Airway resistance full details for md exam answer 10 marks

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Airway Resistance - Complete Answer (10 Marks)

1. Definition

Airway resistance (Raw) is defined as the ratio of the driving pressure for airflow to the rate of airflow along the airways:

Raw = ΔP / V̇

Where:

ΔP = Alveolar pressure (PA) - Mouth/airway opening pressure (Pmo)
V̇ = Rate of airflow (L/s)
Units: cmH₂O/L/s (normal = 0.5 to 2.5 cmH₂O/L/s at FRC)

(Fishman's Pulmonary Diseases & Disorders)

2. Physical Basis - Poiseuille's Law

For laminar airflow through a tube, resistance is governed by Poiseuille's law:

R = 8ηl / πr⁴

Where:

η = viscosity of gas
l = length of the airway
r = radius of the airway

The fourth-power relationship with radius is the most critical point: if the airway radius is halved, resistance increases 16-fold (2⁴ = 16). Conversely, doubling the radius reduces resistance to 1/16th. This explains why even modest bronchoconstriction has a dramatic physiological impact.

(Costanzo Physiology 7th Ed)

3. Distribution of Airway Resistance

This is a frequently tested, counterintuitive concept:

Segment	Contribution to Total Raw
Upper airways (nose, pharynx, larynx, trachea)	~50% (nasal breathing); 20-30% (mouth breathing)
Medium-sized bronchi (lobar, segmental, subsegmental up to ~7th generation)	~40-50%
Small peripheral airways (<2 mm diameter)	Only 10-20%

Key point: Despite being the narrowest airways, the small peripheral airways contribute the LEAST total resistance because they are arranged in parallel - the total cross-sectional area at successive generations increases enormously (from ~2.5 cm² at the trachea to >11,800 cm² at the alveolar level). In parallel circuits, total resistance is less than any individual resistance. This is why small airway disease (e.g., early COPD, peripheral bronchiolitis) is termed the "silent zone" - significant small airway disease can exist before Raw is measurably elevated.

The medium-sized bronchi are the site of highest airway resistance in the normal lung.

(Fishman's Pulmonary Diseases & Disorders; Costanzo Physiology)

4. Factors Affecting Airway Resistance

A. Radius/Caliber of Airways (Most Important)

Due to the r⁴ relationship, any change in airway diameter has a dramatic effect
Bronchoconstriction (asthma, parasympathetic stimulation, irritants) → ↑ Raw
Bronchodilation (sympathetic stimulation, β₂ agonists) → ↓ Raw

B. Lung Volume

↑ Lung volume → ↓ Raw: At high lung volumes, elastic recoil forces increase, pulling on and expanding adjacent airways (radial traction / mechanical tethering). Airways are pulled open.
↓ Lung volume → ↑ Raw: At low volumes, radial traction is lost; small airways may collapse entirely (airway closure).
This is why patients with asthma breathe at higher lung volumes - this is a compensatory mechanism to reduce Raw.
In emphysema, destruction of alveolar walls reduces elastic recoil; transmural airway pressure falls at any given volume, so airways are narrow and Raw is increased even without intrinsic airway disease.
Raw is inversely proportional to lung volume: Raw ∝ 1/Volume.

(Fishman's; Costanzo)

C. Autonomic Nervous System

Parasympathetic (cholinergic/vagal): ACh acts on M₃ muscarinic receptors on bronchial smooth muscle → bronchoconstriction → ↑ Raw. Blocked by atropine/ipratropium.
Sympathetic (adrenergic): Catecholamines act on β₂ receptors → smooth muscle relaxation → bronchodilation → ↓ Raw. This is the basis for β₂ agonists (salbutamol, terbutaline) in asthma.
Non-cholinergic parasympathetic pathways may mediate bronchodilation via VIP and nitric oxide.

D. Viscosity and Density of Inspired Gas

From Poiseuille's law: ↑ gas viscosity → ↑ Raw
Deep-sea divers breathing compressed air (↑ gas density) experience ↑ Raw
Breathing helium-oxygen (Heliox) mixtures reduces gas density → converts turbulent to laminar flow → ↓ Raw. Used therapeutically in upper airway obstruction.
Breathing high concentrations of xenon increases Raw compared to other inhalational agents.

E. Pathological Causes of ↑ Raw

Mucosal edema (allergic rhinitis, bronchitis)
Excessive mucus secretion (chronic bronchitis)
Smooth muscle hypertrophy/hyperplasia
Loss of elastic recoil (emphysema)
Airway collapse (dynamic compression during forced expiration)

5. Dynamic Airway Compression During Forced Expiration

During forced expiration, as intrathoracic pressure rises, a point along the airway is reached where the external pressure (pleural pressure) equals the internal airway pressure - this is the Equal Pressure Point (EPP). Downstream from this point, airway pressure falls below surrounding pleural pressure, causing dynamic compression.

In normal lungs, EPP is located in medium-to-large airways where cartilaginous support prevents collapse.
In emphysema (loss of elastic recoil) or chronic bronchitis, EPP shifts peripherally into unsupported small airways, causing flow limitation - maximal expiratory flow cannot be increased further no matter how hard the patient tries.
This is reflected in reduced FEV₁ and FEV₁/FVC ratio (obstructive pattern on spirometry).

6. Measurement of Airway Resistance

Body Plethysmography (Gold Standard)

Patient sits in a sealed airtight box (body plethysmograph) and pants
Airflow (V̇) measured by pneumotachograph
Alveolar pressure (PA) estimated from box pressure changes (Boyle's law)
Raw = PA / V̇
Introduced by DuBois et al. (1956)

Other Methods

Forced oscillation technique (FOT/IOS) - measures resistance across all frequencies, can detect peripheral airway disease
Interrupter technique (Rint)
Derived from spirometry: elevated Raw manifests as ↓ FEV₁, ↓ PEF, ↓ FEF 25-75%

7. Airway Conductance (Gaw)

The reciprocal of resistance:

Gaw = 1/Raw = V̇/ΔP

Clinically more useful because Gaw is linearly related to lung volume. Therefore, specific conductance (SGaw = Gaw/lung volume) corrects for lung volume and allows valid comparisons between individuals and serial measurements.

8. Clinical Correlations

Condition	Effect on Raw	Mechanism
Asthma	↑↑ (reversible)	Bronchospasm, mucosal edema, mucus
COPD/Emphysema	↑	Loss of elastic recoil, dynamic collapse
Chronic bronchitis	↑	Mucus hypersecretion, wall thickening
Anaphylaxis	↑↑	Histamine-mediated bronchoconstriction
β₂ agonist therapy	↓	Bronchodilation
Increasing lung volume	↓	Radial traction, widened airways

Summary Points for Exam

Raw = ΔP/V̇; units cmH₂O/L/s; normal ~0.5-2.5
Governed by Poiseuille's law; r⁴ relationship is dominant
Medium-sized bronchi = site of highest Raw; small airways contribute only 10-20% (parallel arrangement)
Raw ∝ 1/lung volume; radial traction mechanism
Parasympathetics ↑ Raw; Sympathetics (β₂) ↓ Raw
Measured by body plethysmography
Dynamic compression and EPP determine flow-limitation in disease
"Silent zone" concept - small airway disease missed by routine spirometry

Sources: Costanzo Physiology 7th Ed; Fishman's Pulmonary Diseases & Disorders; Ganong's Review of Medical Physiology 26th Ed; Murray & Nadel's Textbook of Respiratory Medicine

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