In an obese patient with sleep apnea, which plethysmographic lung volumes are typically reduced?

• A. RV and ERV
• B. TLC and RV
• C. TLC, VC, and FRC
• D. IC and FEV1

Answer: (C)

Obesity with sleep apnea creates a restrictive pattern because excess chest wall and abdominal fat limit how much the lungs can expand. On body plethysmography, this shows up as reductions in volumes that reflect overall lung inflation. Total lung capacity decreases since the lungs can’t reach their full volume due to chest wall stiffness. Vital capacity also falls because both inspiratory and expiratory reserves are smaller in a restricted system. Functional residual capacity drops as diaphragmatic elevation and reduced chest wall compliance reduce the amount of air left after a normal exhale. So the combination of reduced TLC, VC, and FRC is typical in this scenario. FEV1 is a flow measure, not a lung volume, so it’s not the volume pattern being described here.