respiratory failure Flashcards
what is hypoxia
– reduced level of tissue oxygenation
what is hypoxaemia
– decreased pp of oxygen (PaO2) in blood
does hypoxia and hypoxaemia always co exist
Don’t always co-exist – can develop hypoxaemia without hypoxia if there a compensatory mechanism
Compensatory increase in Hb or CO (eg COPD)
In cyanide poisoning cells can’t utilise O2 despite having normal blood and tissue oxygen levels
how is arterial oxygen tension used to measure hypoxaemia
PaO2 (arterial oxygen tension) – pp of oxygen indicates dissolved oxygen in plasma (noy bound to Hb)
Measured with arterial blood gas analyser
how is arterial oxygen saturation used to measure hypoxaemia
Arterial oxygen saturation (SaO2) – percentage of Hb saturated with O2
Measured with pulse oximeter and arterial blood gas analyser
how is pulse oximetry used to measure hypoxaemia
Pulse oximetry = 5th vital sign, uses Beer-Lambert-Bougeur law which states that the attenuation of light depends on the properties of materials through which light is travelling (finger probe and light emitting diode, nail varnish can nullify or lowered by pigmentation)
what is respiratory failure
Clinical term to describe failure to maintain oxygenation
Type 1 – reduced PaO2 but no change in PaCO2 – V/Q mismatch
Type 2 – increase PaO2 and reduced PaCO2 – underventilation
what are the mechanisms of hypoxaemia
V/Q mismatch Right to left shunt Diffusion impairment Hypoventilation Low inspired pO2
what does the A-a gradient indicate
the integrity of the alveolocapillarey membrane and the effectiveness of gas exchange – pathology of alveolocapillary unit widens the gradient
Eg Hypoxaemia caused by V/Q mismatch, diffusion limitation and shunt widen the gradient
Hypoxaemia by hypoventilation have a normal gradient
what is V/Q mismatch in relation to hypoxaemia
Most common
aka Regional heterointegrity of V/Q – subatmospheric intrapleural pressure and gravity
Ventilation and perfusion higher at bases and lower at apex
A low V/Q ratio produces hypoxaemia by decreasing PAO2 and subsequent PaO2
Compensatory mechanism – restriction in perfusion in areas of reduced ventilation (hypoxic pulmonary vasoconstriction)
what does a high V/Q ratio indicate
Ventilation in excess of perfusion eg PE (less perfusion so high ratio)
Hypoxaemia is caused if compensatory rise in total ventilation is absent
what are characteristics of V/Q mismatch
Hypoxaemia due to mismatch can be corrected by supplemental oxygen
Wide A-a gradient
Common causes are asthma, COPD, bronchiectasis, CF, ILD and pulmonary hypertension
what is a shunt
Blood from the right side of the heart enters the left side with no gas exchange
Extreme degree of V/Q. mismatch with no ventilation
Hypoxaemia is uncommon in shunt until fraction reaches 50%
Lack of hypercapnia is due to stimulation of the respiratory centre by chemoreceptor
what are characteristics of pulmonary shunt
A-a gradient elevated
pCO2 normal
poor response to oxygen therapy
common causes – pneumonia, pulmonary oedema, ARDS, pulmonary arteriovenous communication
what is diffusion limitation
transport across alveolocapillary membrane impaired (decrease in lung SA for difussion, inflammation and fibrosis, low alveolar oxygen and reduced capillary transit time)
since O2 and CO2 tranfer across this membrane theoretically it should cause hypercapnia
uncommon – CO2 is 20x more soluble than O2 and is less likely to be affected by diffusion limitation
