Unit II Part II – Airway Resistance, Compliance & Flow Dynamics

Vocabulary flashcards covering airway resistance, lung compliance, and flow dynamics to aid exam preparation.

Airway Resistance (RAW)

The opposition to ventilation produced by gas flow through the airways, expressed as the change in pressure divided by the change in flow (ΔP / V̇).

Upper-Airway Resistance

Approximately 80 % of total RAW occurring in the nose, mouth, trachea, and other large airways where flow is mostly turbulent.

Lung Compliance

A measure of the lung’s distensibility; the change in lung volume per unit change in transpulmonary pressure (ΔV/ΔP).

Elasticity

The property of lung tissue that allows it to stretch during inspiration.

Lung Recoil

The lung’s tendency to spring back to its original size during exhalation.

Increased Lung Compliance

Abnormally easy lung expansion (e.g., COPD) that requires little pressure but often leads to hyperinflation.

Decreased Lung Compliance

Stiffer lungs that require greater pressure to inflate, seen in conditions like pulmonary fibrosis and interstitial lung disease.

Hyperinflation

Excessive lung volume associated with obstructive disease; characterized by ↑TLC and ↑RV with normal or reduced FEV1/FVC.

Obstructive Lung Disease

A disorder (e.g., COPD, asthma) marked by airflow limitation, often accompanied by increased compliance and hyperinflation.

Pulmonary Fibrosis

Lung scarring that reduces compliance and lung volumes.

Interstitial Lung Disease

A group of disorders causing inflammation/scarring of the lung interstitium, leading to decreased compliance.

Flow Resistance

Energy loss (pressure drop) that occurs as a fluid or gas moves through a tube due to frictional forces.

Bulk Flow

The macroscopic movement of gas or liquid driven by a pressure gradient.

Laminar Flow

Streamlined flow in concentric layers at low velocities; pressure and flow are linearly related (P ∝ V̇).

Turbulent Flow

Chaotic, disorganized flow occurring at high velocities, large diameters, or high densities; pressure rises with the square of flow (P ∝ V̇²).

Transitional (Tracheobronchial) Flow

A mixture of laminar and turbulent patterns typically found in medium-sized airways; total driving pressure equals laminar plus turbulent components.

Viscosity

The internal friction of a fluid; a key determinant of pressure requirements under laminar conditions.

Driving Pressure

The pressure difference that causes gas to move through the airways; increases with higher flow, viscosity, or tube length.

Poiseuille’s Law

For laminar flow, ΔP = 8ηL V̇ / πr⁴; pressure rises with viscosity, tube length, and flow, and increases dramatically as radius decreases.

Reynolds Number (Re)

A dimensionless value predicting flow type:

Law of Continuity

With constant flow, velocity varies inversely with cross-sectional area; as airway diameter narrows, velocity rises.

Heliox

A low-density helium-oxygen mixture used to lower turbulent resistance in obstructed upper airways, reducing work of breathing.

Work of Breathing

The energy expended to inhale and exhale; rises sharply with increased RAW or decreased compliance.

Artificial Airway Resistance

Extra RAW imposed by narrow endotracheal or tracheostomy tubes; halving radius increases resistance 16-fold (Poiseuille).

Terminal Bronchioles

The smallest conducting airways; RAW increases here at low lung volumes due to diameter reduction.

Clinical Pulmonary Infection Score (CPIS)

A clinical scale using temperature, WBCs, PaO₂/FiO₂, secretions, radiograph, and culture to assess pneumonia severity.

Cross-Sectional Area

The total airway sectional size; increasing area lowers velocity and turbulence for a given flow.

Density (Effect on Flow)

Higher gas density promotes turbulent flow, raising RAW; lowering density (e.g., heliox) reduces turbulence.

Upper-Airway Obstruction

Partial blockage of pharynx, larynx, or trachea that greatly increases turbulent resistance and energy expenditure.

Pneumotachometer

Pulmonary lab device that measures airflow to calculate RAW.

Plethysmograph

Body box instrument measuring lung volumes and airway resistance by detecting pressure changes during breathing maneuvers.

Mini-Clini

A bedside tool referenced for assessing RAW or delivering respiratory therapy (as per course notes).

Total Lung Capacity (TLC)

The maximum volume of air the lungs can hold after a maximal inhalation.

Residual Volume (RV)

Air remaining in the lungs after maximal exhalation; often increased in obstructive disease.

Forced Expiratory Volume in 1 Second (FEV₁)

The volume exhaled in the first second of a forced breath; reduced in airway obstruction.

Forced Vital Capacity (FVC)

Total exhaled volume during a forced maneuver; compared with FEV₁ to diagnose obstruction.