Respiratory failure Flashcards
What is hypoxia?
Reduced level of tissue oxygenation
What is hypoxaemia?
Decrease in partial pressure of oxygen (PaO2) in the blood
What is Pao2?
Arterial oxygen tension (PaO2) – partial pressure of oxygen that indicates the dissolved oxygen in plasma (not O2 bound to Hb)
When do hypoxia and hypoxaemia not coexist?
- 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
How is arterial oxygen tension (PaO2) measured?
Measured by arterial blood gas analyser
What is Sao2?
Arterial oxygen saturation (SaO2) – percentage of haemoglobin saturated with O2
How is Sao2 measured?
Measured with pulse oximeter and arterial blood gas analyser
What is the 5th vital sign?
Pulse oximetry
How does pulse oximetry work?
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
What is respiratory failure?
clinical term used to describe the failure to maintain oxygenation
What is type 1 respiratory failure?
Reduction in PaO2 but no change in PaCO2 – V/Q mismatch
What is type 2 respiratory failure?
Increase in PaCO2 and a reduction in PaO2 - underventilation
What is an algorithm to analyse ABGs?
- 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 acidosis 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
What are the 5 mechanisms of hypoxaemia?
- V/Q mismatch
- Right-to-left shunt
- Diffusion impairment
- Hypoventilation
- Low inspired pO2
What is the A-a gradient?
A-a gradient is the difference between alveolar O2 level (PAO2) and the arterial oxygen level (PaO2)
What is the equation to calculate Pao2?
What is the equation to calculate the A-a gradient?
PAO2 - PaO2
What does the A-a gradient indicate?
A-a gradient indicates the integrity of the alveolocapillary membrane and the effectiveness of gas exchange – pathology of the alveolocapillary unit widens the gradient
What widens the A-a gradient?
Hypoxaemia caused by V/Q mismatch, diffusion limitation and shunt widen the A-a gradient
When will hypoxaemia have a normal A-a gradient?
Hypoxaemia caused by hypoventilation have a normal gradient
What is the most common cause of hypoxaemia?
V/Q mismatch
What is the regional hetergenity of V/Q throughout the lungs due to?
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
Ventilation and perfusion are _______ at the bases and _______ at the apex
Higher
Lower
V/Q ratio is ______ at apex and ______ at the base
Higher
Low
What does a low V/Q ratio produce?
A low V/Q ratio produces hypoxaemia by decreasing the PAO2 and subsequent PaO2
What is hypoxic pulmonary vasoconstriction?
Compensatory mechanism – there is a restriction in perfusion in areas of the lung with reduced ventilation
Which V/Q ratio does a PE cause?
- Ventilation is in excess of perfusion – think PE
- In PE – less perfusion – high V/Q ratio
When does a PE cause hypoxaemia?
Hypoxaemia is caused if the compensatory rise in total ventilation is absent
What are the characteristics of a V/Q mismatch?
- Hypoxaemia due to V/Q mismatch can be easily corrected by supplemental oxygen
- Widened A-a gradient
What are the common causes of V/Q mismatch?
asthma, COPD, bronchiectasis, cystic fibrosis, ILD and pulmonary hyptertension
What is a shunt?
Blood from the right side of the heart enters the left side without taking part in gas exchange
What causes the extreme degree of V/Q mismatch where there is no ventilation?
Shunt
What distinguishes a shunt from other mechanisms?
Poor response to oxygen therapy
When does a patient with a shunt experience hypoxaemia?
Hypoxaemia is uncommon in shunt until the shunt fraction reaches 50%
Why is there a lack of hypercapnia in shunt patients?
Lack of hypercapnia is due to simulation of the respiratory centre by chemoreceptor
What are the characteristics of a pulmonary shunt?
- A-a gradient is elevated
- pCO2 is normal
- Poor response to oxygen therapy
What are common causes of a pulmonary shunt?
pneumonia, pulmonary oedema, ARDS, pulmonary arteriovenous communication
What causes diffusion limitation?
Decrease in lung surface area for diffusion, inflammation and fibrosis, low alveolar oxygen and reduced capillary transit time
What is diffusion limitation?
Transport across the alveolocapillary membrane is impaired
What does diffusion limitation cause?
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.
What are the characteristics of diffusion limitation?
- Hypoxaemia shows a good response to oxygen therapy
- A-a gradient is elevated
- PaCO2 is normal
What are the characteristics of hypoventilation?
- 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
What are the causes of hypoventilation?
- 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
Complete the diagram on the various mechanisms of hypoxaemia
