Theme 4: Lecture 4 - Respiratory failure

Question 1

Hypoxia

Answer 1

Reduced level of tissue oxgyenation

Question 2

Hypoxaemia

Answer 2

Decrease in the partial pressure of oxygen in the blood

Question 3

Why do hypoxia and hypoxaemia not always coexist

Answer 3

• 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

Question 4

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

Answer 4

Arterial blood gas analyser

Question 5

What is arterial O2 saturation

Answer 5

Percentage of haemoglobin saturated with O2

Question 6

How is arterial O2 saturation measured

Answer 6

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

Question 7

Which law does pulse oximetry use

Answer 7

Beer-Lambert-Bougeur law

Question 8

Beer-Lambert-Bougeur law

Answer 8

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

Question 9

What is respiratory failure

Answer 9

clinical term used to describe the failure to maintain oxygenation

Question 10

Type 1 respiratory failure

Answer 10

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

Question 11

Type 2 respiratory failure

Answer 11

increase in PaCO2 and a reduction in PaO2 - underventilation

Question 12

Hypercapnia

Answer 12

Abnormally elevated CO2 levels in the blood

Question 13

Algorithm to review ABGs

Answer 13

• 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

Question 14

What are the mechanisms of hypoxaemia

Answer 14

• V/Q mismatch
• Right-to-left shunt
• Diffusion impairment
• Hypoventilation
• Low inspired pO2

Question 15

What is the A-a gradient

Answer 15

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

Question 16

What does the A-a gradient indicate

Answer 16

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

Question 17

What widens the A-a gradient

Answer 17

• Pathology of the alveolocapillary unit widens the gradient

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

Question 18

What results in a normal A-a gradient

Answer 18

Hypoxaemia caused by hypoventilation

Question 19

Hypoxic pulmonary vasoconstriction

Answer 19

• Compensatory mechanism

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

Question 20

What is the most common cause of hypoxaemia

Answer 20

V/Q mismatch

Question 21

V/Q mismatch

Answer 21

• 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

Question 22

What does a low V/Q ratio cause

Answer 22

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

Question 23

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

Answer 23

PE

Question 24

High V/Q ratio

Answer 24

• 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

Question 25

Characteristics of V/Q mismatch

Answer 25

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

Question 26

Common causes of V/Q mismatch

Answer 26

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

Question 27

Bronchiectasis

Answer 27

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

Question 28

Describe pulmonary shunt

Answer 28

- 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%

Question 29

What is a feature that distinguishes pulmonary shunt from other mechanisms

Answer 29

Poor response to oxygen therapy

Question 30

Why is there a lack of hypercapnia in pulmonary shunt

Answer 30

Due to stimulation of respiratory centre by chemoreceptor

Question 31

Characteristics of pulmonary shunt

Answer 31

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

Question 32

Common causes of shunt

Answer 32

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

Question 33

Describe diffusion limitation

Answer 33

- 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

Question 34

Characteristics of diffusion limitation

Answer 34

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

Question 35

Causes of hypoventilation

Answer 35

- 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

Question 36

Characteristics of hypoventilation

Answer 36

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

Question 37

Describe hypoventilation

Answer 37

- 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