06: Airway Mechanics Resistance

Default status of the airway radius

As wide and relaxed as can be to ensure low resistance and low energy

At a resting state, can you relax/dilate the lower airways

Not to any appreciable degree

How can administering a bronchodilator be a useful diagnostic test

If you administer it and breathing gets easier, then it means that those airways were constricted, and something isn’t normal

What parts of the airway are NOT normally relaxed, and can be actively dilated

• Nares

• Pharynx

• Larynx

Conditions where the upper airway may be actively dilated

• Something is wrong

• Increased exercise (increased minute ventilation need)

When can you evaluate dilation issues in the upper airway

When an animal is exercising

How is ventilation (breathing) driven

By generating and releasing negative thoracic/pleural pressure in cycles

Pressure in the thorax, alveoli, intrathoracic RT, and extrathoracic RT at the end of exhalation

• Thorax: small negative

• Alveoli: 0

• Intra: 0

• Extra: 0

Negative all the way through means no air movement, slight negative to keep the elastic components of the RT dilated

Pressure in the thorax, alveoli, intrathoracic RT, and extrathoracic RT during inhalation

• Thorax: slightly larger negative

• Alveoli: medium negative

• Intra: small negative

• Extra: 0

Negative pressure gradient from out (0) → in (-0.7)

Pressure in the thorax, alveoli, intrathoracic RT, and extrathoracic RT during normal exhalation

• Thorax: small negative

• Alveoli: medium positive

• Intra: small positive

• Extra: 0

Muscles relax and generate a positive pressure gradient from in (0.2) → out (0) so the air moves out but the slight negative in the thorax hold the RT open

Pressure in the thorax, alveoli, intrathoracic RT, and extrathoracic RT during forced exhalation

• Thorax: positive

• Alveoli: large positive

• Intra: medium positive

• Extra: 0

Muscles force a greater positive gradient by making the thoracic pressure positive while generating a positive gradient from in (8.7) → out (0)

What happens if you forcefully exhale too hard

You can collapse parts of the RT by making the thoracic positive pressure greater than the positive pressure in the RT → reduced exhalation volume despite breathing harder

What determines if the RT collapses when you exhale to hard

Where the pressure pushes in and whether that part of the RT has cartilage or not

What test can be done in humans (and has been done in humans) to diagnose asthma by looking at lower airway mechanics

FEV1

How can dogs increase minute ventilation with a pretty low E input

The are equipped with a better capacity to increase minute ventilation by taking lots of shallow breaths

Why does any RT issue in horses become clinically significant

They increase their work of breathing SO fast (basically a vertical curve) and they really can’t increase minute ventilation with any efficiency

What limits the increase in minute ventilation in humans

Our airways collapse pretty quickly

What is the primary cause of increased resistance in the lower RT

Smooth muscle contraction

What does smooth muscle contraction do to airway mechanics if it happens in the bronchioles

Increases resistance AND decreases lung compliance

Effect of edema in the upper RT

Narrows the space significantly because bone doesn’t dilate, so the fluid has no choice but to distend the mucosa into the lumen (shrinks functional cross section), increasing resistance by a lot

Effect of edema in the lower RT

Narrow the space to a lesser degree because there is no bone and the fluid can distend the mucosa on either side of the lumen, still increases resistance

Increasing resistance increases the energy needed for what parts of breathing

Inhalation, exhalation, or both

If there is increased resistance in the lower RT (intrathoracic), what part of breathing will we see problems in first

Exhalation

If there is increased resistance in the upper RT (extrathoracic), what part of breathing will we see problems in first

Inhalation

When will we see issues if there is a nasal mass or tracheal collapse

Inhalation

When will we see issues if the soft palate is displaced

Exhalation (snoring!!)

When will we see issues if there is laryngeal paresis

Inhalation

Common smooth muscle constrictors

• Histamine

• Ach

• Hypertonic saline

• Prostaglandins and leukotrienes (inflammatory molecules)

• Ozone

• Acid vapor → gastric reflux!

Why do some animals have very severe reactions to a normal, subclinical exposure to a bronchoconstrictor

Airway hyperreactivity

Common situation to see chronic hypersensitivity reactions

Chronic respiratory distress

Why does chronic RT disease result in respiratory hyperreactivity

Disease can cause smooth muscle to become more twitchy

How to test for respiratory hyperreactivity

Administer a small, subclinical dose of a bronchoconstrictor and check for a response that shouldn’t be there