Acid Base Disorders Flashcards
Define the pH and What is the formula for pH
pH is the measure of acidity in an aqueous solution.
pH depends on the concentration of H ions
pH = - log10 [H]
log 10 is the logarithm (base 10) and [H] is the molar concentration of Hydrogen ions.
pH is dimensionless. It has no units
What is the H concentration of a person with a pH of 7.4 versus 7.0
pH 7.4: [H] = 40 nmol/L
pH 7.0: [H] = 100 nmol/L
What is the Henderson-Hasselbach equation
pH = pKa + log base/acid
pH = pKa + log [HCO3-] / [H2CO3]
Plug in the bicarbonate equilibrium equation
pH = 6.1 + log [HCO3-] / [0.03 x PaCO2]
Which entities are measured and which are calculated on a blood gas
Measured
pH
PCO2
pO2
Other acid-base related parameters are derived from these
Why is calculated bicarbonate not useful
(Siggard- anderson) Calculated from pCO2 and pH - therefore will change in the presence of either a metabolic or respiratory abnormality and not a good indicator of the presence of either.
What is Standard Bicarbonate
This is the bicarbonate value that has been CORRECTED or respiratory disturbances. Gives a more accurate account metabolic disturbance vs calculated HCO3.
i.e. it is the patients HCO3 under standard conditions:
1. pCO2 of 5.3 kPa
2. T = 37 deg C
3. Fully saturated Hb
What is the base excess
It is the amount of acid or alkali required IN-VITRO to return plasma pH to normal under standard conditions
Inadequate to describe what happens in vivo
What is Standard Base Excess
If available, it is the best indicator of a metabolic disturbance
Standardized to PCO2 of 5.3. Temp 37. Fully saturated Hb.
It is the base excess calculated for anaemic blood (Hb 5 g/dL)
–> This is more representative of whats happening in vivo.
How do you calculate the Anion Gap, what is normal and what are the causes of HAGMA/NAGMA
Anion gap = [Na + K] - [CL + HCO3]
Normal is 12 ± 4
HAGMA (CAT MUDPILES)
Carbon Monoxide, Cyanide
Aminoglycosides
Theophylline, Toluene
Methanol
Uraemia
DKA
Paraldehyde, Paracetamol
Isoniazid, Inborn errors
Lactic acidosis
Ethanol, Ethylene Glycol
Salicylates
NAGMA (HARDUP)
- HCO3 loss replaced by Cl-
- H + gain
- Cl- gain
Hyperchloraemia
Acetazolamide/Addison’s/Acidifying salts
Renal Tubular Acidosis (1, 2 and 4)
Diarrhoea, ileostomies, fistulae (Hyperalimentation)
Ureteroenterostomies
Pancreatoenterostomies
How does albumin affect the anion gap
Albumin is the major unmeasured anion that contributes almost the whole value of the normal anion gap.
every 1g/L decrease in albumin will decrease the anion gap by 0.25 mmol.
Especially NB in ICU to calculate the corrected anion gap = AG + [0.25 x (40 - Albumin)]
