eLFH - Respiratory Physiology Part 3

Question 1

Hypoxia definition

Answer 1

Deficiency of oxygen for tissue respiration

Question 2

Hypoxaemia definition

Answer 2

Arterial PO2 < 12 kPa

Question 3

Hypoxia classification

Answer 3

Hypoxic hypoxia

Anaemic hypoxia

Circulatory (stagnant) hypoxia

Cytotoxic (histotoxic) hypoxia

Question 4

Delivery of oxygen equation and relation to classification of hypoxia

Answer 4

Question 5

Cytotoxic (histotoxic) hypoxia

Answer 5

Occurs at cellular level with no deficit in O2 delivery

E.g. cyanide poisoning

Question 6

Oxygen cascade

Answer 6

Question 7

Causes of Hypoxic hypoxia

Answer 7

Low FiO2

Hypoventilation

Diffusion defect

V/Q mismatch / shunt

Question 8

Causes for low FiO2

Answer 8

Inadvertent hypoxic gas mixture

High altitude

Diffusion hypoxia following nitrous oxide administration

Question 9

Part of oxygen cascade affected by low FiO2

Answer 9

Air

Question 10

Part of oxygen cascade affected by hypoventilation

Answer 10

Alveolus
Pulmonary capillary
Artery
Organ capillary
Mitochondria

(I.e. alveolus onwards)

Question 11

Part of oxygen cascade affected by diffusion defect

Answer 11

Pulmonary capillary onwards

Question 12

Part of oxygen cascade affected by V/Q mismatch / shunt

Answer 12

Artery onwards

Question 13

Typical O2 consumption in an adult

Answer 13

250 ml/min

Question 14

Alveolar gas equation

Answer 14

Question 15

How does hypoventilation lead to hypoxia

Answer 15

Usually secondary to hypercarbia (see alveolar gas equation) as room air with hypoventilation typically sufficient to meet oxygen consumption of 250 ml/min

Question 16

Causes of hypoventilation

Answer 16

Reduced central respiratory drive

Impaired peripheral mechanisms of breathing

Increased dead space

Question 17

Causes of reduced central respiratory drive causing hypoventilation

Answer 17

Drugs
Metabolic alkalosis
Intracranial pathology
Alveolar hypoventilation syndrome
Hypothermia

Question 18

Drugs which reduce central respiratory drive

Answer 18

Opiates
Benzodiazepines

Question 19

Causes of impaired peripheral mechanisms of breathing causing hypoventilation

Answer 19

Airway obstruction
Restriction
Chest disease
Muscular weakness
Neuromuscular junction impairment
Nerve lesions

Question 20

Examples of lung restriction

Answer 20

Pain
Obesity
Ascites

Question 21

Examples of chest diseases

Answer 21

COPD
Asthma
Flail chest

Question 22

Examples of muscular weakness

Answer 22

Dystrophies
Electrolyte imbalance
Critical illness neuropathies

Question 23

Examples of neuromuscular junction impairment

Answer 23

Muscle relaxants
Myasthenia gravis

Question 24

Examples of nerve lesions

Answer 24

Phrenic nerve / spinal cord injuries
Guillain-Barre syndrome
Polio

Question 25

Causes of increased dead space

Answer 25

Use of anaesthetic equipment

Question 26

Causes of diffusion defect

Answer 26

Pulmonary fibrosis Pulmonary oedema

Question 27

How diffusion defect causes hypoxia

Answer 27

Diffusion defects increases time spent in capillary required for O2 diffusion to be complete When pulmonary blood flow increases (e.g. exercise), time spent in capillary reduces can can cause clinically relevant hypoxia Severely abnormal cases are apparent at rest

Question 28

How far along pulmonary capillary is O2 transfer complete in healthy individuals

Answer 28

~ one third of the way along pulmonary capillary

Question 29

How much does exercise reduce time blood spends in pulmonary capillary by

Answer 29

Around one third reduced

Question 30

Normal healthy lung unit with ventilation and perfusion matched

Answer 30

End capillary blood and alveolus have the same PO2 and PCO2

Question 31

How does dead space manifest clinically

Answer 31

Increased PaCO2-EtCO2 difference Dead space alveoli not involved in gas exchange so no CO2, therefore dilutes EtCO2 from other alveoli

Question 32

How does shunt manifest clinically

Answer 32

Hypoxaemia Unventilated lung unit means end pulmonary capillary blood has same PO2 and PCO2 as venous blood, therefore dilutes PO2 in arterial blood

Question 33

Lung volumes top vs bottom of lung

Answer 33

Top > Bottom due to gravity

Question 34

Ventilation top vs bottom of lung

Answer 34

Bottom > top Therefore V/Q matched

Question 35

Perfusion top vs bottom of lung

Answer 35

Bottom > top due to gravity Therefore V/Q matched

Question 36

Effect of decreased FRC under GA on V/Q matching

Answer 36

FRC reduced Lung moves down compliance curve V/Q no longer match as ventilation now better at top (as compliance curve steeper) but perfusion still best at bottom of lung I.e. a shunt has developed

Question 37

Closing capacity definition

Answer 37

Volume at which airway closure occurs Closing capacity = Closing volume + Residual volume I.e. lung volume above residual volume at which airway closes

Question 38

Relevance of closing capacity

Answer 38

Usually CC much lower than FRC If CC increases, or FRC decreases, eventually airway closure occurs earlier causing shunt

Question 39

Factors which increase closing capacity

Answer 39

Age Increased intrathoracic pressure (e.g. asthma) Smoking

Question 40

At which ages does closing capacity = FRC

Answer 40

Neonates and infants Supine 45 year old Upright 65 year old

Question 41

Factors which decrease closing capacity

Answer 41

PEEP / CPAP

Question 42

Factors which reduce FRC

Answer 42

Head down position Obesity / pregnancy GA Restrictive lung disease Female Youth

Question 43

Measuring closing volume

Answer 43

Fowler's method After plateau phase, EtN2 rises again - this stage represents closing volume

Question 44

Clinical relevance of closing volume

Answer 44

Used more often than closing capacity as can be measured with Fowler's method Residual volume needs helium dilution or total body plethysmography to be measured and therefore closing capacity

Question 45

Why does second N2 rise occur on graph using Fowler's method

Answer 45

Ventilation better at bottom of lungs Therefore with only a single breath of 100% O2, bottom alveoli have greater proportion of O2 Top alveoli have higher N2 proportion as have greater volume Therefore as closing volume reached, bottom alveoli collapse first as they are lower volume When this occurs, higher proportion of N2 exhaled with lower proportion of O2, therefore get second rise in graph

Question 46

Usual PaCO2-EtCO2 difference in healthy adults

Answer 46

0.7 kPa

Question 47

Reason for there being a PaCO2-EtCO2 difference in healthy adults

Answer 47

There is always some dead space

Question 48

Why does V/Q ratio decrease in bottom vs top of lung

Answer 48

Both ventilation and perfusion increase in bottom of lung, but perfusion more so than ventilation Vice versa for top of lung Therefore bottom of lung tends more towards shunt and top of lung tends more towards dead space

Question 49

Non respiratory functions of the lung

Answer 49