L6. Pathology and Consequences of Airway Obstruction

Question 1

What are the major consequences of airflow obstruction? [6]

Answer 1

1. Increased sensation of breathlessness
2. Increased respiratory muscle effort
3. Active exhalation
4. Longer time to inspire
5. Longer time to expire
6. Reduced maximum ventilation

Question 2

What is breathlessness due to?

Answer 2

It is due to increase in either the LOAD or the DRIVE for breathing. If it is an appropriate change (exercise or stress) then it is a sensation . It if is inappropriate then it is a symptom.

Question 3

What is the difference between a load and a drive?

Answer 3

The Load a function of the lung that increases load: stiffness, narrowed airways, chest wall problems or diaphragm dysfunction.
The Drive is the control of breathing: Higher limbic centres, mechanoreceptor, chemoreceptor, baroreceptor, irritant receptors causing an increased drive to breath.

Question 4

What is the normal work of inspiration?

Answer 4

Inspiration is normally a very efficient process due to small amounts of friction and compliance of the lungs. So the work of breathing only uses up ~3% of total oxygen usage.

Question 5

What happens to the work of breathing if airway obstruction?

Answer 5

The work of breathing substantially increases as the inspiratory muscles need to generate higher pressures to overcome the obstruction.

1. obstruction generates friction (increased resistive work)
2. expiration is made active because air cannot pass out
3. ventilation is severely impacted

Question 6

What are the consequences of increasing the work of breathing?

Answer 6

1. Recruitment of accessory muscles of breathing (scalene and sternomastoid muscles)
2. Increasing oxygen consumption of breathing
3. Increased risk of respiratory muscle fatigue: to the point where ventilation is insufficient for metabolic needs disturbing blood haemostasis.

Question 7

How is ventilation failure defined?

Answer 7

PaO2 < 60 mmHg

paCO2 > 50 mmHg

Question 8

How is prolonged inspiration and expiration measured?

Answer 8

Spirometry measures the forced expiratory volume (FEV) over time.

Question 9

What are the normal spirometry readings for spirometry? Describe the curve. How is this different in obstruction?

Answer 9

There is a rapid increase in the volume to maximal after 2-3 seconds and the FEV1 (the volume exhaled in 1 second) is 70% of the FVC. And the ratio is >70%.

In obstruction the curve has a smaller slope such that the FEV1 is markedly reduced. The FVC is kept generally the same but it takes longer to reach it.

Question 10

What are the flow-volume loops, and what do they measure?

Answer 10

Measure both the peak expiratory flow rate and the peak inspiration. Normally the expiratory flow is triangular and the inspiration is the semicircular.

Question 11

What happens to the flow-volume loop in airway obstruction

Answer 11

There is plateauing of the expiratory curve with a decrease in the flow. The inspiration is generally preserved.

Question 12

What is ventilatory failure?

Answer 12

When the requirements of oxygen demand and/or carbon dioxide clearance is larger that that the ventilation capacity of the lungs is able to keep up with

Question 13

What is meant by ventilation/perfusion (V/Q) matching

Answer 13

Ventilation is the amount of air flowing in and out of the alveoli and perfusion is about of blood flowing through the pulmonary capillaries. Thus the amount of oxygen matches the Hb coming through the capillaries.
Both ventilation and perfusion are homologous and equalled out.

Question 14

What happens to the V/Q matching in airways obstruction?

Answer 14

Perfusion is not affected in airways obstruction. Some areas (depending on the site of obstruction) have poor ventilation and so there are patchy areas of low V/Q.
SHUNTS are formed: where deoxygenated blood enter the alveoli without receiving any oxygenation. Despite the patchiness of this, deoxygenated blood and oxygenated blood mix at the pulmonary veins decreasing the overall O2 content.

Question 15

What is the Alveolar-arterial (A-a) gradient? What is the normal range?

Answer 15

It is a measure of the overall efficiency of gas exchange across ALL the units.
Aa gradient = PAO2 -PaO2
Normal range is A-a < 15-30 mmHg
Any deviations from this normal range suggests abnormal gas exchange

Question 16

Alveolar O2 concentration is difficult to measure. How do we estimate it?

Answer 16

PAO2 = (PiO2 - PaO2)/0.8

Question 17

What is the pathogenesis of asthma in causing respiratory problems?

Answer 17

Chronic airway inflammation, increased mucous production and bronchospams which lead to a decrease in the airways diameter (obstruction). This is an uneven process causing patchy areas of V/Q mismatching but generally preserves the diffusing capacity of the alveoli.

Question 18

What is the pathogenesis of COPD in causing respiratory problems?

Answer 18

Also called emphysema - a smoking related disease. Inflammation of the bronchial mucosa that leads to a loss in elastic support of small airways and subsequent distribution of the alveolar - capillary membrane. This is both airway obstruction (collapse of airways due to tissue loss), impaired gas exchange and reduced capillary bed integrity.