Hypoxia and Respiratory Failure Flashcards
What is Hypoxia?
Hypoxia is oxygen deficiency at the tissue level – if it persists tissues will undergo ischaemic damage or necrosis.
What are the 4 categories of hypoxia?
Four categories of hypoxia – anaemic hypoxia is due to issues with the number or function of haemoglobin. Hypoxaemia hypoxia is due to some sort of respiratory failure. Stagnant hypoxia is due to poor perfusion. Cytotoxic hypoxia is when the cells are unable to utilise the oxygen.
What is respiratory failure?
Respiratory Failure is when not enough oxygen enters the blood OR not enough CO2 leaves the blood (not necessarily both at the same time).
What are the normal levels of O2 saturation and pO2 of oxygen?
Normal range of O2 saturation is 94-98% and pO2 11.1-14.4kPa. Any levels below this is hypoxia but tissue damage doesn’t occur until below 90% saturation and pO2<8kPa.
What is type 1 respiratory failure and what physiological defect causes it?
pO2 of arterial blood is below 8kPa but pCO2 is normal
Caused by poor diffusion across alveolar membrane or mismatching of ventilation and perfusion.
What diffusion defects can cause type 1 respiratory failure and how does it effect oxygen and carbon dioxide?
Increased thickness of the lung such as in fibrosis.
Available surface area is reduced such as emphysema.
Rate of diffusion of oxygen is less than that of CO2. In diseases causing diffusion impairment O2 is affected but Co2 isn’t. Hypoxia stimulates peripheral chemoreceptors, resulting in hyperventilation which further increases CO2 removal hence pCO2 may be lower than normal.
How does Ventilation-perfusion mismatching cause type 1 respiratory failure?
Normal V/Q ratio is about 1. When the ratio is < 1 the alveolar pO2 falls and pCO2 rises. The low alveolar pO2 hypoxic vasoconstriction to divert blood to better ventilated areas but some blood continues to perfuse these alveoli.
The blood diverted to perfuse other areas of the lung will get rid of lots more CO2 but the pO2 will not rise much above normal because although there will be an increase in ventilation of these areas the blood is mostly saturated already so can’t change much.
When perfusion decreases such as due to PE this results in a V/Q ratio that is greater than 1. As a result blood is diverted to the rest of the lung. This means that most the lung now has a V/Q ratio that is less than one so you become hypoxic.
What causes ventilation-perfusion mismatching?
Ventilation-perfusion mismatching can be due to either reduced ventilation or reduced perfusion such as with a PE. Reduced ventilation can be due to: Pneumonia, acute asthma and RDS of a new-born. Net result is that you become hypoxic but because pCO2 doesn’t rise it is a type one respiratory failure.
What is type 2 respiratory failure and what physiological defect causes it?
pO2 of arterial blood is low , 8kPa and pCO2 is high (above 6.4kPa)
This is caused by Ventilatory failure (pump failure) – unable to move sufficient air in and out of the lungs.
What are the acute effects of type 2 respiratory failure?
pCO2 rises and pO2 falls. Central chemoreceptors are stimulated. Breathlessness due to some compensation by resultant increase in ventilation but due to disease you are unable to completely correct deficit. High CO2 causes respiratory acidosis. Acute type 2 usually needs assisted ventilation.
What is a flail segment?
Flail segment = multiple ribs fractured in 2 places. During inspiration, the segment gets sucked inwards preventing adequate lung expansion.
Discuss some causes of type 2 respiratory failure?
Brain stem - opiates and head injury
Spinal cord - trauma
Phrenic and intercostal nerve damage
Chest wall - severe obesity, kyphoscoliosis and flail segments
Pleural cavity - PNX, pleural effusions
Poor lung compliance - RDS, lung fibrosis
High airway resistance - severe acute asthma, late stages of COPD
How is respiratory failure managed?
Treat the cause
Oxygen therapy improves hypoxia in type 1 respiratory failure because it increases alveolar pO2 which improves oxygenation. This improves partial pressure gradient in diffusion defects and improves O2 uptake in areas with V/Q ratio < 1.
In type two respiratory failure the problem of hypercapnia often requires assisted ventilation (particularly if acute).
How can hypoxia present?
Hypoxia may present as an exercise intolerance, tachypnoea (may be perceived as breathlessness), confusion due to cerebral hypoxia and central cyanosis.
What is cyanosis and how does it present depending on the severity?
Purplish discolouration of skin and mucous membranes due to the colour of desaturated Hb. It becomes present when >50mg/L of desaturated Hb in the blood.
Central cyanosis indicates arterial hypoxia SaO2 < 85% and is seen in the tongue and oral mucosa. Naturally accompanying this is peripheral cyanosis.
Peripheral cyanosis occurs in the finger tips, toes, ear lobes and outer lips. Can occur without central cyanosis such as in peripheral vascular disease, cold climate.
