Flashcards in 16. Respiratory failure Deck (22):
Define respiratory failure
A defect in pulmonary gas exchange leading to hypoxia, with or without hypercapnia
PaO2 less than 8kPa (60mmhg)
PaCO2 more than 6.7kPa (50mmHg)
Deficiency in the amount of oxygen reaching the tissues
Abnormally elevated CO2 levels in the blood
Give and define the different types of hypoxia
Hypoxic hypoxia - PaO2 reduced
Anaemic hypoxia - reduced haemoglobin
Stagnant hypoxia - blood flow is reduced
Histotoxic hypoxia - oxygen cannot be used
What are the two different types of respiratory failure?
Type 1 - hypoxic with normal or low PaCO2
Type 2 - Raised PaCO2 with or without hypoxia
Type 1 - hypoxia and hypocapnia
Type 2 - hypoxia and hypercapnia
Describe type 1 respiratory failure
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
What is the cause of type 1 respiratory failure?
Due to ventilation/perfusion mismatch
The lung is perfused and not ventilated or the lung is ventilated and not perfused
The provision of air i.e. O2 to the lungs
The provision of blood to the lungs
What can cause the ventilation/perfusion mismatch? (airways and 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
What other causes may there be for ventilation/perfusion mismatch?
Reduced chest expansion due to muscular weakness e.g. the diaphragm
Stiff lungs with reduced compliance
Stiff chest wall with reduced compliance
What diseases can lead to a ventilation/perfusion mismatch?
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
What can result from reduced perfusion of the lungs?
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
What is the treatment of type 1 respiratory failure?
Provide the hypoxic patient with oxygen
Provide enough oxygen to bring up the PaO2
Blood gasses should be checked regularly
Where and how in the body are the oxygen and CO2 levels monitored?
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
What is the effect of CO2 on breathing and ventilation?
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)
What happens when arterial PCO2 is elevated?
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
What variations can occur in patients with regards to response to CO2 levels?
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
What is meant by type 2 respiratory failure?
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
What are the causes of type 2 respiratory failure?
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
What is the treatment for type 2 respiratory failure?
Remove the cause if possible
Use of controlled oxygen therapy
Increased ventilation with ventilatory support