ABG Flashcards
What is the normal range for PaCO2
4.6-6.0 kPa
What is the normal range for PaO2
10-13 kPa
What is the normal range for bicarbonate?
22-26 mmol/L
What is the normal anion gap?
8-16 mmol/L
What is the normal A-a gradient
<2.6 kPa
How do you calculate A-a gradient?
PAO2 (alveolar pressure O2) - PaO2 (arterial pressure O2)
The difference should be less than 2.6 kPa
Under what circumstances might a patient be hypoxic with a NORMAL A-a gradient?
Alveolar hypoventilation
Low PiO2 (either due to reduced air pressure, or due to reduced FiO2)
What might be the cause of a patient’s hypoxia with a RAISED A-a gradient?
V/Q mismatch
Right to left shunt
Increased O2 consumption
Diffusion defect (rare)
What does a reduced PaO2 usually suggest?
A problem with gas exchange
How do you interpret PaO2 in the context of the FiO2?
Calculate the P:F ratio:
PaO2 / FiO2
Normal P:F ratio is approximately 60 (in kPa).
e.g. healthy patient:
PaO2 of 12, divided by FiO2 of 0.21 (room air) = 57
What determines oxygen content?
Haemoglobin concentration and saturation.
What is the timeframe for chronic compensation?
Hours to days only.
Evidence of a “chronic compensation” on a blood gas doesn’t necessarily imply a longstanding disorder.
What are the two different types of metabolic acidosis?
Increased anion gap metabolic acidosis
Normal anion gap metabolic acidosis
How is anion gap calculated?
Na - Cl - HCO3
What is a normal anion gap?
8-16
e.g. Na of 140 - Cl of 100 - HCO3 of 24
= 16
What makes up most of the anion gap?
Albumin (negatively charged protein)
What causes increased anion gap metabolic acidosis?
Lactic acidosis
Ketoacidosis (diabetes, alcohol)
Renal failure
Poisonin (e.g. salicylate)
What causes normal anion gap metabolic acidosis?
GI bicarbonate loss (diarrhoea)
Renal bicarbonate loss
What is the most common cause of lactic acidosis?
Inadequate tissue perfusion.
How much can the respiratory system compensate for a metabolic acidosis?
Complete compensation does not occur.
Therefore a normal pH in a pateitn with a metabolic acidosis suggests a concomitant alkalosis.
What is the typical cause for a metabolic alkalosis?
Volume (and therefore chloride) depletion: vomiting, diuresis
Hyperadrenocoticoidism (Cushing’s, Conns, steroid therapy)
Severe K depletion
What should be corrected first: alkalosis or volume depletion?
Volume depletion.
Alkalosis will sustain until volume depletion is corrected with NaCl
How do you distinguish between causes of metabolic alkalosis?
Measure urinary chloride
Urinary chloride is low when alkalosis is due to volume contraction.
Urinary chloride is high when alkalosis is due to hyperadrenocorticoidism or severe K depletion.
What can cause respiratory alkalosis?
Non-hypoxic: Anxiety, sepsis, salicylate intoxication, pregnancy
Hypoxic: pneumonia, asthma, oedema, fibrosis, high altitude
