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Module 103: The Heart, Lungs and Blood > Theme 4: Lecture 4 - Respiratory failure > Flashcards

Theme 4: Lecture 4 - Respiratory failure Flashcards

1
Q

Hypoxia

A

Reduced level of tissue oxgyenation

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2
Q

Hypoxaemia

A

Decrease in the partial pressure of oxygen in the blood

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3
Q

Why do hypoxia and hypoxaemia not always coexist

A
  • Individuals can develop hypoxaemia without hypoxia if there is a compensatory eg increase in Hb level or cardiac output
  • In cyanide poisoning, cells are unable to utilise O2 despite having normal blood and tissue oxygen levels
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4
Q

What is the arterial O2 tension (AKA partial pressure of arterial O2) - partial pressure of O2 that indicates the dissolved oxygen in the plasma (not O2 bound to Hb) measured by

A

Arterial blood gas analyser

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5
Q

What is arterial O2 saturation

A

Percentage of haemoglobin saturated with O2

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6
Q

How is arterial O2 saturation measured

A

Measured with pulse oximeter and arterial blood gas analyser (pulse oximetry = the 5th vital sign)

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7
Q

Which law does pulse oximetry use

A

Beer-Lambert-Bougeur law

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8
Q

Beer-Lambert-Bougeur law

A

states that the attenuation (the reduction of the force, effect, or value of something) of light depends on the properties of the materials through which the light is travelling

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9
Q

What is respiratory failure

A

clinical term used to describe the failure to maintain oxygenation

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10
Q

Type 1 respiratory failure

A

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

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11
Q

Type 2 respiratory failure

A

increase in PaCO2 and a reduction in PaO2 - underventilation

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12
Q

Hypercapnia

A

Abnormally elevated CO2 levels in the blood

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13
Q

Algorithm to review ABGs

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

What are the mechanisms of hypoxaemia

A
  • V/Q mismatch
  • Right-to-left shunt
  • Diffusion impairment
  • Hypoventilation
  • 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)
  • A-a gradient = PAO2 - PaO2
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16
Q

What does the A-a gradient indicate

A

indicates the integrity of the alveolocapillary membrane and the effectiveness of gas exchange

17
Q

What widens the A-a gradient

A
  • Pathology of the alveolocapillary unit widens the gradient

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

18
Q

What results in a normal A-a gradient

A

Hypoxaemia caused by hypoventilation

19
Q

Hypoxic pulmonary vasoconstriction

A
  • Compensatory mechanism

- there is a restriction in perfusion in areas of the lung with reduced ventilation

20
Q

What is the most common cause of hypoxaemia

A

V/Q mismatch

21
Q

V/Q mismatch

A
  • Regional heterogeneity of V/Q – subatmospheric intrapleural pressure and gravity
  • Ventilation and perfusion are higher at the bases and lower at the apex
  • V/Q ratio is higher at apex and low at the base
22
Q

What does a low V/Q ratio cause

A

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

23
Q

What do you think of when you see a high V/Q ratio

A

PE

24
Q

High V/Q ratio

A
  • Ventilation is in excess of perfusion – think PE
  • In PE – less perfusion – high V/Q ratio
  • Hypoxaemia is caused if the compensatory rise in total ventilation (ie to other areas where the blood is redirected) is absent
25
Q

Characteristics of V/Q mismatch

A
  • Hypoxaemia to to V/Q mismatch can be easily corrected by supplemental oxygen
  • Widened A-a gradient
26
Q

Common causes of V/Q mismatch

A

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

27
Q

Bronchiectasis

A

Bronchiectasis is a long-term condition where the airways of the lungs become abnormally widened, leading to a build-up of excess mucus that can make the lungs more vulnerable to infection

28
Q

Describe pulmonary shunt

A
  • Blood from the right side of the heart enters the left side without taking part in gas exchange
  • Shunt is the extreme degree of V/Q mismatch where there is no ventilation
  • Hypoxaemia is uncommon in shunt until the shunt fraction reaches 50%
29
Q

What is a feature that distinguishes pulmonary shunt from other mechanisms

A

Poor response to oxygen therapy

30
Q

Why is there a lack of hypercapnia in pulmonary shunt

A

Due to stimulation of respiratory centre by chemoreceptor

31
Q

Characteristics of pulmonary shunt

A
  • A-a gradient is elevated
  • pCO2 is normal
  • Poor response to oxygen therapy
32
Q

Common causes of shunt

A

pneumonia, pulmonary oedema, ARDS (acute respiratory distress syndrome), pulmonary arteriovenous communication

33
Q

Describe diffusion limitation

A
  • Transport across the alveolocapillary membrane is impaired (decrease in lung surface area for diffusion, inflammation and fibrosis, low alveolar oxygen and reduced capillary transit time)
  • 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
34
Q

Characteristics of diffusion limitation

A
  • Hypoxaemia shows a good response to oxygen therapy
  • A-a gradient is elevated
  • PaCO2 is normal
35
Q

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
36
Q

Characteristics of hypoventilation

A
  • Hypoxaemia shows good response to oxygen therapy
  • A-a gradient is normal
  • PaCO2 is high
37
Q

Describe 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

Module 103: The Heart, Lungs and Blood (51 decks)

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