FRC: Why Supine Patients Desaturate

Functional residual capacity = the volume of air left in the lungs at the end of normal expiration. Average adult: ~2.5-3.5 L (varies by height + body habitus). It's the gas you have available for oxygen exchange between breaths — and during apnea (induction, intubation, prolonged laryngoscopy), it's the only oxygen reservoir keeping you out of trouble.

In the supine patient, the abdominal contents push the diaphragm cephalad. The chest wall reaches its end-expiratory position with less lung volume left below it. Net effect: FRC drops 0.7-1.2 L just from rolling onto the back. This brings FRC closer to closing capacity (the lung volume below which small airways collapse). When FRC < closing capacity, basal lung units get atelectatic + intrapulmonary shunt rises → A-a gradient widens → less efficient O₂ exchange.

BMI 30+ patients have an even larger drop in FRC supine (1.5-2 L+). FRC routinely sits BELOW closing capacity in obese supine patients — meaning small airways are closed at the start of every inspiration. Apneic oxygenation reserve drops from a healthy ~6 minutes to <2 minutes. Pre-oxygenation that achieves end-tidal O₂ ≥85-90% is mandatory. Ramp position (ear-to-sternal-notch) restores ~30% of the FRC loss. CPAP 10 cmH₂O during pre-oxygenation prevents end-expiratory collapse + extends apnea time meaningfully.

Induction adds another ~15-20% drop in FRC (loss of inspiratory muscle tone, diaphragmatic shift, paralysis). Atelectasis appears within minutes of induction in 90% of patients on chest CT — mostly basal, dependent. The fix during the case: PEEP (5-8 cmH₂O baseline; up to 10-15 cmH₂O for obesity), recruitment maneuvers (40 cmH₂O sustained for 7-10 sec), avoid 100% FiO₂ for prolonged periods (denitrogenation atelectasis), reverse Trendelenburg in obese patients when surgical position allows.

FRC explains: (1) apnea desaturation curves — why a healthy patient drops below 90% in 6 min and an obese patient in 90 sec. (2) Why ramped + CPAP pre-O₂ extend the safe-apnea window. (3) Why post-op atelectasis is universal + is the proximate cause of post-op hypoxemia + pneumonia. (4) Why incentive spirometry + early mobilization + PEEP-equivalent CPAP work — they all restore FRC. (5) Why pregnant patients desaturate fast (gravid uterus pushes diaphragm cephalad even more than abdominal fat).