Gunn: Acid/Base Physiology 3

Question 1

What is metabolic alkalosis?

Answer 1

• If plasma bicarbonate > 24mmols/L, HCO3 is excreted by the kidneys
  
  • Rapid correction of circulationg alkali
• Thus a metabolic alkalosis requires both:
  
  • An initiator
  • Impaired renal correction
• Steady state reflects
  
  • reabsorption of all filtered HCO3
  • Excretion of filtered fixed acid/regeneration of plasma HCO3

Question 2

Initator of matabolic alkalosis?

Answer 2

• Gain of alkali in the ECF
  
  • exogenous (eg. IV NaHCO3 infusion, citrate in transfused blood)
  • endogenous (eg. matabolism of ketoanions to produce bicarbonate)
• Loss of H+ from ECG
  
  • Kidneys (eg. use of diuretics)
  • Gut (vomiting gastric fluid, NG suction) - (eg. pyloric stenosis in babies)

Question 3

Chloride’s role in Metabolic alkalosis?

Answer 3

Lack of Cl- leads to increased HCO3- reabsorption

• Cl- nad HCO3- are the only anions present in appreciable quantities in ECF
• When Na+ and K+ are reabsorbed we need a balancing anion
• Thus a deficiency of one leads to the increased reabsorption of the other

(eg. Diuretics such as frusemide resuly in a loss of NaCl and there is a risk if the patient is already volume depleted leaving them with elevated alosterone, and they have a low dietary chlorine intake)

Question 4

Potassium depletion results from? leads too?

Answer 4

Hyperaldosteronism: Primary: Conn’s; secondary: Bartters

• Increased distal tubular Na+ reabsorption and Increased K+, H+ loss
• Increased HCO3- reabsorption matches the incresed H+ loss
• *Indirect**: Increased Na+ reabsorption leads to an increased nagative cell voltage promoting H+ secretion
• *Direct**: Stuimulates H+ ATPase

Upregulaters anion exchanger, facilitating HCO3/Cl exchange

Question 5

What is the anion gap?

Answer 5

The anion gap is the difference between primary measured cations (sodium Na+ and potassium K+) and the primary measured anions (chloride Cl- and bicarbonate HCO3-) in serum.

Anion Gap = (Na⁺ + K⁺) - (Cl^- + HCO₃^-)

• If K⁺ is not used in the measurement the normal range is 8 - 16 mmol.L^-1

Question 6

What is the purpose of the anion gap calculation?

Answer 6

It is a useful way of identifying the cause of metabolic acidosis.

• If the anion of the non-volatile acid is Cl^- (metabolic acidosis due to diarrhoea or renal dysfunction) the anion gap will be normal.
• If the anion of the non-volatile acid is not Cl^- (e.g lactate or beta-hydroxybutyrate) the anion gap will increase.

Question 7

The alveolar gas equation and estimating the Alveolar O2?

Answer 7

We know that the PaCO2 = PACO2 and so:

PAO2 = PIO2 - PACO2/R + F (R = resp exchange ratio)

PAO2 = 150 - 40/0.8 + 2

Alveolar O₂ = 102 mmHg

Question 8

What value difference between P_AO2 and P_aO2 indicates impaired oxygen diffusion in the lungs? Some common examples of this?

Answer 8

Young Adults = >12mmHg at sea level

Elderly = >25mmHg at sea level

• A widened gap would make a major contribution to a patients severe hypoxaemia. COPD is commonly associated with diffusion defects which give rise to such gradients.
• These is also common in HF as the lungs become oedematous and CO2 diffuses normally but O2 struggles with the increased diffusion distance

Question 9

What are the steps involved in the analysis and interpretation of arterial blood gases?

Answer 9

1. pH - Normal, acidaemic, alkalaemic?
2. P_CO2, H_CO3^- and BE - Acidosis, Alkalosis, Respiratory or Metabolic?
3. P_CO2, H_CO3^- and BE - Compensated or Uncompensated? Simple/mixed?
4. Anion Gap - Normal or Increased?
5. P_O2 - Normal, Low or High? (relative to the F_IO2 and therfore alveolar PO2)

Question 10

Symptoms felt from Alkalosis and acidosis?

Answer 10

Alkalosis: Nerve effects are common such as tingling of the fingers

Acidosis: Non-specific, general unwell/decreased consciousness with nausea especially in cases of acute ketone acidosis

DKA: pretty much everyone presents with 2-3 weeks of polyuria, polydypsia and waking in the night multiple times to urinate (not caused by much else)