07: Airway Mechanics Compliance

How do changes in lung compliance increase the work of breathing

It uses more energy to actually expand the lung

When we put in E to stretch the lungs, what happens to it

A lot of it is stored in the lung tissue, and the recoil of the elastic tissue is used to exhale

If you had to waste energy as increased resistance or decreased compliance, which would be less bad

Compliance → at least you get some of that energy back

T/F: lung compliance during breathing is linear and constant

False

How does lung compliance change throughout a breath

The lung is more compliant at the beginning of inhalation than at the end, and so by the end of a breath you need more force to stretch the tissues

How does the body know when to stop using (wasting) energy to fight lower compliance at the end of a breath

Pulmonary and thoracic stretch receptors

What animals use their leg muscles alongside the diaphragm to drive inhalation

Horses

T/F: the compliance curve for inhalation and exhalation is the same

False, the “stretch and contract” energy curves don’t match up (a lot like tendon hysteresis)

Pattern of compliance during inhalation

Starts low → very high compliance → very low compliance at the end of inhalation

Pattern of compliance during exhalation

Starts with very little compliance → takes E to get exhalation started → high compliance, deflates rapidly

Physiological application of the Law of LaPlace

A small, wet, air filled structure takes energy to keep it from collapsing

Abbreviated equation for the Law of LaPlace

P = 2T/R

Explanation of the Law of LaPlace

The amount of energy it takes to prevent structural collapse is directly related to the amount of tension and inversely proportional to the radius of the structure

If the alveoli are small, what does that mean for the energy required to keep them open

Smaller structures have a greater force trying to collapse it, and so it takes more energy to keep them open

If the alveolar radius has to be small for gas exchange, how do we reduce the amount of energy it requires to keep the alveoli open

The surface tension is reduced by surfactant

Chemical structure of surfactant

Phospholipids

What secretes surfactant in the lungs

T2 alveolocytes

How does a normal breath affect the amount of force it takes to open the lungs

At the beginning of a breath, the alveolar radius is very small, and it takes a lot of force to make them larger. As you inhale and the radius increases, the amount of force it takes to open and stretch the lungs decreases

At the end of the breath, the alveoli are very large. Why does it then start taking more energy to inflate the lung

You start fighting the bone in the thoracic wall for expansion

Why doesn’t the fetus have surfactant

It doesn’t need its lungs, so the body doesn’t waste energy manufacturing surfactant

When does surfactant start being manufactured in fetuses

Last 5-10% of gestation

BIG problem with the RT in premature neonates

There is no surfactant → lung compliance is abysmal (pasta al dente)

Medical issues that decrease lung compliance

• Pulmonary fibrosis

• Pulmonary inflammation

Why does fibrosis/inflammation decrease compliance

Makes it harder to stretch the interstitial tissue → stretch receptors detect threshold force much earlier during inhalation

Result of decreased compliance due to fibrosis/inflammation

Shallower breathing (decreased V_T) → increased RR to maintain minute alveolar ventilation

How does a surfactant deficiency decrease lung compliance

Increases surface tension in the alveoli and bronchioles

Cause of primary surfactant deficiency

Lungs aren’t mature yet (premature neonate)

• “like trying to inflate a brisket”

Cause of secondary surfactant deficiency

The patient has normal surfactant production, but there is an inflammatory condition that eats up the surfactant faster than it’s being used

Ways to acquire surfactant

• Synthetic surfactant

• Harvest from a fresh lung by dissolving in chloroform

Summary of the ways to decrease lung compliance

• Occlude lumen with debris/mucus

• Edema in the mucosa

• Pulmonary emphysema

Pulmonary emphysema pathology

Breakdown of the interstitial tissue that means you can’t pull on the tissues and there is no recoil in the tissues anymore

Clinical signs of pulmonary emphysema

Hyperinflated thorax with shallow breathing