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103 Theme 4 - Lungs > Respiratory failure > Flashcards

Respiratory failure Flashcards

1
Q

What is hypoxia?

A

Reduced level of tissue oxygenation

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2
Q

What is hypoxaemia?

A

Decrease in partial pressure of oxygen (PaO2) in the blood

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3
Q

What is Pao2?

A

Arterial oxygen tension (PaO2) – partial pressure of oxygen that indicates the dissolved oxygen in plasma (not O2 bound to Hb)

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4
Q

When do hypoxia and hypoxaemia not coexist?

A
  • Individuals can develop hypoxaemia without hypoxia if there is a compensatory
  • In cyanide poisoning, cells are unable to utilise O2 despite having normal blood and tissue oxygen levels
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5
Q

How is arterial oxygen tension (PaO2) measured?

A

Measured by arterial blood gas analyser

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6
Q

What is Sao2?

A

Arterial oxygen saturation (SaO2) – percentage of haemoglobin saturated with O2

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7
Q

How is Sao2 measured?

A

Measured with pulse oximeter and arterial blood gas analyser

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8
Q

What is the 5th vital sign?

A

Pulse oximetry

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9
Q

How does pulse oximetry work?

A

Pulse oximetry uses Beer-Lambert-Bougeur law – which states that the attenuation of light depends on the properties of the materials through which the light is travelling

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10
Q

What is respiratory failure?

A

clinical term used to describe the failure to maintain oxygenation

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11
Q

What is type 1 respiratory failure?

A

Reduction in PaO2 but no change in PaCO2 – V/Q mismatch

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12
Q

What is type 2 respiratory failure?

A

Increase in PaCO2 and a reduction in PaO2 - underventilation

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13
Q

What is an algorithm to analyse ABGs?

A
  1. pH – is there acidosis or alkalosis
  2. pCO2 – is it contributing or attempting to compensate for the abnormality identified in the pH
  3. Bicarbonate – sHCO3- (standardised) or BE (base excess). If primary metabolic problem sHCO3- will hold no surprise. Metabolic acidosis it will be low, metabolic acidosis it will be high. Respiratory problem sHCO3- maybe normal (therefore acute issue), attempting to correct the respiratory effect on the pH (chronic problem)
  4. pO2 – allows you to determine weather time 1 or type 2 respiratory failure
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14
Q

What are the 5 mechanisms of hypoxaemia?

A
  1. V/Q mismatch
  2. Right-to-left shunt
  3. Diffusion impairment
  4. Hypoventilation
  5. Low inspired pO2
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15
Q

What is the A-a gradient?

A

A-a gradient is the difference between alveolar O2 level (PAO2) and the arterial oxygen level (PaO2)

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16
Q

What is the equation to calculate Pao2?

A
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17
Q

What is the equation to calculate the A-a gradient?

A

PAO2 - PaO2

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18
Q

What does the A-a gradient indicate?

A

A-a gradient indicates the integrity of the alveolocapillary membrane and the effectiveness of gas exchange – pathology of the alveolocapillary unit widens the gradient

19
Q

What widens the A-a gradient?

A

Hypoxaemia caused by V/Q mismatch, diffusion limitation and shunt widen the A-a gradient

20
Q

When will hypoxaemia have a normal A-a gradient?

A

Hypoxaemia caused by hypoventilation have a normal gradient

21
Q

What is the most common cause of hypoxaemia?

A

V/Q mismatch

22
Q

What is the regional hetergenity of V/Q throughout the lungs due to?

A

Subatmospheric intrapleural pressure and gravity

Apex of lung under high stretch

Base of lung under low stretch

Ventilation wants to go up

Perfusion wants to go down

23
Q

Ventilation and perfusion are _______ at the bases and _______ at the apex

A

Higher

Lower

24
Q

V/Q ratio is ______ at apex and ______ at the base

A

Higher

Low

25
Q

What does a low V/Q ratio produce?

A

A low V/Q ratio produces hypoxaemia by decreasing the PAO2 and subsequent PaO2

26
Q

What is hypoxic pulmonary vasoconstriction?

A

Compensatory mechanism – there is a restriction in perfusion in areas of the lung with reduced ventilation

27
Q

Which V/Q ratio does a PE cause?

A
  • Ventilation is in excess of perfusion – think PE
  • In PE – less perfusion – high V/Q ratio
28
Q

When does a PE cause hypoxaemia?

A

Hypoxaemia is caused if the compensatory rise in total ventilation is absent

29
Q

What are the characteristics of a V/Q mismatch?

A
  • Hypoxaemia due to V/Q mismatch can be easily corrected by supplemental oxygen
  • Widened A-a gradient
30
Q

What are the common causes of V/Q mismatch?

A

asthma, COPD, bronchiectasis, cystic fibrosis, ILD and pulmonary hyptertension

31
Q

What is a shunt?

A

Blood from the right side of the heart enters the left side without taking part in gas exchange

32
Q

What causes the extreme degree of V/Q mismatch where there is no ventilation?

A

Shunt

33
Q

What distinguishes a shunt from other mechanisms?

A

Poor response to oxygen therapy

34
Q

When does a patient with a shunt experience hypoxaemia?

A

Hypoxaemia is uncommon in shunt until the shunt fraction reaches 50%

35
Q

Why is there a lack of hypercapnia in shunt patients?

A

Lack of hypercapnia is due to simulation of the respiratory centre by chemoreceptor

36
Q

What are the characteristics of a pulmonary shunt?

A
  • A-a gradient is elevated
  • pCO2 is normal
  • Poor response to oxygen therapy
37
Q

What are common causes of a pulmonary shunt?

A

pneumonia, pulmonary oedema, ARDS, pulmonary arteriovenous communication

38
Q

What causes diffusion limitation?

A

Decrease in lung surface area for diffusion, inflammation and fibrosis, low alveolar oxygen and reduced capillary transit time

39
Q

What is diffusion limitation?

A

Transport across the alveolocapillary membrane is impaired

40
Q

What does diffusion limitation cause?

A

Since O2 and CO2 occur across the alveolocapillary membrane – theoretically it should cause hypoxaemia and hypercapnia.

Hypercapnia is uncommon – CO2 is 20x more soluble than O2 and is less likely to be affected by diffusion limitation.

41
Q

What are the characteristics of diffusion limitation?

A
  • Hypoxaemia shows a good response to oxygen therapy
  • A-a gradient is elevated
  • PaCO2 is normal
42
Q

What are the characteristics of hypoventilation?

A
  • Hallmark – high PaCO2
  • Leads to low PAO2 and subsequent low PaO2
  • Normal A-a gradient
  • In healthy lungs hypoventilation does not cause significant hypoxaemia but does in the presence of lung disease
  • If hypoxaemia is present it is easily corrected by oxygen therapy but hypoventilation and hypercapnoea can persist
43
Q

What are the causes of hypoventilation?

A
  • Impaired central drive – drug over dose, brainstem infarction, primary alveolar hypoventilation
  • Spinal cord – ALS
  • Nerve – Guillian-Barre syndrome
  • Neuromuscular junction – Myasthenia gravis
  • Respiratory muscles – myopathy
44
Q

Complete the diagram on the various mechanisms of hypoxaemia

A

103 Theme 4 - Lungs (21 decks)

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