L24 Acid Base 2

Question 1

What are the responses( pH, CO2, HCO3-) to respiratory alkalosis and metabolic acidosis (1’ and compensatory)

Answer 1

R. Alkalosis
1’: hyperventilation reduces PCO2, and increases pH (Bb line)
2’: Reduced renal net acid excretion to restore pH which reduces HCO3- (down isobar)

Metab acidosis:
1’: Accumulation of lactic acid (non-volatile acid) reduces HCO3- and pH (down isobar)
2’: Increased ventilation increases pH but reduces PCO2 (Bb line)

Question 2

How do you tell the difference between chronic respiratory alkalosis and mixed respiratory acidosis and metabolic acidosis

Answer 2

Chronic resp alkalosis

• pH would be high, PCO2 would be low.
• Compensation with mild metabolic acidosis would mean HCO3- is low.
• Low base excess

Mixed resp + metab acidosis

• very low pH
• Increase in CO2 and decrease in HCO3- (looks like no compensation)
• low base excess

Question 3

How does metab alkalosis come about

Answer 3

1. Initiator:
   a) Loss of H+ from ECF:
   - overuse of diuretics (triggers aldosterone release–> H+ excretion leading to HCO3- regeneration)
   - vomiting, NG suction

b) Gain of alkali in ECF: eg
- exogenous infusion, endogenous metabolism of ketoanions after DKA

2: Impaired renal correction (normally HCO3- is excreted by kidney quickly to compensate)

Question 4

What effect does metabolic alkalosis have on Cl and why

Answer 4

As Cl- and HCO3- are the only 2 anions in ECF they need to balance each other for electroneutrality.
- So when there is increased reabsorption of HCO3- there will be a decrease in Cl-

• also when Na+ and K+ are reabsorbed they need a balancing anion to maintain electroneutrality.
  eg. in diuretics: loss of Na+ also leads to loss of Cl-

Question 5

How does Hyperaldosteronism: 1’ conns or 2ndary Bartters effect on K+ affect HCO3-

Answer 5

1. Leads to increased distal tubular Na reabsorption and increased K, H loss
2. Indirect: increased Na+ reabsorption –> negative lumen voltage promoting H+ secretion
3. Direct: stimulation of H+ ATPase secretion
4. The increased loss of H+ is matched by Increased HCO3- reabsorption
5. 1’ and 2’ upregulate anion exchanger, facilitating HCO3/Cl exchange

Question 6

What is the anion gap, what does a high gap and a normal gap mean

Answer 6

Anion gap is the difference between plasma concentrations of the major cation (Na+) and major anions (Cl- and HCO3-). It can be used to determine cause of metabolic acidosis

1. If normal: the anion of non-volatile acid is Cl-: due to diarrhoea, renal dysfunction
2. If high: anion of nonvolatile acid is not Cl- (lactate or B-hydroxybutyrate); due to lactic acidosis caused by ischaemia or DKA

Question 7

What is the alveolar gas equation used for. Therefore how can increased PaCO2 effect PAO2

Answer 7

To estimate the alveolar partial pressure of O2 from Parterial Co2 (=PACO2 as solubility high).

PAO2 = Pressure of inspired O2 - PACO2/ Respiratory exchange ration + correction factor.

• Increased PaCO2 can decrease/displace PAO2 causing low PaO2.

Question 8

What is the alveolar arterial gradient, and what does a increase in the gradient mean

Answer 8

It is the difference between PO2 in the alveoli - arterial blood.
An increased gradient indicates impaired oxygen diffusion in the lung
eg. COPD : thickening and loss of alveoli (effects O2> CO2 as CO2 is very soluble.)
eg. Pneumonia: protein rich pulmonary oedema increases diffusion distance.