Acid Base Control

Question 1

1kPa = how many mmHg

Answer 1

7.5

Question 2

When do acid-base disturbances occur?

Answer 2

• If there is a problem with ventilation,
• Problem with renal function,
• Overwhelming acid or base load

Question 3

What is standard bicarbonate?

Answer 3

It reflects the metabolic component of acid base balance. It is calculated from the actual bicarbonate but assuming 37 degrees and paCO2 of 5.3kPa

Question 4

What is base excess?

Answer 4

Amount of acid required to restore a litre of blood to its normal pH at PCO2 of 5.3. It becomes more negative in metabolic acidosis

Question 5

Describe the stepwise approach to the assessment of ABGs

Answer 5

1) Assess PO2 and oxygenation (P/F ratio)
2) Assess pH, acidaemia or alkalaemia?
3) Determine primary problem,
4) Is compensation occurring?

Question 6

What is a good clue to assess if compensation is occuring?

Answer 6

It is likely to be occurring if pCO2 and bicarb are moving in the same direction.

Question 7

What is the anion gap?

Answer 7

It is the sum of routinely measured cations in venous blood minus routinely measured anions. It can be early indicator of acidaemia.

Question 8

What is the normal anion gap?

Answer 8

16. If it is increased then is signals presence of metabolic acidaemia

Question 9

Explain how endogenous acidaemia can occur from lactate

Answer 9

Results from any condition causing hypoperfusion eg, Shock or emboli. This will result in increased anaerobic metabolism with subsequent increased production of lactic acid. Lactic acidaemia can also result from severe acute hypoxia, severe convulsions or strenuous exercise.

Question 10

explain how endogenous acidaemia can occur from an increase in insulin and glucagon

Answer 10

• In uncontrolled diabetes (can lead to ketoacidosis), alcoholic or starvation ketoacidosis

Question 11

Why would you you use venous blood instead of arterial in DKA?

Answer 11

Unless respiratory problems indicate then there is no interest in PO2. Since venous samples are less painful, faster and lower risk of harm then it is preferred.

Question 12

Name some examples of exogenous acid overload

Answer 12

Accidental/deliberate ingestion of methanol or ethylene glycol.

Question 13

What are some causes of metabolic acidosis?

Answer 13

• Renal failure (increase anion gap)
• Renal tubular acidosis (normal anion gap)
• Diarrhoea - cells below pylorus secrete bicarbonate into lumen and for every bicarb ion into gut, an H+ ion enters ECF. (normal anion gap)

Question 14

Describe the compensation for metabolic acidosis

Answer 14

Respiratory. pCO2 must fall so minute volume must increase. Therefore Kussmaul respiration occurs which is a laboured deep, rapid breathing patter.

Question 15

What 2 processes happen for metabolic alkalosis to occur?

Answer 15

An initiating process and a maintaining process

Question 16

What is the most common initiating processes in metabolic alkalosis

Answer 16

• Loss of H+ ions from the gut (above pylorus)
• Less common is loss of H+ from the kidney due to loop and thiazide diuretics.
• Gain of exogenous alkali is much less common

Question 17

Explain the maintenance of alkalosis

Answer 17

• Occurs via a process which impairs the kidneys ability to excrete bicarbonate (HAV);
• Hypokalaemia,
• Aldosterone excess,
• Volume and chloride depletion

Question 18

Describe the compensation for metabolic alkalosis

Answer 18

pCO2 must increase and so minute volume must fall.