Acid-Base + Blood Gases

Question 1

Causes of HAGMA

Answer 1

CATMUDPILES
C - Carbon Monoxide, Cyanide
A - Alcohol, Alcoholic Ketoacidosis
T - Toluene
M - Metformin, Methanol
U - Uraemia
D - Diabetic Ketoacidosis
P - Paraldehyde, Phenformin, Paracetamol, Propylene glycol
I - Iron, Isoniazid
L - Lactic acidosis (any cause)
E - Ethylene glycol
S - Salicylates

Or KULT
K - Ketones
U - Uraemia
L - Lactate
T - Toxins/Toxic alcohols

Question 2

Causes of Lactic Acidosis

Answer 2

Type A: Imbalanced oxygen supply and demand
Carbon monoxide
Shock
Severe anaemia
Severe hypoxia
Excessive oxygen demand: fever, seizure, exercise, shivering

Type B: Metabolic
“BLACK MIST”
B - Beta-2 agonists
L - Liver failure, renal failure
A - Alcohols
C - Cyanide
K - Ketoacidosis
M - Metformin
I - Inborn errors of metabolism
S - Sepsis
T - Thiamine deficiency

Question 3

Causes of NAGMA

Answer 3

USED CRAP

U - Ureteroenterostomy
S - Small bowel fistula
E - Extra chloride (NS 0.9% hydration)
D - Diarrhoea
C - Carbonic anhydrase inhibitors (acetozolamide, topiramate)
R - Renal tubular acidosis
A - Adrenal insufficiency
P - Pancreatic fistula

Mechanisms: bicarb loss vs chloride gain

Top cases: diarrhoea, normal saline, rental tubular acidosis

Question 4

Cases of a low anion gap

Answer 4

Increased cations - Ca, Mg, Li, multiple myeloma

Decreased anions - Dilution, hypoalbuminaemia

Artifactual - Bromism, iodism, propylene glycol, triglycerides

Question 5

Causes of metabolic alkalosis

Answer 5

CLEVERR
C - Contraction (volume)
L - Licorice, diuretics
E - Endocrine (Hyperaldosteronism, Bartter’s, Cushing’s, Conn’s)
V - Vomiting, NG suction (chloride loss)
E - Excess alkali (antacids, dialysis, milk-alkali syndrome)
R - Refeeding alkalosis
R - Renal bicarb retention (hypochloraemia, hypokalaemia, chronic hypercapnoea)

Usually vomiting or diuretics +/- potassium depletion (kidney retains K+ > H+)

Question 6

Causes of respiratory acidosis

Answer 6

Acute

Resp vs neuro:
Airway obstruction
Aspiration
Bronchospasm
Pulmonary disease
CNS depression
Muscle weakness

Chronic

Chronic lung disease
Neuromuscular disorders
Obesity

Question 7

Causes of respiratory alkalosis

Answer 7

CHAMPS

C - CNS disease disease (raised ICP)
H - Hypoxia (altitude, anaemia, VQ mismatch)
A - Anxiety
M - Mechanical hyperventilation
P - Progesterone, pregnancy
S - Sepsis, salicylates (+ toxins, nicotine, xanthines)

Question 8

Anion gap equation

Answer 8

AG = Na - Cl - HCO3
Normal <12

With K
AG = Na - Cl - HCO3 + K
Normal <16

Correct for hypoalbuminaemia
+2.5 mEq/L for each 10 below 40g/L

Question 9

Expected PCO2 in metabolic acidosis (Winter’s formula)

Answer 9

Expected pCO2 = 1.5 x HCO3 + 8 (+/- 2)
Lower limit of compensation ~10 mmHg

Question 10

Expected pCO2 in metabolic alkalosis

Answer 10

Expected pCO2 = 0.7 x HCO3 + 20 (+/- 5)
Upper limit of compensation ~60 mmHg

Question 11

Expected HCO3 in respiratory acidosis (acute + chronic)

Answer 11

For every 10 mmHg increase in pCO2 should increase HCO3 by:
- 1 mEq/L (acute)
- 4 mEq/L (chronic)

Question 12

Expected HCO3 in respiratory alkalosis (acute + chronic)

Answer 12

For every 10 mmHg decrease in pCO2 should decrease HCO3 by:
- 2 mEq/L (acute)
- 5 mEq/L (chronic)

Question 13

Delta gap calculation (metabolic acidosis)

Answer 13

Change in AG - change in bicarb
i.e. = (AG - 12) - (24 - HCO3)

-6 = Mixed high and normal anion gap acidosis
-6 to 6 = Only ahigh anion gap acidosis exists
over 6 = Mixed high anion gap acidosis and metabolic alkalosis

Question 14

Delta ratio calculation (metabolic acidosis)

Answer 14

Increase in anion gap (AG) divided by decrease in HCO3
i.e. = (AG-12) / (24 - HCO3)

<0.4 NAGMA
0.4 - 0.8 HAGMA+NAGMA
1-2 HAGMA
>2 HAGMA + metabolic alkalosis

Question 15

Osmolar gap calculation (+ calculated osmolality)

Answer 15

Calculated osmolality = 2[Na] + urea + glucose + (EtOH x 1.25)
Osmolar gap (OG) = measured - calculated osmolality

Normal OG <10

Question 16

Aa-gradient calculation

Answer 16

PAO2 = (713 x FiO2) - (PCO2 / 0.8)

Room air
Calculated PAO2 = 150 - (PCO2 / 0.8)

Aa gradient = PAO2 - measured PaO2 (ABG)

Normal Aa gradient
* <age/4
* <10 in the young
* <20 in the elderly
* Increased in the supine patient (due to VQ mismatch)

Gradient varies with age and FiO2:
FiO2 0.21 – 7 mmHg in young, 14 mmHg in elderly
FiO2 1.0 – 31 mmHg in young, 56 mmHg in elderly

OR
age/4 + 4 (i.e. age 40, normal Aa gradient <14)

Question 17

Sodium correction for hyperglycaemia

Answer 17

Corrected NA = measured Na + 0.3 x (glucose - 5.5)

i.e. 3mmol/L Na added for every 10mmol of glucose above 5.5

e.g Na 124, Glu 25.5; corrected Na = 124 + 0.3 * 20 = 130

Question 18

Interpretation of Aa gradient

Answer 18

Normal A-a gradient

1. Alveolar hypoventilation (elevated PACO2)
2. Low PiO2 (FiO2 < 0.21 or barometric pressure < 760 mmHg)

Raised A-a gradient

1. Diffusion defect (rare)
2. V/Q mismatch
3. Right-to-Left shunt (intrapulmonary or cardiac)
4. Increased O2 extraction (CaO2-CvO2)

Question 19

Potassium correction for pH

Answer 19

K+ corrected = [K+]measured - 0.6 ([7.4 - pH] / 0.1))