12 - Respiratory Failure Flashcards
What are some examples of where tissues are hypoxic without hypoxaemia?
- Anaemia
- Poor circulation
At what oxygen saturation does tissue damage occur?
- O2 <90%
- pO2 < 8kPa
How can hypoxaemia occur?
For normal O2 need:
- Normal inspire pO2
- Normal alveolar ventilation
- V/Q = 1
- Normal alveolar capillary membrane
- Cardiac output through lungs
What are some effects of hypoxaemia in the body?
- Impaired CNS function (confusion, agitation)
- Central cyanosis (mucous membranes >50gm/L of unsaturated Hb)
- Cardiac arrhythmias
- Hypoxic vasoconstriction
What is type 1 and type 2 respiratory failure?
Type 1: low pO2 (<8kPa) , normal or low pCO2
Type 2: low pO2 and high pCO2 (>6.7kPa)
Respiratory failure is when arterial pO2 falls below 8kPa when breathing air at sea level
What are some effects of hypercapnia on the body?
- Respiratory acidosis
- Impaired CNS function: drowsiness, confusion, coma, flapping tremors
- Peripheral vasodilation: warm hands, bounding pulse
- Cerebral vasodilation so headache
What are some compensatory mechanisms to chronic hypoxia and what are some issues with this compensatory mechanism?
- Increased EPO from kidney so increased Hb
- Increased 2,3 DPG to shift Hb saturation curve
- Hypoxia induced vasoconstriction of pulmonary arterioles which can lead to pulmonary hypertension, right heart failure and cor pulmonale
How can low inspired pO2 lead to hypoxia and how is this compensated?
Example and what are long term solutions?
- People acutely at high altitudes can get mountain sickness
- Hyperventilation so low pO2 and pCO2
- Chronic high altitudes compensate by polycythemia, increased capillary density, 2,3 DPG
How can hypoventilation lead to hypoxia?
- Type 2 respiratory failure (acute high pCO2 needs urgent ventilation as life threatening)
- Entire lung poorly ventilated
- Can fix pO2 by administering oxygen
How do you work out alveolar ventilation?
Alveolar volume x Respiration rate
What are some causes of hypoventilation?
- Obesity
- MD
- Polio
Why does someone with chronic type 2 respiratory failure not need to be ventilated?
- Compensate, will need to be ventilated as disease progresses though
How do you treat hypoxia in someone with chronic type 2 respiratory failure?
Controlled oxygen therapy
- 24-28% oxygen to achieve sats of 88-92%
Why do you need to give controlled oxygen therapy when a patient has hypoxia with hypercapnia (e.g COPD)
- Correcting hypoxia removes pulmonary hypoxic vasoconstriction so perfusion increased to poorly ventilated alveoli and respiratory stimulus removed
- Because the pO2 will increase so respiration rate will lower leading to hypercapnia and low blood pH
- Check bicarb and CO2 over 24-48 hours
How can a V/Q mismatch lead to respiratory failure and how is this compensated?
- Reduced ventilation: V/Q<1 in pneumonia, asthma, COPD early, RDS
- Reduced perfusion: V/Q>1 in PE causes diverted blood changing the V/Q elsewhere
Type 1 respiratory failure due to chemoreceptors causing a hyperventilation
Why doesn’t hyperventilation in a V/Q mismatch correct hypoxaemia?
- Because the haemoglobin is fully saturated
- Does correct pCO2
Type 1 Respiratory Failure (can administer oxygen)
What are some causes of V/Q mismatch?
- PE
How can a diffusion impairment lead to hypoxia?
- Thick barrier, e.g lung fibrosis and pulmonary oedema
- Low surface area
- O2 diffuses less readily than CO2 so this is impaired the most
- Type 1 respiratory failure
What are some causes of diffuse lung fibrosis?
What happens if you give someone with a shunt in the respiratory system oxygen?
- Air can’t get in so makes no difference
- Need positive pressure ventilation to adjust O2
e. g RDS there is no surfactant so alveolar atelectasis
What is the danger of type 1 respiratory failure?
- Can lead to type 2 e.g asthma exacerbation and end stage COPD
What causes of hypoxaemia can be corrected by administration of oxygen?
Shunts don’t respond to oxygen
How does the body increase bicarbonate levels when trying to compensate a respiratory acidosis?
- Kidneys retain more
- PCT makes more from amino acids
