Respiratory Mechanics, Compliance, Airflow & Lung Disease

What is respiratory mechanics?

How pressure, volume, and forces work together to move air.

What direction do the lungs and chest wall naturally recoil?

Lungs recoil inward; the chest wall springs outward.

What happens during inspiration and expiration?

Inspiration expands the lungs; expiration allows the lungs to recoil and push air out.

What is compliance, and what is its formula?

How easily something stretches. C = change in volume ÷ change in pressure.

What do high and low compliance mean?

High = easy to inflate. Low = stiff and difficult to inflate.

What causes increased and decreased lung compliance?

Increased: emphysema and aging. Decreased: fibrosis, atelectasis, edema, and ARDS.

What is static compliance?

Compliance measured with no airflow; it reflects the elastic properties of the lungs and chest wall.

What is dynamic compliance?

Compliance measured during airflow; it reflects elastic forces and airway resistance.

How does dynamic compliance compare with static compliance?

Dynamic compliance is usually equal to or lower than static compliance.

What is chest wall compliance, and what affects it?

How easily the chest expands; affected by bone flexibility, muscle strength, and tissue elasticity.

What is elastance, and what is its formula?

Resistance to stretching and the tendency to recoil. E = change in pressure ÷ change in volume.

What do high and low elastance mean?

High = stiff, strong resistance to stretching. Low = less elastic recoil.

How are compliance and elastance related?

They are opposites: high compliance generally means low elastance.

What is Hooke's law?

Within an elastic object's limits, more force causes more stretching.

What is a pressure gradient?

A difference in pressure between two points that causes airflow.

In which direction does air flow?

From high pressure to low pressure.

What happens to alveolar pressure during inspiration, expiration, and when airflow stops?

Inspiration: below atmospheric. Expiration: above atmospheric. No airflow: equal to atmospheric.

What is transrespiratory pressure?

The pressure difference between the alveoli and body surface; it reflects the whole respiratory system.

What is transpulmonary pressure?

The pressure difference between alveolar and pleural pressure that helps keep the lungs open.

What is transthoracic pressure?

The pressure difference between the pleural space and body surface; it reflects the chest wall.

What is airway resistance, and what factors affect it?

Opposition to airflow; affected by airway radius, length, flow velocity, flow pattern, and gas properties.

How does airway radius affect resistance?

Smaller radius greatly increases resistance. Halving the radius increases resistance 16-fold during laminar flow.

How does flow velocity affect resistance?

Higher velocity can increase turbulence and resistance.

What are laminar, turbulent, and transitional flow?

Laminar = smooth. Turbulent = chaotic. Transitional = a mixture of both.

What is Poiseuille's law?

It explains how pressure, radius, airway length, and gas viscosity affect laminar airflow.

How do pressure, radius, length, and viscosity affect airflow?

Increased pressure and radius increase flow; increased length and viscosity decrease flow.

What are gas density and viscosity?

Density = mass per volume. Viscosity = resistance to flowing.

What is heliox, and why can it help with narrowed airways?

Helium and oxygen; helium's low density can reduce turbulent resistance and improve airflow.

What is a time constant, and what is its formula?

How quickly a lung unit fills or empties. Time constant = resistance × compliance (τ = R × C).

What percentages are completed at one through five time constants?

1 = 63%, 2 = 86%, 3 = 95%, 4 = 98%, 5 = 99%.

What causes a long or short time constant?

Long: increased resistance or compliance. Short: decreased resistance or compliance.

What are examples of restrictive lung diseases?

ARDS and pulmonary fibrosis.

What happens to compliance in restrictive lung diseases?

Compliance decreases because the lungs are stiff.

What happens to resistance and time constants in restrictive diseases?

Resistance is often normal or decreased; time constants are often shorter if resistance is normal.

What breathing pattern is associated with restrictive diseases?

Rapid, shallow breathing.

What are examples of obstructive lung diseases?

Asthma and COPD.

What happens to airway resistance in obstructive diseases?

Resistance increases, making airflow more difficult, especially during expiration.

What happens to compliance in emphysema?

Compliance increases because the lungs lose elastic recoil.

What happens to the time constant in obstructive diseases?

It often increases because airway resistance increases.

What is the main difference between restrictive and obstructive diseases?

Restrictive = difficulty expanding the lungs. Obstructive = difficulty moving air, especially out of the lungs.