Lesson 11

Lesson 11: Functional Residual Capacity (FRC)

• Volume of air in lungs at end-expiration.

• Acts as oxygen reservoir to prevent hypoxemia during apnea.

• At FRC, inward elastic recoil of lungs is balanced by outward elastic recoil of chest wall (static equilibrium).

Factors Affecting FRC

Decreasing FRC:

• General Anesthesia (GA)

• Obesity

• Pregnancy

• Supine Position

• Fluid Overload

Increasing FRC:

• Positive End-Expiratory Pressure (PEEP)

• Advanced Age

• Sigh Breaths

• Prone or Sitting Positions

Measuring FRC

• Indirect Measurement Methods:

  • Nitrogen washout

  • Helium wash-in

  • Body plethysmography

• Cannot be measured via spirometry due to residual volume (RV) presence in FRC.

• Normal FRC value: $35 ext{ mL/kg}$.

FRC Duration During Apnea

• Estimated using:
  $ext{Time until desaturation} = rac{ ext{FRC}}{ ext{VO}_2}$

• Example for FRC = 2300 mL, VO2 = 250 mL/min:

  • Breathing 100% O2: $rac{2300 ext{ mL}}{250 ext{ mL/min}} ext{ = 9.2 minutes}$

  • Breathing 21% O2: $rac{483 ext{ mL}}{250 ext{ mL/min}} ext{ = 1.9 minutes}$.

Clinical Implications of Altered FRC

• Reduced FRC shortens apnea tolerance; critical during airway management.

• Conditions that reduce outward lung expansion or lung compliance increase intrapulmonary shunt (West zone 3).

• Alveolar recruitment maneuvers and PEEP help restore FRC and reduce shunting.

Common Conditions and Their Effects on FRC

• General Anesthesia: Decreases FRC due to diaphragm shift and lung compliance.

• Obesity & Pregnancy: Decrease FRC due to abdominal pressure influence.

• Positioning: Supine, lithotomy, and trendelenburg position reduce FRC; upright position tends to increase it.

• Neuromuscular Blockers & IV Fluids: May decrease FRC by impairing lung volume.

• PEEP: Increases FRC, enhances lung recruitment, and prevents atelectasis.