2011B14 Describe the changes in respiratory function tests that occur with long term increases in small airways resistance.

Small airways

·   <2mm diameter

·   ≥8th generation

·   <20% total resistance in health due to large total surface area

·   Silent zone: large increase in resistance before detectable

Air flow physiology

·   Flow rate = (P₁ – P₂) / airway resistance

o Inspiration: P_Mouth – P_Alv

o Expiration: P_Alv - P_Mouth

·   Laminar flow in small airways (small radius, low Reynolds number)

o Resistance = (8 x length x viscosity) / (π x radius⁴)
(radius power of 4, most important)

Small airway obstruction

·   e.g. asthma, COPD

·   Obstruction in expiration (>inspiration) due to loss of radial traction

o ↓Radius -> ↑airway resistance

§ Variable resistance -> variable alveolar time constants

o Earlier and worse dynamic airways collapse (DAC)

§ ↑Resistance -> ↑Pressure drop -> distal migration of equal pressure point (P_intrapleural = P_airway)

·   Occurs in forced expiration, +/- in tidal expiration if severe disease

o -> ↑RV, ↑FRC, closing capacity, ↓flow rate

Clinical implications

·   Require higher peak airway pressure

·   Require longer expiratory time

·   Risk of gas trapping, breath stacking, volutrauma and barotrauma

Test summary

·   Most sensitive in early disease: ↑CV

·   Most specific to small airways: ↓FEF25-75%

·   Most quantitative of obstruction: ↓FEV1 (80-100% mild, 50-80% mod, <50% sev), ↓PEFR

Bedside PEFR

·   ↓ (e.g. 4L/s c.f. 8L/s)

Spirometry

·   ↓FER

·   ↓FEV1

·   ↔↓FVC (less affected)

Static lung volume testing

·   ↑TLC, ↑FRC, ↑RV

·   Helium dilution: underestimates FRC, TLC due to gas trapping (+/- bullae)

·   Whole body plethysmography: accurate

Forced flow-volume curve

·   ↔↓PIFR (affected if severe)

·   ↓PEFR, scooped-out expiratory curve, ↓FEF25-75

·   ↓FVC (less affected)

·   (↑TLC, ↑RV – unable to measure here)

Tidal volume-pressure curve

·   ↑Pressure work

·   ± Active tidal expiration: if
Expiratory work > elastic inspiratory work
i.e. area ABCG’A > area ABCDEA

Single breath nitrogen washout

·   ↑Time constant variation -> ↑slope phase 3

o Poorly ventilated alveoli empty late

·   ↑DAC -> ↑Closing volume (10% in health)

Arterial blood gas

·   If chronic hypoventilation:

o ↑PaCO₂ (PACO₂ ∝ VCO₂/V_A)

o ↔↓pH (0.03 per mmHg ↑PaCO₂)

o Renal compensation: ↑HCO₃^- = 24 + 0.4(PaCO₂ – 40)

·   If closing capacity > FRC:

o Shunt -> ↓PaO₂

Exercise testing

·   ↓Max voluntary minute ventilation -> ↓DO₂ and ↑respiratory work -> ↓ peak aerobic power