5. Acid-base disturbances

Question 1

what are alkalaemia and acidaemia and why are these nefarious

Answer 1

1. Alkalaemia = plasma pH > 7.45
   Decreased solubility of calcium salts… free calcium leaves ECF, binding to bone and proteins… hypocalcaemia… increased nerve excitability (as calcium ions are divalent so contribute to charge shielding which makes membranes less excitable)… paraesthesia and eventually tetany.
2. Acidaemia = plasma pH <7.35
   Increased [H+] denatures proteins… affects enzyme function in many tissues… affects muscle contractility, glycolysis and hepatic function.
   Causes K+ efflux from cells (H/K antiporter)… hyperkalaemia… affects excitability, esp. cardiac muscle… arrhythmia.

Question 2

what is the effect of diabetic ketoacidosis on plasma [K+]

Answer 2

Diabetic ketoacidosis can cause total K+ body depletion as K+ moves out of cells but osmotic diuresis means K+ lost in urine.

Question 3

state the acid-base status:

• decreased pH
• normal pCO2
• decreased HCO3-

Answer 3

metabolic acidosis

Question 4

state the acid-base status:

• normal pH
• increased pCO2
• increased HCO3-

Answer 4

compensated respiratory acidosis or compensated metabolic alkalosis

Question 5

state the acid-base status:

• increased pH
• decreased pCO2
• normal HCO3-

Answer 5

respiratory alkalosis

Question 6

state the acid-base status:

• normal pH
• decreased pCO2
• decreased HCO3-

Answer 6

compensated respiratory alkalosis or compensated metabolic acidosis

Question 7

state the acid-base status:

• increased pH
• normal pCO2
• increased HCO3-

Answer 7

metabolic alkalosis

Question 8

state the acid-base status:

• decreased pH
• increased pCO2
• normal HCO3-

Answer 8

respiratory acidosis

Question 9

how can one differentiate between metabolic and renal causes of metabolic acidosis

Answer 9

Anion gap (difference between measured cations and anions):

• if increased = metabolic cause
• if normal = renal cause

Question 10

suggest causes of metabolic acidosis (with normal anion gap)

Answer 10

1. severe persistent diarrhoea: loss of HCO3- (replaced by Cl- so anion gap unaltered)
2. renal tubular acidosis (rare): problems with transport mechanisms in tubules:
   - type 1 (distal) - inability to pump out H+
   - type 2 (proximal, v. rare) - probs. with HCO3- reabsorption

Question 11

suggest causes of respiratory acidosis and alkalosis

Answer 11

Resp. acidosis: hypoventilation, T2RF

Resp. alkalosis: hyperventilation (panic attack), T1RF

Question 12

suggest causes of metabolic alkalosis

Answer 12

• severe vomiting causing loss of H+

- K+ depletion/mineralocorticoid excess, e.g. loop and thiazide diuretics