Theme 4: Lecture 4 - Respiratory failure Flashcards
Hypoxia
Reduced level of tissue oxgyenation
Hypoxaemia
Decrease in the partial pressure of oxygen in the blood
Why do hypoxia and hypoxaemia not always coexist
- 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
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
Arterial blood gas analyser
What is arterial O2 saturation
Percentage of haemoglobin saturated with O2
How is arterial O2 saturation measured
Measured with pulse oximeter and arterial blood gas analyser (pulse oximetry = the 5th vital sign)
Which law does pulse oximetry use
Beer-Lambert-Bougeur law
Beer-Lambert-Bougeur law
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
What is respiratory failure
clinical term used to describe the failure to maintain oxygenation
Type 1 respiratory failure
reduction in PaO2 but no change in PaCO2 – V/Q mismatch
Type 2 respiratory failure
increase in PaCO2 and a reduction in PaO2 - underventilation
Hypercapnia
Abnormally elevated CO2 levels in the blood
Algorithm to review 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 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
What are the 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)
- A-a gradient = PAO2 - PaO2
What does the A-a gradient indicate
indicates the integrity of the alveolocapillary membrane and the effectiveness of gas exchange
What widens the A-a gradient
- Pathology of the alveolocapillary unit widens the gradient
- Hypoxaemia caused by V/Q mismatch, diffusion limitation and shunt widen the gradient
What results in a normal A-a gradient
Hypoxaemia caused by hypoventilation
Hypoxic pulmonary vasoconstriction
- Compensatory mechanism
- there is a restriction in perfusion in areas of the lung with reduced ventilation
What is the most common cause of hypoxaemia
V/Q mismatch
V/Q mismatch
- 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
What does a low V/Q ratio cause
A low V/Q ratio produces hypoxaemia by decreasing the PAO2 and subsequent PaO2
What do you think of when you see a high V/Q ratio
PE
High V/Q ratio
- 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
Characteristics of V/Q mismatch
- Hypoxaemia to to V/Q mismatch can be easily corrected by supplemental oxygen
- Widened A-a gradient
Common causes of V/Q mismatch
asthma, COPD, bronchiectasis, cystic fibrosis, ILD and pulmonary hypertension
Bronchiectasis
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
Describe pulmonary shunt
- 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%
What is a feature that distinguishes pulmonary shunt from other mechanisms
Poor response to oxygen therapy
Why is there a lack of hypercapnia in pulmonary shunt
Due to stimulation of respiratory centre by chemoreceptor
Characteristics of pulmonary shunt
- A-a gradient is elevated
- pCO2 is normal
- Poor response to oxygen therapy
Common causes of shunt
pneumonia, pulmonary oedema, ARDS (acute respiratory distress syndrome), pulmonary arteriovenous communication
Describe diffusion limitation
- 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
Characteristics of diffusion limitation
- Hypoxaemia shows a good response to oxygen therapy
- A-a gradient is elevated
- PaCO2 is normal
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
Characteristics of hypoventilation
- Hypoxaemia shows good response to oxygen therapy
- A-a gradient is normal
- PaCO2 is high
Describe 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
