arterial blood gases Flashcards
What are the normal ranges?
pH: 7.35 – 7.45 PaCO2: 4.7 – 6.0 kPa || 35.2 – 45 mmHg PaO2: 11 – 13 kPa || 82.5 – 97.5 mmHg HCO3–: 22 – 26 mEq/L Base excess (BE): -2 to +2 mmol/L
Why is a normal PaO2 for a patient on oxygen flow a bad thing?
You would expect PaO2 to be higher/above normal they are receiving oxygen.
What does a normal PaCO2 in a hypoxic asthmatic signal?
Tiring
Maybe need ITU
What does a very low PaO2 and normal everything else mean?
It’s a venous sample
Healthy oxygen?
> 10kPa
What should oxygen saturation be for someone on oxygen?
Should be 10kPa less than the concentration of the inspired oxygen.
What is considered hypoxaemic and severely hypoxaemic?
<10kPa hypoxia
<8kPa is severe
Type 1 vs type 2 resp failure?
Type one is hypoxia but normocapnia whereas, type 2 is hypoxia and hypercapnia.
What causes type 1 failure?
Ventilation perfusion mismatch, volume of air flow is not matched by blood flow. Initial increase in PaCO2 is is compensated by increased alveolar ventilation hence normocapnia
What are some examples of V/Q mismatch?
Reduced ventilation and normal perfusion (e.g. pulmonary oedema, bronchoconstriction)
Reduced perfusion with normal ventilation (e.g. pulmonary embolism)
Why does type 2 occur?
Alveolar hypoventilation
What are some reasons for alveolar hypoventilation?
Resistance due to airway obstruction - COPD
Less compliance of lung tissue and chest wall - pneumonia and rib fracture
reduced strength of resp muscles - MND
Drugs acting on resp centre - opioids
What are the two driving forces for changes in pH?
metabolic (HCO3-) and respiratory (CO2)
What is PaCO2 like for resp acidosis/alkalosis?
CO2 increased in acidosis with a decrease in pH and vice versa for alkalosis.
There is compensation when HCO3- is involved and the pH can be returned to normal/slightly abnormal.
What does a high base excess show?
Higher than normal HCO3- which indicates primary metabolic alkalosis or compensated resp acidosis and vice versa for low base excess.
How is compensation achieved for a metabolic change?
Quickly by either blowing off more CO2 or reducing respiratory rate, all with the idea of increasing alveolar ventilation (getting rid of CO2) or reducing alveolar ventilation to retain CO2.
How is a metabolic compensation different?
Takes a few days
Kidneys are required to increase/reduce HCO3- production.
What should never happen?
Overcompensation, if this is the case there is a pathology causing it.
What is mixed acidosis/alkalosis and how is it corrected?
The PaCO2 and HCO3- will be moving in opposite directions. You need to correct each disturbance individually.
What is the basis for the cause of resp acidosis?
Alveolar hypoventilation
What are some causes of resp acidosis?
Asthma, COPD, Resp depression by opiates,
Guillain Barre which is paralysis that leads to inability to ventilate.
What are some causes of resp alkalosis?
Excessive alveolar ventilation
anxiety, pain, hypoxia, PE, pnuemothorax
What is the basis of metabolic acidosis?
Increased acid production/ingestion or decreased excretion/HCO3-
What is an anion gap?
Tells you how to differentiate the causes of metabolic acidosis.
Anion gap = Na+ – (Cl- + HCO3-)
