ARTERIAL BLOOD GASES Flashcards
What is respiratory failure?
Failure of the respiratory system to maintain adequate gas exchange
Whats the difference between hypoxia and Hypoxaemia?
Hypoxia is lack of oxygen in tissues whereas Hypoxaemia is lack of oxygen in blood
What is normal PaO2?
11-13KPa
Should be >10 on room air
If on O2 it should be 10KPa less than %inspired concentration FiO2
What is type 1 and type 2 respiratory failure?
Type 1 - Hypoxaemia
Type 2 - Hypoxaemia and hypercapnia
What causes type 1 respiratory failure?
Low V/Q - Asthma, pneumothorax, pneumonia, HF
High V/Q - PE
What causes type 2 respiratory failure?
Obstructive lung diseases - COPD
Restrictive lung diseases - IPF
Depression of respiratory centre - opiates or TBI
Respiratory muscle weakness. Guillan-barre syndrome or motor neurone disease
Decreased lung compliance - rib fractures or obesity
Whats the aetiology of type 1 respiratory failure?
Ventilation-perfusion mismatch
Whats the aetiology of type 2 respiratory failure?
Alveolar hypoventilation
The lungs fail to effectively oxygenate and blow off CO2
What causes hypocapnia?
Pain, anxiety attack, iatrogenic e.g. ventilating a pt too much
Whats the problem with hypocapnia?
It causes vasoconstriction and cerebral ischaemia
Whats the problem with hypercapnia?
It increases cerebral blood flow and volume which increases ICP
How do we manage type 1 respiratory failure?
Mechanical ventilation
How do we manage type 2 respiratory failure?
Non-invasive ventilation - BiPAP
How do type 1 and type 2 respiratory failure typically present differently?
Type 1 likely to be agitate
Type 2 more likely to have lowered GCS and so are barely rousable
What are the issues with pulse oximetry?
Requires reasonable circulation to the finger i..e wont work if pt is cold or in shock
Foetal Hb, meth Hb and carboxy Hb wont show
Motions e.g. seizures or AF will affect results
Imaging or surgery dyes can affect it
High ambient lighting e.g. in jaundice treatment of neonates can affect it
Nail varnish can impact the results
What causes the oxygen dissociation curve to shift to the left?
Low CO2, high pH and low temps
How should you interpret an ABG?
- PH
- PaCO2 (does it make sense i..e if acidosis is it high - if so then resp cause?)\
- paO2 - type 1 or type 2 resp failure if any?
- HCO3- (does this make more sense to pH? Or is there compensation)
- Base excess (should be same as HCO3-)
- Anion gap?
How are resp and metabolic compensation different?
Resp acts much faster to changes in pH (sec-mins) whereas metabolic can take hours or days to compensate
What does it mean if CO2 and HCO3- move in different directions?
Likely mixed
How is lactate a prognostic factir?
The higher it is the worse the prognosis
What causes raised lactate?
Impaired tissue oxygenation, tissue hypoperfusion
How do you calculate the anion gap?
(Concentration of Na+ and K+)- (concentration of Cl- and HCO3-)
Whats a normal anion gap?
8-14mmol/L
What can cause a normal anion gap metabolic acidosis?
GI bicarbonate loss e.g. diarrhoea
Renal tubular acidosis
Drugs - acetazolamide (CA inhibitor)
Ammonium chloride injection
Addisons disease
What causes raised anion gap metabolic acidosis?
Lactic acidosis
Ketones - DKA and alcoholics
Urate - renal failure
Acid poisoning - salicylates and methanol
5-oxoproline overdose e.g. chronic paracetemol ingestion
