11 - Acid Base 2

Question 1

42 year old semi-comatose with seizures
normal O2
acidaemia 
low pCO2
low HCO3-
-ive BE

Answer 1

Metabolic acidosis with resp compensation
Anion gap later shown to be very large suggesting unmeasured protein OR acid i.e. lactate-.
Suspected ethylene glycol poisoning (provided the unmeasured anions) and was treated with an ethanol infusion.
The ethanol competitively inhibits the enzyme alcohol dehydrogenase that metabolises ethylene glycol into the toxic glycolic and oxalic acids.

Question 2

9 year old presents with growth delay. Rickets.
Raised Cl-
Low ph
Low pCO2
Low HCO3-
-ive BE
Normal AG

Answer 2

Metabolic normal AG acidosis with resp compensation. The increased Cl also suggests normal AG acidosis/hyperchloremic acidosis)

Next step - is it a renal or non-renal cause? I.e. is the kidney acidifying the urine like it should be?

Test urine pH and urine ammonia. If the urine pH is HIGH and urine ammonia is LOWER than expected than it is a renal cause.

Also check the URINE anion gap:

Question 3

acidosis is associated with …. alkalosis is associated with…

Answer 3

Hyperkalaemia

Hypokalaemia

Question 4

What are the exceptions to the acidosis and hyperkalaemia rule?

Answer 4

1. Diarrhoea as there is loss of both HCO3- and K+

2. Renal tubular acidosis where there is hypokalaemia

Question 5

Muscle weakness. No periods for a year.
Hypokalaemia
Low CL-
High pH
High HCO3-
\+ive BE

DENIED any history of vomiting

Answer 5

Metabolic alkalosis with chloride depletion

Urine electrolytes showed low Cl-

Found that she was actually vomiting and using laxatives (causing both H+ and Cl- loss as well

Question 6

Pathogenesis of metabolic alkalosis with chloride depletion?

Answer 6

1. Loss of HCl due to vomiting causing alkalosis
2. Hypokalaemia is due to both the alkalosis and laxative use
3. The hypokalaemia is causing the muscle weakness as it hyperpolarises cells

Question 7

Arterial puncture?

Answer 7

painful and occasionally has severe complications like thrombosis.

Question 8

When would you use venous blood for blood gases?

Answer 8

In patients with reasonable perfusion, pH, bicarbonate and BE are almost identical in venous and arterial blood. Can be used when pO2 is not wanted (i.e. not worried about a resp disorder) and severe circulatory failure is not present. May use in metabolic acidoses like DKA, renal acidosis, poisoning

Question 9

Blood measurement artifacts?

Answer 9

If there is air in a blood-gas syringe it can give a low pCO2 and so an apparent resp alkalosis. If there is delayed separation of plasma from the RBCs then the RBCs produce lactic acid > high AG > metabolic acidosis

Question 10

EXAM Q
Baby with pyloric stenosis. Severe vomiting.
Hypokalaemia
High HCO3-

What is the primary cation and anion in gastric juice

Answer 10

H+ and Cl-

Question 11

EXAM Q
Baby with pyloric stenosis. Severe vomiting.
Hypokalaemia
High HCO3-

Why is the K+ low?

Answer 11

Due to the alkalosis - shifts of H+ and K+ into and out of cells, and H+ and K+ compete with each other for secretion

Question 12

EXAM Q
Baby with pyloric stenosis. Severe vomiting.
Hypokalaemia
High HCO3-

What acid-base disorder?

Answer 12

Metabolic alkalosis - chloride responsive (volume depleted)