Blood gas analysis

Question 1

What is the concentration of H+ in blood? how is this converted to pH? How are H+ levels maintained in the blood?

Answer 1

• 40nmol
• pH=-log10([H+]) or log10(1/[H+])
• intracellular (proteins, phosphate, Hb) and extracellular (proteins, HCO3) buffers

Question 2

Define BE and SBE

Answer 2

Base excess definition

• Dose of acid or base required to return the pH of a blood sample to 7.40
• Measured at standard conditions - 37°C and 40mmHg PaCO2
• Thus, isolates the metabolic disturbance from the respiratory

Standard base excess

• Dose of acid or base required to return the pH of an anaemic blood sample
• Calculated for a Hb of 50g/L
• Haemoglobin buffers both the intravascular and the extravascular fluid
• Thus, SBE assesses the buffering of the whole extracellular fluid, not just the haemoglobin-rich intravascular fluid

Question 3

What could cause a negative base excess with normal anion gap?

Answer 3

normal anion gap metabolic acidosis (e.g. acetazolide, hypercholoremia, GI losses of HCO3, renal tubular acidosis)

Question 4

What could cause a normal base excess with abnormal anion gap?

Answer 4

• lactic acidosis (or other high anion gap metabolic acidosis) with pre-existing metabolic alkalosis
• HAGMA masked by hypoalbuminemia (if anion gap is uncorrected)
• salicylate toxicity – respiratory alkalosis plus in increased anion gap metabolic acidosis

Question 5

how should an adequate PaO2 be approximated when the pt is breathing supplemental O2?

Answer 5

• PaO2 should be roughly 5x FiO2
  (PaO2 100 at FiO2 21%)
  P:F ratio of 5:1

Question 6

How is anion gap calculated?

Answer 6

Anion Gap = Na+ – (Cl- + HCO3-)

The Anion Gap (AG) is a derived variable primarily used for the evaluation of metabolic acidosis to determine the presence of unmeasured anions
The normal anion gap depends on serum phosphate and serum albumin concentrations

An elevated anion gap strongly suggests the presence of a metabolic acidosis
The normal anion gap varies with different assays, but is typically 4 to 12mmol/L (if measured by ion selective electrode; 8 to 16 if measured by older technique of flame photometry)
If AG > 30 mmol/L then metabolic acidosis invariably present
If AG 20-29mmol/L then 1/3 will not have a metabolic acidosis
K can be added to Na+, but in practice offers little advantage

ALBUMIN AND PHOSPHATE

the normal anion gap depends on serum phosphate and serum albumin
the normal AG = 0.2 x [albumin] (g/L) + 1.5 x [phosphate] (mmol/L)
albumin is the major unmeasured anion and contributes almost the whole of the value of the anion gap.
every 1g/L decrease in albumin will decrease anion gap by 0.25 mmoles
a normally high anion gap acidosis in a patient with hypoalbuminaemia may appear as a normal anion gap acidosis.
this is particularly relevant in ICU patients where lower albumin levels are common

Question 7

Causes of HAGMA

Answer 7

MUDPILES
Metformin, Methanol
Uremia
DKA
Pyroglutamic acidosis, paracetamol, phenformin, propylene glycol, paraladehyde
Iron, Isoniazid
Lactic acidosis
Ethylene glycol
Salicylates

Causes (LTKR)
Lactate
Toxins
Ketones
Renal

Question 8

Causes of NAGMA

Answer 8

NAGMA results from loss of HCO3- from ECF

Causes (CAGE)
Chloride excess
Acetazolamide/Addisons
GI causes – diarrhea/vomiting, fistulae (pancreatic, ureters, billary, small bowel, ileostomy)
Extra – RTA

Causes (ABCD)
Addisons (adrenal insufficiency)
Bicarbonate loss (GI or Renal)
Chloride excess
Diuretics (Acetazolamide)