Hypoxia and Respiratory Failure Flashcards
What are the two types of respiratory failure and how are they classified
Type 1 respiratory failure - low pO2, normal or low pCO2
Type 2 respiratory failure - low pO2, high pCO2
Why can giving O2 to treat type 2 respiratory failure worsen the patients conditions
Giving oxygen removes the pulmonary hypoxic vasoconstriction, increasing perfusion of poorly ventilated alveoli -> directs blood away from better ventilated alveoli
Giving O2 removes stimulus for hypoxic respiratory drive meaning alveolar ventilation drops -> worsens hypercapnia
What can hypoxia be caused by
Low inspired pO2
Hypoventilation
Ventilation/perfusion mismatch
Diffusion defect
Right to left shunt
How does a low pO2 cause hypocapnia
Low inspired pO2 level stimulates peripheral chemoreceptors causing hyperventilation
This increases CO2 washout
Results in low pO2 and low pCO2
How does hypoventilation result in hypoxia
Lung becomes poorly ventilated so there is decreased alveolar ventilation
Alveolar pO2 falls -> arterial pO2 falls -> hypoxia
Alveolar pCO2 rises -> arterial pCO2 rises -> hypercapnia
Causes type 2 respiratory failure
What are some causes of hypoventilation
Trauma to cervical spinal cord or head
Opiates
Myopathy
MND
Obesity
Kyphoscoliosis
Myasthenia gravis
Pneumothorax
Lung fibrosis
Pleural effusion
Respiratory distress of new born
Late stage COPD
Head injury
What are the effects of hypoxaemia
Impaired CNS function - confusion and irritability
Cyanosis - central and/or peripheral
Cardiac arrhythmias
Hypoxic pulmonary vasoconstriction which may cause: pulmonary hypertension, right heart failure, cor pulmonale
What are the effects of hypercapnia
Respiratory acidosis
Impaired CNS function - drowsiness, confusion, coma, flapping tremors
Peripheral vasodilation - warm hands, bounding pulse
Cerebral vasodilation
How does a ventilation/perfusion mismatch result in hyperventilation
In conditions where some alveoli are poorly ventilated, affected alveoli will have a V/Q < 1 -> alveolar pO2 falls while pCO2 rises
This causes hypoxic vasoconstriction -> diverts blood to better ventilated areas
Will then have mixing of blood from affected and non-affected alveoli resulting in a lower arterial pO2 and high pCO2
This stimulates central chemoreceptors causing hyperventilation
How does a ventilation/perfusion mismatch cause type 1 respiratory failure
V/Q ratio < 1 stimulates hyperventilation in both affected and un-affected alveoli
O2 content of the blood from un-affected alveoli does not significantly increase as there is only a tiny amount of extra dissolved O2
However, this is insufficient to compensate for low pO2 from segments with V/Q < 1
Hyperventilation is able to decrease pCO2
This results in a low pO2 with a normal/low pCO2 -> type 1 respiratory failure
What causes a mismatched V/Q ratio
Occurs in disorders where some alveoli are poorly ventilated
Asthma
Pneumonia
RDS in new-borns
Pulmonary oedema
Pulmonary embolism
How does a diffusion defect cause hypoxia and what type of respiratory failure is it
In diffusion defects there is a problem with the alveolar capillary membrane -> poor diffusion across alveolar membrane
Results in type 1 respiratory failure as CO2 diffusion is less affected than O2 diffusion -> low pO2 but normal/low pCO2
Name some diffusion defects that result in hypoxia
Fibrotic lung disease - thicker alveolar membrane
Pulmonary oedema - fluid in interstital space increases diffusion distance
Why can type 1 respiratory failure lead to type 2 respiratory failure
Over time a patient’s ventilation rate can decrease due to exhaustion or another factor
This means alveolar ventilation is no longer sufficient to remove CO2 -> pCO2 rises
So there is a low pO2 and a high pCO2 -> type 2 respiratory failure
