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Anaesthetic FRCA Primary - eLFH Physiology > eLFH - Respiratory Physiology Part 3 > Flashcards

eLFH - Respiratory Physiology Part 3 Flashcards

1
Q

Hypoxia definition

A

Deficiency of oxygen for tissue respiration

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2
Q

Hypoxaemia definition

A

Arterial PO2 < 12 kPa

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3
Q

Hypoxia classification

A

Hypoxic hypoxia

Anaemic hypoxia

Circulatory (stagnant) hypoxia

Cytotoxic (histotoxic) hypoxia

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4
Q

Delivery of oxygen equation and relation to classification of hypoxia

A
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5
Q

Cytotoxic (histotoxic) hypoxia

A

Occurs at cellular level with no deficit in O2 delivery

E.g. cyanide poisoning

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6
Q

Oxygen cascade

A
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7
Q

Causes of Hypoxic hypoxia

A

Low FiO2

Hypoventilation

Diffusion defect

V/Q mismatch / shunt

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8
Q

Causes for low FiO2

A

Inadvertent hypoxic gas mixture

High altitude

Diffusion hypoxia following nitrous oxide administration

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9
Q

Part of oxygen cascade affected by low FiO2

A

Air

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10
Q

Part of oxygen cascade affected by hypoventilation

A

Alveolus
Pulmonary capillary
Artery
Organ capillary
Mitochondria

(I.e. alveolus onwards)

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11
Q

Part of oxygen cascade affected by diffusion defect

A

Pulmonary capillary onwards

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12
Q

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

A

Artery onwards

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13
Q

Typical O2 consumption in an adult

A

250 ml/min

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14
Q

Alveolar gas equation

A
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15
Q

How does hypoventilation lead to hypoxia

A

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

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16
Q

Causes of hypoventilation

A

Reduced central respiratory drive

Impaired peripheral mechanisms of breathing

Increased dead space

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17
Q

Causes of reduced central respiratory drive causing hypoventilation

A

Drugs
Metabolic alkalosis
Intracranial pathology
Alveolar hypoventilation syndrome
Hypothermia

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18
Q

Drugs which reduce central respiratory drive

A

Opiates
Benzodiazepines

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19
Q

Causes of impaired peripheral mechanisms of breathing causing hypoventilation

A

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

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20
Q

Examples of lung restriction

A

Pain
Obesity
Ascites

21
Q

Examples of chest diseases

A

COPD
Asthma
Flail chest

22
Q

Examples of muscular weakness

A

Dystrophies
Electrolyte imbalance
Critical illness neuropathies

23
Q

Examples of neuromuscular junction impairment

A

Muscle relaxants
Myasthenia gravis

24
Q

Examples of nerve lesions

A

Phrenic nerve / spinal cord injuries
Guillain-Barre syndrome
Polio

25
Q

Causes of increased dead space

A

Use of anaesthetic equipment

26
Q

Causes of diffusion defect

A

Pulmonary fibrosis
Pulmonary oedema

27
Q

How diffusion defect causes hypoxia

A

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

28
Q

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

A

~ one third of the way along pulmonary capillary

29
Q

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

A

Around one third reduced

30
Q

Normal healthy lung unit with ventilation and perfusion matched

A

End capillary blood and alveolus have the same PO2 and PCO2

31
Q

How does dead space manifest clinically

A

Increased PaCO2-EtCO2 difference

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

32
Q

How does shunt manifest clinically

A

Hypoxaemia

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

33
Q

Lung volumes top vs bottom of lung

A

Top > Bottom
due to gravity

34
Q

Ventilation top vs bottom of lung

A

Bottom > top

Therefore V/Q matched

35
Q

Perfusion top vs bottom of lung

A

Bottom > top
due to gravity

Therefore V/Q matched

36
Q

Effect of decreased FRC under GA on V/Q matching

A

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

37
Q

Closing capacity definition

A

Volume at which airway closure occurs

Closing capacity = Closing volume + Residual volume

I.e. lung volume above residual volume at which airway closes

38
Q

Relevance of closing capacity

A

Usually CC much lower than FRC

If CC increases, or FRC decreases, eventually airway closure occurs earlier causing shunt

39
Q

Factors which increase closing capacity

A

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

40
Q

At which ages does closing capacity = FRC

A

Neonates and infants

Supine 45 year old

Upright 65 year old

41
Q

Factors which decrease closing capacity

A

PEEP / CPAP

42
Q

Factors which reduce FRC

A

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

43
Q

Measuring closing volume

A

Fowler’s method

After plateau phase, EtN2 rises again - this stage represents closing volume

44
Q

Clinical relevance of closing volume

A

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

45
Q

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

A

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

46
Q

Usual PaCO2-EtCO2 difference in healthy adults

A

0.7 kPa

47
Q

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

A

There is always some dead space

48
Q

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

A

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

Anaesthetic FRCA Primary - eLFH Physiology (13 decks)

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