Which of the following changes occurs in the functional residual capacity (FRC) immediately after induction of general anaesthesia in a healthy adult?

• A FRC increases by approximately 20% due to diaphragmatic relaxation upward
• B FRC remains unchanged as long as positive-pressure ventilation is maintained
• C FRC increases because chest wall compliance rises with muscle relaxation
• D FRC decreases by approximately 15–20% due to cephalad diaphragm shift and loss of tonic muscle activity ✓

Explanation

General anaesthesia causes a reduction in FRC of approximately 15–20% in healthy adults, primarily due to cephalad displacement of the diaphragm from loss of end-expiratory muscle tone and change in chest wall configuration. This reduction predisposes to atelectasis, intrapulmonary shunting, and hypoxaemia. The effect is exacerbated in obese patients and in the supine position. Positive-pressure ventilation does not restore FRC to awake levels because the underlying thoracic geometry change persists.

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.