72 - Physiological Consequences of Airway Obstruction

Question 1

Sensation of airflow obstruction
1)
2)
3)
4)

Answer 1

1) Increased sensation of breathing
2) Increased respiratory muscle effort
3) Active exhalation
4) Longer time to inspire and expire

Question 2

Things that can increase load on breathing 
1
2
3
4

Answer 2

Stiff lungs
Narrow airways
Chest wall
Diaphragm

Question 3

Things that can increase the drive to breathe 
1
2
3
4
5
6

Answer 3

Higher centres (limbic system)
Mechanoreceptors
Irritant receptors
Chemoreceptors
Baroreceptors
Temperature

Question 4

Factors contributing to the work required to breathe
1)
2)

Answer 4

1) Load on lungs.

2) Drive to breathe

Question 5

When is difficulty breathing a sensation?

Answer 5

When appropriate

Question 6

When is difficulty breathing a symptom?

Answer 6

When inappropriate

Question 7

Nerve stimuli leading to inspiration

Answer 7

Stimulation of the diaphragm by phrenic nerves.

External intercostal stimulation by intercostal nerves.

Question 8

Contraction of the diaphragm does what to thoracic dimensions?

Answer 8

Increases the longitudinal and lateral dimensions of the thorax

Question 9

Amount of breathed oxygen that goes to respiratory muscles at rest

Answer 9

~3%

Question 10

When does the intra-alveolar pressure equal atmospheric pressure?

Answer 10

At the end of inspiration and expiration

Question 11

Intra-pleural pressure relative to intra-alveolar pressure

Answer 11

Intra-pleural is always lower than intra-alveolar, because of elastic recoil of lungs and chest wall (pleura held together by fluid tension, upon inspiration negative pressure within lungs pulls inwards on pleura. Fluid pressure resists this, leading to decreased pressure in pleural space).

Question 12

Intrapleural pressure during inspiration

Answer 12

Decreases pressure in pleural cavity

Question 13

Pleural cavity pressure during one inspiration/expiration cycle.

Answer 13

Decreases until change from inspiring to expiring, then increases in pressure again. Frequency is ~1/2 that of inspiration/expiration pressure.

Question 14

Why is work of breathing increased when there is an airway obstruction

Answer 14

Inspiratory muscles need to generate higher pressures to overcome obstruction to airflow

Question 15

Consequences of obstruction
1
2
3

Answer 15

1) Recruitment of accessory muscles of inspiration
2) Increased oxygen consumption by respiratory muscles
3) Risk of respiratory muscle fatigue (if obstruction is severe)

Question 16

Effect of respiratory muscle fatigue

Answer 16

Too little O2 dissolved in blood

Question 17

Ventilatory failure

Answer 17

When rate of O2 entry into body is below rate of CO2 expiration.

Question 18

Arterial partial pressures considered to be ventilatory failure

Answer 18

PaO2 under 60mmHg, PaCO2 over 50mmHg

Question 19

How is exhalation normally passive?

Answer 19

Elastic recoil of lungs generates positive intrapulmonary pressure, pushing air out.

Question 20

Muscles involved in active exhalation

Answer 20

Abdominals, internal intercostals

Question 21

TLC

Answer 21

Total lung capacity.

Greatest possible breath.

Question 22

Amount of time for a normal FVC

Answer 22

2-3 seconds

Question 23

Spirometric measurement of total lung capacity

Answer 23

FVC

Question 24

% of vital capacity in FEV1

Answer 24

~80%

Question 25

FEV1 percentage of FVC considered normal

Answer 25

Over 70% (if older)

~80% in a young person

Question 26

Appearance of a FVC graph for someone with airway obstruction

Answer 26

Flatter, increases less quickly than that of a healthy person (person able to expire less quickly)

Question 27

Flow-volume loop

Answer 27

Measurement of flow rate vs volume during a FVC, followed by a forced inspiratory manoeuvre.

FVC is on the upper part of y-axis, inspiratory pressure is on the lower part of y-axis

Question 28

WOB

Answer 28

Work of breathing

Question 29

Effect of obstruction leading to uneven ventilation

Answer 29

Leads to impaired gas exchange

Question 30

Examples of obstructions that can lead to uneven ventilation

Answer 30

COPD, asthma

Question 31

Ventilation/perfusion matching

Answer 31

Gas exchange is most efficient when ventilation and perfusion are matched (V/P=1) in all alveolar-capillary units.

Question 32

How can you image whether ventilation and perfusion match?

Answer 32

Ventilation and perfusion scans.
Ventilation - inhale radioactive particles, which are non-absorbable.
Perfusion - Inject radioactive particles into systemic vein which lodge in small pulmonary arterioles.

Question 33

Appearance of COPD on ventilation/perfusion scan

Answer 33

Non-homogenous ventilation and perfusion in lungs

Question 34

Effect of V/Q mismatch

Answer 34

O2 binding sites on Hb not filled.

Some blood returning to left atrium not fully oxygenated.

Question 35

Most clinically important cause of reduced PaO2

Answer 35

Low V/Q units

Question 36

Shunt

Answer 36

A unit of blood that goes through a non-ventilated area of lungs, so isn’t oxygenated.
An extreme form of low V/Q unit.

Question 37

Compensatory mechanism for regions of reduced ventilation (causing low V/Q)

Answer 37

Vasoconstriction occurs in areas of low ventiation to reduce the hypoxaemic effects of low V/Q units.

This is effective in reducing altered gas exchange, but not usually fully successful

Question 38

Potential downstream effect of compensatory mechanism for low V/Q units

Answer 38

Increased pulmonary arterial pressure

Question 39

Things that airflow obstruction can be due to
1)
2)

Answer 39

1) Spasm of bronchial smooth muscle

2) Airway inflammation

Question 40

Things that can be used to treat spasm of bronchial smooth muscle
1)
2)
3)

Answer 40

1) Beta agonists
2) Anti-cholinergics
3) Methyl-xanthines

Question 41

COPD

Answer 41

Smoking related disease
Causes inflammation of bronchial mucosa.
Inflammation is not steroid-responsive
Causes destruction of lung parenchyma

Question 42

Differences between asthma and COPD

Answer 42

COPD inflammation is not steroid responsive
COPD leads to destruction of airway parenchyma
Asthma is steroid responsive and doesn’t destroy airway parenchyma

Question 43

Effects of COPD
1)
2)
3)

Answer 43

1) Collapse of small airways (destruction of elastic tissue in airways), particularly in inspiration
2) Impaired gas exchange from V/Q mismatch, loss of alveolar-capillary membrane
3) Reduced capillary bed, from pulmonary hypertension

Question 44

What are asthma inflamed airways responsive to?

Answer 44

Corticosteroids

Leukotriene antagonists

Question 45

Alveolar-arterial gradient for oxygen

Answer 45

Measure of the overall efficiency of gas exchange across all alveolar-capillary units

Question 46

How is alveolar pressure calculated for A-a gradient of O2?

Answer 46

Estimated using the ideal gas equation:
PAO2 = PiO2 - PACO2/RQ

R=~0.8
PiO2 = Partial pressure of inspired oxygen
PACO2 = Alveolar CO2

Question 47

Effect of hypoventilation on O2 and CO2

Answer 47

Decrease O2, increase CO2

Question 48

How to tell if someone is hypoxic from hypoventilation, so something more.

Answer 48

Calculate the amount of hypoxia from alveolar-arterial gradient for O2, see if there is more in a patient than is expected.

Question 49

RQ

Answer 49

0.8

Question 50

PiO2 at sea level

Answer 50

150

Question 51

Normal A-a gradient

Answer 51

15-30

Question 52

How can cellular O2 consumption be calculated?

Answer 52

Fick equation

QO2 (cellular use of O2) = Cardiac output x (arterial O2 content - venous O2 content)