16. Respiratory failure

Question 1

Define respiratory failure

Answer 1

A defect in pulmonary gas exchange leading to hypoxia, with or without hypercapnia

PaO2 less than 8kPa (60mmhg)
PaCO2 more than 6.7kPa (50mmHg)

Question 2

Define hypoxia

Answer 2

Deficiency in the amount of oxygen reaching the tissues

Question 3

Define hypercapnia

Answer 3

Abnormally elevated CO2 levels in the blood

Question 4

Give and define the different types of hypoxia

Answer 4

Hypoxic hypoxia - PaO2 reduced
Anaemic hypoxia - reduced haemoglobin
Stagnant hypoxia - blood flow is reduced
Histotoxic hypoxia - oxygen cannot be used

Question 5

What are the two different types of respiratory failure?

Answer 5

Type 1 - hypoxic with normal or low PaCO2

Type 2 - Raised PaCO2 with or without hypoxia

TUTOR NOTES:
Type 1 - hypoxia and hypocapnia
Type 2 - hypoxia and hypercapnia

Question 6

Describe type 1 respiratory failure

Answer 6

Characterised by PaO2 reduced to below 8kPa

Reduced PaO2 leads to increased ventilation
Increased ventilation leads to reduced PaCo2

Caused by the reduced passage of O2 into the blood

Question 7

What is the cause of type 1 respiratory failure?

Answer 7

Due to ventilation/perfusion mismatch

The lung is perfused and not ventilated or the lung is ventilated and not perfused

Question 8

Define ventilaiton

Answer 8

The provision of air i.e. O2 to the lungs

Question 9

Define perfusion

Answer 9

The provision of blood to the lungs

Question 10

What can cause the ventilation/perfusion mismatch? (airways and alveoli)

Answer 10

Obstructed airways
Collapsed alveoli
Collapsed alveoli due to pressure from pleural fluid in pleural effusions
Alveoli full of fluid in LVF
Alveoli full of inflammation in pneumonia
Alveoli with thickened walls in fibrosis

Question 11

What other causes may there be for ventilation/perfusion mismatch?

Answer 11

Reduced chest expansion due to muscular weakness e.g. the diaphragm
Stiff lungs with reduced compliance
Stiff chest wall with reduced compliance

Question 12

What diseases can lead to a ventilation/perfusion mismatch?

Answer 12

Emphysema - reduced elastic recoil, reduced surface area of alveoli walls and damage to the capillaries of the lungs - means that there is a reduced local blood flow

Pneumonia - solid lung has good blood flow by reduced ventilation

Question 13

What can result from reduced perfusion of the lungs?

Answer 13

Pulmonary emboli - clots that block the pulmonary arteries
Pulmonary hypertension - pulmonary arteries become narrowed
Pulmonary vasoconstriction due to hypoxia - local hypoxia causes vasoconstriction in patients with obstructed bronchi

SO the obstructed pulmonary vessels lead to increased right heart work (have to pump the blood through the pulmonary vein against the obstruction)
Right ventricle dilates and may fail
Tricuspid valve leaks
Jugular venous pressure rises (right heart failure)
Back pressure from failing heart leads to odoema

Question 14

What is the treatment of type 1 respiratory failure?

Answer 14

Provide the hypoxic patient with oxygen
Provide enough oxygen to bring up the PaO2
Blood gasses should be checked regularly

Question 15

Where and how in the body are the oxygen and CO2 levels monitored?

Answer 15

In the carotid and aortic bodies
There are O2 sensitive K+ channels and haem based mitochondrial cytochrome enzymes that respond to the local PO2 concentration
There is then nervous output from the carotid body via the vagus nerve to the brainstem

The ventilation response from this starts within a few seconds and ventilation generally increases for 5-10 minutes
PCO2 and PH responses also occur but these are far less important than the central effects

Question 16

What is the effect of CO2 on breathing and ventilation?

Answer 16

CO2 is the most important stimulant to breathing

Most of the effect of CO2 is via central chemo-sensitive areas (only a small amount is via the carotid body)

Question 17

What happens when arterial PCO2 is elevated?

Answer 17

Elevation of arterial PCO2 increases CSF PCO2
Increased CSF PCO2 leads to increased H+ ion concentration and CSF acidosis
This increases the rate and volume of breathing, mainly within five minutes - ventilation doubles

Question 18

What variations can occur in patients with regards to response to CO2 levels?

Answer 18

The response to PCO2 is reduced during sleep and with increased age
Patients with persistently raised PCO2 become insensitive to changes with PCO2 and instead are dependent on hypoxic drive e.g. patients with COPD

Question 19

What is meant by type 2 respiratory failure?

Answer 19

Increased PaCO2 greater than 6.7kPa
If the relevant response of increased ventilation and changes in pH does not occur then the PaCO2 rises further and leads to type 2 respiratory failure

Question 20

What are the causes of type 2 respiratory failure?

Answer 20

SO is caused by decreased ventilation:

Loss of lung ventilation capacity e.g. major pulmonary emboli

Loss of chest wall ability to ventilate e.g. chest wall shape, muscle weakness, spinal cord trauma, neurological trauma

Failure of central control of respiration e.g. due to drugs, head injury, brainstem injury, acclimatisation to high PaCO2 sleep

Question 21

What is the treatment for type 2 respiratory failure?

Answer 21

Remove the cause if possible
Use of controlled oxygen therapy
Increased ventilation with ventilatory support

Question 22

What are the different types of ventilatory support that can be used on a patient?

Answer 22

Drugs to increase ventilation (but these are not very effective and have side effects): doxapram, acetaxolamide, progesterone
If the airway is compromised: use an airway, CPAP to keep airway open, intubate, trachesotomy
If the airway is okay: use BIPAP
Facemask ventilation - either up the nose or by full facemask