Arterial Blood Gases Flashcards
How are blood gases measured?
Using a heparinised blood sample from the radial artery.
Measures inspired [O2], PaO2, PaCO2, arterial pH, and HCO3 levels.
What considerations must be made when measuring blood gases?
A heparinised blood gas syringe expels most heparin, since it is acidic.
The skin may be anaesthetised for comfort.
Expel any excess gas bubbles.
Heparinise the sample to avoid clotting.
What are typical values of arterial blood gases?
PaO2 = 10-12.
PaCO2 = 4.5-6.
pH = 7.35-7.45.
HCO3 = 23-27.
What do changes in arterial pH cause?
Low = acidosis. High = alkalosis.
Due to respiratory / metabolic factors.
What do changes in PaCO2 cause?
High = a feature of respiratory acidosis.
Low = a feature of respiratory alkalosis.
What causes abnormal PaCO2?
Primary - inadequate ventilation.
Secondary - metabolic acidosis for some reason unconnected with the lungs. The subject hyperventilates to try and restore arterial pH.
What do changes in HCO3 cause?
High = feature of metabolic alkalosis.
Low = feature of metabolic acidosis.
What causes abnormal HCO3?
Primary - acid accumulates in diabetes.
Secondary - respiratory acidosis occurs, due to inadequate ventilation. The kidneys retain HCO3 to try and restore arterial pH.
What does a low PaO2 result in?
A failure of the lung to take up enough O2.
Describe breathing during sleep.
The normal stimuli to respiration and the tone of the upper airway are reduced.
This may be accentuated by medications or alcohol.
What happens in sleep apnoea syndromes?
Sleep breathing changes are exaggerated.
Subjects undergo repeated periods where ventilation is impaired during sleep.
What do sleep apnoea syndromes cause?
PaO2 falls. Quality of sleep is impaired.
Chronic - hypertension can occur.
What is the most common form of sleep apnoea?
Obstructive sleep apnoea.
The upper airway becomes obstructed. Diaphragm and intercostal movement increase until the obstruction is overcome. This cycle repeats many times per hour.
What makes sleep apnoea syndromes more common?
Obese people.
People with short, thick necks.
People with a posterior / poorly developed lower jaw.
How are sleep apnoea syndromes detected?
Nocturnal sleep monitoring.
O2 sats are monitored and respiratory effort is measured by flow detectors worn at the nose, or coils placed around the chest.
