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Closing Volume (CV) and Residual Volume (RV) — Full Detail (15-Mark Answer)

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RESIDUAL VOLUME (RV)

Definition

Why RV Exists — Physiological Basis

1. Airway closure and gas trapping: At low lung volumes, pleural pressure at the lung bases exceeds airway pressure, causing terminal airways to close. Gas trapped distal to these closed airways cannot be expelled.
2. Chest wall elastic recoil: The chest wall's outward recoil limits further inward movement beyond a certain point, even with maximal expiratory muscle effort. The balance between expiratory muscle force and outward chest wall recoil sets the RV.

Normal Values and Relationships

Measurement of RV — Cannot Be Measured by Spirometry Alone

• Patient sits in an airtight box and pants against a closed shutter at FRC.
• Based on Boyle's Law: P₁ × V₁ = P₂ × V₂
• Pressure changes at the mouth (= alveolar pressure) vs. box pressure changes allow calculation of thoracic gas volume.
• RV = FRC − ERV (ERV measured separately by spirometry)
• Advantage: Most accurate; measures ALL intrathoracic gas including trapped gas
• Disadvantage: May overestimate if panting maneuver is too rapid (compresses abdominal gas)

• Patient breathes from a closed circuit containing a known volume (V₁) and concentration (C₁) of helium from FRC.
• Helium equilibrates with alveolar gas: C₁ × V₁ = C₂ × (V₁ + FRC)
• FRC = [(C₁ × V₁)/C₂] − V₁; then RV = FRC − ERV
• Disadvantage: Underestimates RV in patients with obstructive lung disease where trapped gas behind closed airways does not communicate with the breathing circuit (bullae, severe COPD)

• Patient breathes 100% O₂ from FRC until exhaled N₂ reaches a plateau (~<1.5%).
• Knowing initial N₂ concentration (~75%) and total N₂ washed out, FRC is calculated.
• RV = FRC − ERV
• Same underestimation limitation as helium dilution in obstruction

Clinical Significance of RV

• RV/TLC ratio > 35% suggests air trapping (obstructive pattern)
• An elevated RV (>150% predicted) confirms hyperinflation
• In restrictive disease, RV may be reduced along with TLC

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CLOSING VOLUME (CV)

Definition

• Closing Capacity (CC) = CV + RV
• Normal CV in healthy young adults: ~10% of VC
• CC is therefore a more commonly reported value because it is independent of airway closure position relative to RV

Physiological Basis — Why Airways Close

• At high lung volumes (near TLC): Basal alveoli are on the lower, steep part of their pressure-volume curve → more compliant → better ventilated
• As lung volume falls toward RV: Pleural pressure at the lung bases exceeds airway pressure → terminal bronchioles (non-cartilaginous airways, <2 mm) in the dependent zones collapse and close
• Gas trapped beyond these closed airways cannot be expelled

Measurement: Single-Breath Nitrogen Washout Test

1. Subject expires fully to RV
2. Inhales a single breath of 100% oxygen to TLC
3. Exhales slowly to RV while expired N₂ concentration and volume are continuously recorded

• At the start of the O₂ inspiration (from RV), basal airways are already closed. The O₂ breath thus enters preferentially to the upper (apical) zones, diluting N₂ there more than at the bases.
• Net result: After the full O₂ inspiration and mixing, basal alveoli have higher N₂ concentration than apical alveoli (because they received proportionally less O₂).
• During expiration, as lung volume approaches CV, basal airways close again. Only apical units (with higher N₂) continue to empty — hence the abrupt rise in expired N₂ concentration = Phase IV.
• The lung volume at which Phase IV begins = CV (volume above RV); adding RV gives CC.

Normal Values and Age Changes

When CV > FRC: Clinical Consequence

• If CC rises above FRC, airways close during normal tidal breathing (not just at RV)
• The closed lung units are still perfused but not ventilated → V/Q mismatch → arterial hypoxemia
• This is the principal cause of the age-related fall in PaO₂
• Supine position: FRC falls by ~0.5–1 L; CC does not change → threshold crossed earlier (explains why supine elderly patients and post-operative patients desaturate more easily)

Factors Increasing CV

CV as a Test of Early Small Airway Disease

• CV/VC > 20% (age-adjusted): abnormal
• CC/TLC > 30%: abnormal
• Also measured alongside Phase III slope (ΔN₂): normal ΔN₂ <1.5% per 750–1250 mL exhaled

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RELATIONSHIP BETWEEN RV, CV, FRC, AND CC — Summary Diagram

TLC ──────────────────
      ↑
      IRV
      ↑
EIV (end-inspiration of tidal breath)
      ↑
      TV (tidal volume)
FRC ─────────────────── ← CV threshold crosses FRC in aging/disease
      ↑
      ERV
      ↑
CC  ─────────────────── ← (CV + RV): onset of airway closure
      ↑
      CV  (above RV)
RV  ─────────────────── ← maximal expiration; airways already closed

• CC = CV + RV
• When CC > FRC → airway closure during tidal breathing → hypoxemia
• Normal: CC < FRC (airways remain open throughout tidal breathing)
• In normal young adults: CV ≈ 10% VC; CC ≈ RV + 0.5 L

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SUMMARY TABLE FOR EXAM

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