respiratory failure Flashcards
what is hypoxia?
reduced level of tissue oxygenation
what is hypoxaemia?
decrease in partial pressure of oxygen in the blood
why do hypoxia and hypoxaemia not always coexist?
individuals can develop hypoxaemia without hypoxia is there is a compensatory
what is arterial oxygen tension?
the partial pressure of oxygen that indicates the dissolved oxygen in plasma (not oxygen bound to Hb)
what measures hypoxia and hypoxaemia?
uses an arterial blood gas analyser
what law does pulse oximetry use?
Beer-Lambert-Bougeur law
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 reduction in PaO2 - underventilation
what are the 5 mechanisms of hypoxaemia?
1) V/Q mismatch
2) right-to-left shunt
3) diffusion impairment
4) hypoventilation
5) low inspired pO2
what is the A-a gradient?
the difference between the alveolar O2 level and the arterial O2 level
what does the A-a gradient indicate?
the integrity of the alveolocapillary membrane and the effectiveness of gas exchange
if hypoxaemia has a widened A-a gradient what is it caused by?
V/Q mismatch, diffusion limitation and shunt
if hypoxaemia has a normal A-a gradient what is it caused by?
hypoventilation
what is the most common cause of hypoxemia?
V/Q mismatch
what is regional heterogeneity of V/Q?
sub atmospheric intrapleural pressure and gravity
how does V/Q mismatch cause hypoxaemia?
- ventilation and perfusion are higher at the bases and lower at the apex
- V/Q ratio is higher at the apex and low at the base
- a low V/Q ratio produces hypoxemia by decreasing the PAO2 and subsequent PaO2
- hypoxemia is caused if the compensatory rise in total ventilation is absent
what is the compensatory mechanism of a V/Q mismatch?
there is a restriction in perfusion in areas of the lung with reduced ventilation known as hypoxic pulmonary vasoconstriction
what are the characteristics of a V/Q mismatch?
- hypoxaemia due to V/Q mismatch can easily be corrected by supplemental oxygen
- widened A-a gradient
what are the common causes of a V/Q mismatch?
asthma, COPD, bronchiectasis, cystic fibrosis, ILD and pulmonary hypertension
what is a shunt?
blood from the right side of the heart enters the left side without taking part in case exchange
what feature of a shunt distinguishes it from other mechanisms?
there is a poor response to oxygen therapy
what fraction does the shunt have to reach for hypoxemia to be more common?
50%
why is there a lack of hypercapnia in a shunt?
due to stimulation of the respiratory centre by chemoreceptors
what are the characteristics of a pulmonary shunt?
- A-a gradient is elevated
- pCO2 is normal
- poor response to oxygen therapy
what are the common causes of a shunt?
pneumonia, pulmonary oedema, ARDS, pulmonary arteriovenous communication
what is diffusion limitation?
transport across the alveolocapillary membrane is impaired due to a decrease in lung surface area for diffusion, inflammation and fibrosis, low alveolar oxygen and reduced capillary transit time
why is hypercapnia uncommon with diffusion limitation?
CO2 is 20x more soluble in 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 is the hallmark of hypoventilation?
high PaCO2
when does hypoventilation cause significant hypoxaemia?
in the presence of lung disease
how do you correct hypoxaemia due to hyperventilation?
oxygen therapy however hypoventilation and hypercapnia can persist
what are the causes of hypoventilation?
- impaired central drive: drug overdose, brain stem infarction, primary alveolar hypoventilation
- spinal chord: ALS
- nerve: Guillan-Barre syndrome
- neuromuscular junction: Myasthenia graves
- respiratory muscles: myopathy
what are the characteristics of hypoventilation?
- hypoxaemia shows good response to oxygen therapy
- A-a gradient is normal
- PaCO2 is high
