Lecture 4 - Ventilation/Alveolar gases

Question 1

What is the function of ventilation?

Answer 1

Delivers O2 and removes CO2 that is exchanged across the respiratory membrane.

Question 2

What is defined as the work of breathing?

Answer 2

Elastic recoil in lung (and chest wall) to change volume

AND

Airways resistance (friction) to get airflow

So we need to overcome the elastic recoil of the lung, and the resistance of the airways.

Question 3

What is the elastic recoil of the lungs?

And what factors affect this?

Answer 3

The elastic recoil can also be called compliance.

Compliance is a measure of the elastic properties (“stiffness”) of the lung.

Compliance varies with:

• Lung volume - as the lung gets larger it’s harder to stretch
• Age - lungs are more compliant with age (but chest wall is stiff, and the resp. muscles are weaker - decreased VC)
• Alveolar surface tension - depends on surfactant (surface tension of water molecules constitutes 2/3rds of effort to expand the lung).

Question 4

Describe what factors affect airways resistance (physics), and is air flow laminar during quiet breathing?

Answer 4

Question 5

Why are the bronchi the main site of airways resistance?

Answer 5

Since there is a reduced cross sectional area

Question 6

In relation to diameter, what physiological factors determine airway resistance?

Answer 6

Bronchoconstriction: can be caused by activity in vagal parasympatetic fibers (reflex from stimulating irritant and cough receptors in airways). Or caused by local mediators (histamine, leukotrienes)

Bronchodilation: Activation of B2-adrenoreceptors via circulating adrenaline or administration of sympathomimetics (e.g. isoprenaline and salbutamol)

Dynamic Compression of airways: when we breath out really hard we can close off airways

Question 7

When does dynamic compression of airways limit airflow?

And how to patients counteract this?

Answer 7

Limits air flow in subjects during a forced expiration.

Counteract with “purse-lipped breathing” - maintain airway pressure with “splints” airway open

Question 8

What happens to the work of breathing in restrictive lung disease, obstructive lung disease and pulmonary oedema?

Answer 8

Restrictive: There is an increased work due to decreased lung compliance

Obstructive lung disease: Increased work due to increase in airways resistance

Pulmonary oedema: Increased work due to decreased lung compliance

Question 9

What does the work of breathing equate to?

Answer 9

Work of breathing is pretty much change in intrapleural pressure.

A fall in intrapleural pressure during inspiratio equates with WORK done to move air.

Question 10

Descibe the dashed line (1, 2) and the curved line (1, 2, 3)

Answer 10

Line 1, 2 is when the work of breathing includes only the compliance of the lungs. The graph becomes non-linear when air ways resistance also needs to be overcome.

Question 11

What do pressure volume loops give us an indication of?

Answer 11

Give us an idea of work of breathing.

The slope of line AC indicates amount of lung compliance, and width of loop is proportional to airway resistance.

Question 12

Describe this

Answer 12

When we inspire we have to do work of breathing, which is the energy require to overcome the compliance of the lungs and the airways resistance - this work is represented by the entire figure. AECDA is the energy stored in the elastic tissue in the lungs (ABCA not included since you can’t store energy thats required to overcome airways resistance).

AECFA is the work of respiration, and since this fits with AECDA expiration is a passive process, since work is done by energy stored in stretched lung tissue.

We only have to overcome airways resistance as exhaling, the energy for this is from elastic recoil.

Question 13

Why is the width of this pressure volume loop wider in this patient with asthma?

Answer 13

The width of the circle is proportional to the airways resistance - we do the extra work to overcome this extra resistance by using secondary respiratory muscles

Question 14

What is the typical O2 consumption?

And CO2 production?

Answer 14

O2 consumption: 250-300ml/min

CO2 Production: 200-250ml/min

Need to exchange these across the alveolar membrane to maintain homeostasis, and needs to be able to increase during exercise (Due to increases O2 consumption and CO2 production).

Question 15

What is the respiratory exchange ratio?

Answer 15

VCO2/VO2

200/250 = 0.8 (depends on diet)

Question 16

Gas exchange is driven by what?

Answer 16

Partial pressure gradients of PAO2 and PACO2

These set the alveolar ‘end’ of the partial pressure gradient.

Question 17

What are the partial pressures “PAO2 and PACO2 determined by?

Answer 17

• Composition of inspired air
• Alveolar ventilation
• O2 consumption or CO2 production
• Matching of alvolar ventilation to pulmonary capillary blood flow

Question 18

How can we use PACO2 to calculate alveolar ventilation

Answer 18

Assume VCO2 = 200ml/min @ resting metabolism

863 factor corrects for pressures and allows for combination of volume and pressure values

shows that VA in inversely proportional to PACO2

Question 19

How can we calculate P_AO2

Answer 19

“the alveolar gas equation”

the typical PiO2 si 150mmHg

Question 20

What use does measuring PAO2 and PaO2 have?

Answer 20

If there is a difference between these two that is too large then it indicates a problem with gas exchange

This difference is the A-a gradient, which is normally <10mmHg

Increased A-a gradient indicates decreased gas exchange (difference between PAO2 and PaO2)

Question 21

How do you calculate the A-a gradient

Answer 21

Substitute alveolar gas equation, then take away PaO2 to get difference