Lecture 4: Resp failure Flashcards by Emily Densham

What are the 4 types of hypoxia?

Cytotoxic - reduced ability to utilise O2

Circulatory - reduced ability to deliver O2

Anaemic - reduced ability to deliver O2

Hypoxaemic - reduced ability to deliver O2

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What are the 5 mechanisms of hypoxaemia?

Reduced PIO2
Hypoventilation
Diffusion limitations
Mismatch 
Shunt

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Where is ventilation lowest?

At the apex

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Where is ventilation highest?

At the base

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Where is perfusion lowest?

At the apex

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Where is perfusion highest?

At the base

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What is the order of Pa, Pv and PA at the apex (zone 1)?

PA>Pa>Pv

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What is the order of Pa, Pv and PA in the centre (zone 2)?

Pa>PA>Pv

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What is the order of Pa, Pv and PA at the base (zone 3)?

Pa>Pv>PA

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Which is greater at the apex, ventilation or perfusion?

Ventilation > Perfusion

V/Q > 1

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Which is greater in the middle of the lung, ventilation or perfusion?

Ventilation = Perfusion

V/Q = 1

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Which is greater at the base, ventilation or perfusion?

Ventilation < Perfusion

V/Q < 1

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What is the normal range of V/Q ratios?

0.3 to 10

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What is meant by V/Q = 0?

When no ventilation occurs in a given area

Extreme version of V/Q mismatch that is called shunt

Decreased PAO2 - hypoxaemia

Unable to compensate with increased FIO2

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What is meant by V/Q &laquo_space;1?

Ventilation substantially reduced in a given area - V/Q mismatch

Decreased PAO2 - hypoxaemia

Able to compensate with increased FIO2

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What is meant by V/Q&raquo_space; 1?

Wasted ventilatory effort

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How would lung failure present?

Gas exchange failure manifested by hypoxaemia

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How would pump failure present?

Ventilatory failure manifested by hypercapnia

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Will giving oxygen with a higher FIO2 increase the PaO2 in hypoxaemia due to reduced PIO2?

Yes

Will giving oxygen with a higher FIO2 increase the PaO2 in hypoxaemia due to hypoventilation?

Yes

As long as there is some ventilation

Will giving oxygen with a higher FIO2 increase the PaO2 in hypoxaemia due to diffusion limitation?

Yes

Will giving oxygen with a higher FIO2 increase the PaO2 in hypoxaemia due to V/Q mismatch?

Yes

Will giving oxygen with a higher FIO2 increase the PaO2 in hypoxaemia due to shunt?

What might cause hypoventilation?

Central nervous system disease

Drug overdose: narcotics, other depressants, alcohol with impair function of the respiratory control centre

Weakness

What might cause diffusion limitation?

Interstitial oedema Inflammation Scarring Increase diffusion distance and slow gas diffusion

What is the effect of diffusion limitation of PAO2 and PaO2?

Normal PAO2 but reduced PaO2

In hypoxamia from V/Q mismatch, what occurs in two alveoli where one is experiencing reduced ventilation?

1st alveolus: PA02 is elevated above normal due to increased ventilation causing the PCO2 to be reduced 1st alveolus: Ca02 is normal. The higher PAO2 adds hardly any more content to the blood leaving that capillary. 2nd alveolus: PA02 is decreased from increased CO2 in the alveolus 2nd alveolus: Ca02 is much reduced due to lower PaO2 - lower amount of dissolved O2 and lower amount of O2 bound to Hb

What limits the amount of V/Q mismatch in the lungs?

Hypoxic pulmonary vasoconstriction directs blood flow away from poorly ventilated areas, and helps maintain overall V/Q matching

What are the examples of abnormal shunts?

Intrapulmonary shunt - Arteriovenous malformations - Lung consolidation (for ex, pneumonia) Intracardiac shunt - Congenital or acquired heart defects

Which causes of hypoxaemia result in increased PA-aO2?

Diffusion limitation Low V/Q Shunt

Which causes of hypoxaemia maintain normal PA-aO2?

Hypoventilation Decreased PIO2

Which causes of hypoxaemia result in decreased PAO2?

Hypoventilation Decreased PIO2 Low V/Q (locally)

What is anatomic dead space?

Typically, 150 mL out of a 500 mL (~ 30%) tidal volume remains in the airways

What is alveolar dead space?

Air reaches the alveoli but can’t participate in gas exchange Normally minimal but can increase in amount in lung disease

What is VD?

Dead space

What is VT?

Tidal volume

What is VD/VT?

Fraction of dead space

What may cause hypercarbic respiratory failure?

Due to weakness, loss of drive from the respiratory centre, or failure to compensate for increased dead space

What are the treatments for hypercarbic failure?

RR and tidal volume

What are the treatments for hyperaemic failure?

FIO2 (21% to 100%) Added pressure

What is minute ventilation (VE)?

Amount of air, per minute, breathed into the lung

What is VE?

Minute ventilation

What is the equation of VE?

VE = VT x RR

What is VA?

Amount of ventilation

Why is VA not equal to VE?

Because of dead space