Lecture 11: Acid-Base Balance #2 Flashcards
What is base excess?
The amount of acid or base needed to restore the pH to 7.4 (at a pCO2 of 5.3kPa) Calculated from pH, pCO2 and haemaglobin and it reflects ALL buffers in plasma (not just HCO3-)
Normal base excess is?
Normal base excess = 0 (-2 to +2)
BE + in metabolic alkalosis
BE - in metabolic acidosis
This is a patient with chronic lung disease. What kind of acid-base disturbance is this?
On admission: Respiratory acidosis with renal compensation
On Ventilator: Metabolic alkalosis (as the respiratory component is corrected, but the ‘compensating’ metabolic alkalosis remains, and takes ~3-5 days to return to normal)
Metabolic Acidosis: the anion gap (AG)
AG = (sum of cations) - (sum of main anions)
= (Na+ + K+) - (Cl- + bicarb)
= (140 + 4) - (104 + 24)
= 16 (the left over protein anions!)
Normal range AG = 14-18
Therefore increased AG reflects the presence of unmeasured anions (eg lactate-!)
This shows an anion gap due to metabolic acidosis. Why is it that although 10 lactate was added, it is ony increased by 8??
Because some of the HCO3- is buffering the H+
Most common causes of acidosis with increased AG?
- Lactic Acidosis
- Ketoacidosis: betahydroxybutyrate is main anion
- Renal Failure: only slightly increased AG due to phosphate + sulfate retention
What type of Acid-base disturbance is this?
What else can we deduce
Metabolic acidosis (low pH, low HCO3-. -BE) with respiratory compensation (low pCO2)
Check lactate, betahydroxybutyrate (ketoacidosis), Ethylene glycol poisoning?
Why are patients with ethylene glycol poisoning given alcohol as treatment??
Ethylene glycol is non-toxic but metabolised → toxic acids via alcohol dehydrogenase.
Ethanol and fomepizole competitively inhibit this
What type of acid-base disturbace is this?
What further tests should be performed?
Metabolic acidosis with respiratory compensation.
Normal AG therefore normal AG acidosis
Tests:
- Check urine pH and NH4+ (if pH high and NH4+ low → RTA)
- Urine Anion Gap
as urine pH should be <5.5 and NH4+ >100mmol/L this indicates Renal Tubular Acidosis, defects in H+ secretion)
What’s normal Anion Gap Acidoses?
Where the acidosis is occuring due to an endogenous acid (not added).
GI HCO3- loss: poops, fistula
Renal Tubular acidosis (RTA): can’t secrete H+, urine pH >5.5 and urine NH4+ not increased
Aldosterone deficiency: addisons disease
Why are those with a Normal AG acidosis hyperchloremic?
When bicarb. is low, extra Cl- needs to be reabsorbed to maintain Na+ balance
What is the linkage between K+ and acid-base?
- the general rule is
Acidosis ⇔ hyperkalemia
Alkalosis ⇔ hypokalemia
These cause shifts of H+ or K+ in/out of the cells
** H+ and K+ compete with each other for secretion
What are the 2 exceptions to the potassium and acid-base balance rule?
- Diarrhoea (HCO3- and K+ loss)
- Renal Tubular acidoses: DT and PT both associated with hypokalemia
Causes of metabolic alkalosis
If you decrease acid and nothing else you still can’t get alkalosis without a secondary issue as the kidneys can compensate. If you also lose Cl- then the kidneys cannot remove HCO3- .
Chloride depletion alkalosis is?
In metabolic alkalosis, the kidney attempts to increase HCO3- loss. If Cl- depletion is present, HCO3- reabsorbtion becomes obligatory to preserve Na+ balance.
The kidney can’t correct the alkalosis until Cl- is replaced
Causes of Cl- depletion: gastric Fluid loss, diuretics (laxitaves and vomiting)
