oxygenation in obesity is a reduction in functional residual capacity (FRC) and increased 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 by employing CPAP during induction

Note that 100% FiO2 may not be the best management decision – it promotes atelectasis and if the right-to-left shunt is larger than 30%, will not improve PaO2

Obesity and Hypoxemia

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


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