Arterial blood gases Flashcards
Why do an arterial blood gas?
Acid base balance
Ventilatory status
Write the equation that shows why CO2 is a volatile acid
Why is CO2 not an acid but acts like one?
It acts like one because when CO2 goes up the pH goes down (via the production of H2CO3).
Carbonic acid is the actual acid, in terms of the substance able to donate a H+
Which acid makes up the most produced by the body?
Among all the acids produced in our body, > 90% (in terms of moles) is CO2.
What happens when CO2 elimination is insufficient?
When CO2 elimination is insufficient, retained CO2 will drive the equation to the right, thereby increasing [H+] and decreasing the pH.
What is respiratory acidosis?
CO2 is called a volatile acid, and the build up or “retention” of CO2 is called RESPIRATORY ACIDOSIS.
What are fixed acids?
“Fixed” or non-volatile acids are products from the oxidation of dietary substrates.
Have to be physically eliminated from the body, typically via the kidneys or liver (where lactate is converted to glucose)
If we make so many acids (volatile + fixed) every day, why isn’t our pH low?
Buffers
3 most important buffers in body -
- Bicarbonate
- Proteins circulating in the blood
- Phosphates
What is the pH of our blood?
7.4
How do the kidneys eliminate fixed acid?
How do we detect an abnormal accumulation of fixed acids?
The anion gap
What does the anion gap measure?
Fundamental principle of biochemistry: for electroneutrality, number of cations = number of anions
What ions are included in the anion gap?
Na + K = 144 mEq/L
Cl + bicarb = 128 mEq/L
There are more uncounted anions than uncounted cations. The uncounted anions minus the uncounted cations is called the ANION GAP.
Which of these is the ion gap?
How is the anion gap calculated?
Two equation options, but the second is most commonly used:
- Anion gap = (Na + K) – (Cl + bicarbonate)
- Anion gap = Na – (Cl + bicarbonate)
Normal AG with this equation = 12
K+ concentration so small it’s not always included in calculation.
What are cations?
Cations = Na+ and K+ plus some uncounted cations
What are anions?
Anions = Cl- and bicarbonate plus some uncounted anions
What is the acronym for the main causes of anion gap acidosis?
What are the most common causes?
- Glycols (ethylene and propylene)
- Oxoproline
•L-lactate
- D-lactate
- Methanol
- Aspirin
•Renal failure
•Ketoacidosis
“GOLD MARK”
Why does the anion gap happen?
What are the 2 catagories of metabolic acidosis?
- Addition of acid (anion gap acidosis)
- Loss of bicarbonate (non anion gap acidosis)
Interpret each anion gap result
Both metabolic acidosis -
- Addition of acid (anion gap acidosis)
- Loss of bicarbonate (non anion gap acidosis)
Where can loss of bicarbonate occur?
In kidney’s or in the pancreatic ducts in the gut
What are the 4 ways loss of bicarbonate can occur?
- Renal tubular acidosis (RTA) - all types result in urinary loss of bicarbonate and a hyperchloremic acidosis
- GI losses
- Acetazolamide
- Excessive chloride administration (intravenous fluids with NaCl)
What is the 6 step approach to analysing an ABG?
- Step 1: Examine the pH, PCO2 and HCO3 –. If they are abnormal:
- Step 2: Determine the primary process. Does the patient have an acidaemia or alkalaemia based on the pH? If so, what type is it?
- Step 3: If a metabolic acidosis is present, calculate the anion gap
- Step 4: Identify the compensatory process
- Step 5: Determine if a mixed acid-base disorder is present
- Step 6: Determine the cause
How would you carry out step 1 - examine the pH, PCO2 and HCO3?