Methods to Measure Functional Residual Capacity (FRC)

Three Principal Methods

1. Closed-Circuit Helium Dilution Method

• A spirometer of known volume (V₀) is filled with a gas mixture containing a known concentration of helium (F₀He, ~10%).
• The patient breathes room air. At the end of a normal quiet expiration (i.e., at FRC), the patient is switched to the spirometer circuit.
• The patient rebreathes from the closed circuit while:
  • CO₂ is absorbed by soda lime
  • O₂ is added to compensate for metabolic consumption
• Helium, confined initially in the spirometer, mixes with the gas in the lungs.
• Helium concentration falls progressively until equilibration (endpoint: change <0.02% over 30 s; normally ~7 min).

F₀He × V₀ = FFHe × V_F

FRC = [(F₀He / FFHe) – 1] × V₀

• Underestimates FRC in patients with obstructive lung disease (poorly ventilated or trapped gas units do not equilibrate with the circuit).
• Equilibration time is prolonged in severe COPD.

2. Open-Circuit Nitrogen Washout Method

• Starting at end-normal expiration (FRC), the patient breathes 100% oxygen.
• All expired gas is collected until the expired N₂ concentration falls to zero (typically 7 min).
• Total volume of expired gas (V_E) and mean N₂ concentration of expired gas (F_EN₂) are measured.

F₀N₂ × V₀ = F_EN₂ × V_E

FRC = (F_EN₂ × V_E) / F₀N₂

• Underestimates lung volume in patients with airway obstruction or poorly communicating air spaces (trapped gas not washed out).
• Open-circuit method; technically requires careful gas collection.

3. Body Plethysmography (Whole-Body Plethysmograph)

• The patient sits inside a sealed, airtight box (plethysmograph) with a mouthpiece, noseclip, and pneumotachograph.
• The patient breathes quietly. At end-expiration, a shutter is closed at the mouthpiece.
• The patient is instructed to pant gently against the closed shutter.
• Panting causes alveolar pressure changes (measured as mouth pressure, Pm) and reciprocal volume changes in the plethysmograph box (measured as box pressure, Pbx).
  • Inspiratory effort → mouth pressure falls, lung volume expands → box pressure rises
  • Expiratory effort → mouth pressure rises, lung volume decreases → box pressure falls
• The slope of the Pm vs. ΔV_box loop is used with Boyle's Law to calculate Thoracic Gas Volume (TGV), which at end-expiration equals FRC.

P_alv × FRC = (P_alv + ΔP) × (FRC – ΔV)

FRC (TGV) = P_alv × (ΔV / ΔP)

• Measures all intrathoracic gas, including trapped or non-communicating gas (bullae, closed airways) — therefore gives the most accurate absolute lung volume.
• Preferred method, especially in obstructive disease.

• Can overestimate FRC if panting is too rapid (abdominal gas compression/decompression contributes).
• Requires patient cooperation and enclosed space (claustrophobia may be an issue).
• A plethysmographic TLC >150% of predicted should raise suspicion of measurement error.

Comparison Table

Additional Methods (Briefly)

• Imaging methods (chest X-ray, CT scan): FRC can be estimated radiologically by measuring lung volumes from cross-sectional images. CT-based FRC measurement is increasingly used in research and ICU settings.
• Sulfur hexafluoride (SF₆) or other inert gases: Used by the same dilution principle as helium; SF₆ is useful in infants and special situations.

Body Plethysmography — Simply Explained

The Basic Idea

The Setup

• Patient sits inside a sealed, airtight glass/perspex box (the plethysmograph)
• A mouthpiece is held between the lips, with a noseclip on
• There's a shutter — a valve that can be closed to block airflow

Step by Step — What Happens

• No air moves in or out of the lungs
• But the chest wall still expands and contracts
• This compresses and decompresses the gas already trapped in the lungs

The Key Logic (Boyle's Law)

Boyle's Law: If you squeeze a fixed amount of gas → pressure goes up, volume goes down. P₁V₁ = P₂V₂

• Alveolar pressure falls (mouth pressure falls)
• Lung volume tries to expand → chest pushes outward
• This expansion compresses the air in the sealed box → box pressure rises

P_alv × FRC = (P_alv + ΔP) × (FRC − ΔV)

Simple Analogy

• If the balloon slightly inflates → the box gets a bit more pressurized
• If you know exactly how much the box pressure changed, and you know the box size, you can calculate how much the balloon grew
• That "growth" tells you the starting size of the balloon

Why is it Better than Helium/Nitrogen Methods?

One-Line Summary

The patient sits in a sealed box, pants against a closed valve, and the machine uses Boyle's Law (pressure × volume = constant) to calculate exactly how much air is in the lungs at rest — that's the FRC.

Step 3 — "Panting Against a Closed Shutter"

First, what is the shutter?

Now imagine this:

What happens when you try to BREATHE IN (against the closed shutter)?

What happens when you try to BREATHE OUT (against the closed shutter)?

So in summary — two things happen together:

Try to inhale  →  Lung pressure ↓  AND  Box pressure ↑
Try to exhale  →  Lung pressure ↑  AND  Box pressure ↓

Why panting and not normal breathing?

One-line version of Step 3:

You try to breathe in and out but nothing moves — your chest just slightly squeezes and unsqueezes the trapped air, and those tiny pressure changes are measured to calculate your lung volume.

FRC Changes Across Age Groups

FRC = the point where lung's inward recoil (pulls inward) = chest wall's outward recoil (pushes outward)

🍼 Neonates / Newborns

FRC is LOW (but similar per kg to adults — with important functional problems)

• Minute ventilation/FRC ratio is 2–3× higher than adults
• This means oxygen reserve in the FRC is very small
• If a neonate becomes apnoeic → oxygen levels drop very rapidly (much faster than in an adult)

• Rapid respiratory rate (30–50 breaths/min)
• Laryngeal braking — partially closes the larynx during expiration to slow airflow and maintain lung volume (like breathing out through pursed lips)
• Active contraction of inspiratory muscles during expiration

👦 Children (growing)

FRC progressively increases as the child grows

• Alveoli multiply until about 8 years of age
• Chest wall becomes more rigid (cartilage → bone)
• Elastic recoil of lung increases
• Closing capacity decreases relative to FRC — so airways no longer close during tidal breathing (by ~6–8 years, similar to adults)
• By adulthood, FRC stabilizes at ~2.5 L (or ~30 mL/kg)

👴 Elderly

FRC is INCREASED

• V/Q mismatch increases → baseline PaO₂ falls with age
• Diffusion capacity (DLCO) decreases
• Diminished response to hypoxia and hypercapnia
• More prone to respiratory failure under stress

Summary Table

• Neonate → floppy chest + no surfactant → FRC too low, collapses easily
• Elderly → loose lungs + stiff walls → FRC too high, air gets trapped