Chempath 3: Acid base disorders

Question 1

Which acid/base disorder do you get in pyloric stenosis ?

Answer 1

Hypokalaemic hypochloraemic Metabolic Alkalosis

Question 2

What happens to HCO3 in Pyloric stenosis and why ?

Answer 2

HCO3 increases because of relative fluid loss in vomiting. Also carbonic acid dissociates and only the H+ ions are lost, HCO3 is retained.

H+ further decreases due to exchange for intracellular K+ to adjust the hypokalaemia.

Question 3

What two electrolytes are involved in short-term H+/pH buffering in the ECF?

Answer 3

Either bicarbonate or phosphate:
HCO3- + H+ –> H2CO3

HPO4- + H+ –> H2PO4

(Note - hb also plays a role in this -> HHb)

Question 4

How is pH control achieved in the proximal convuluted tubule?

Answer 4

1. H+ and HCO3- combine in tubule lumen to form H2CO3
2. Carbonic anhydrase on tubule tumen membrane converts H2CO3 to H2O and CO2 and absorbs it into the tubule wall cell
3. H2O + CO2 –> H2CO3 again inside the cell, via carbonic anhydrase II
4. Bicarbonate is exchanged with chloride ions, and releasing into the capillary
5. H+ ions can be actively secreted into the tubule lumen, or transported via a sodium-proton exchanger

Question 5

Recall 3 broad mechanisms of aetiology of metabolic acidosis

Answer 5

1. H+ prodution (eg DKA)
2. Decreased H+ excretion (eg renal tubular acidosis)
3. Bicarbonate loss (eg intestinal fistula)

Question 6

Describe the change in the acid-base equilibrium in a metabolic acidosis

Answer 6

Equilibrium =
HCO3- + H+ = H2CO3 = H2O + CO2
Extra H+ produced by acidosis pushes reaction RIGHT
CO2 production increases –> blown out by increased ventilation

Question 7

Describe the change in the acid-base equilibrium in a respiratory acidosis

Answer 7

Equilibrium =
HCO3- + H+ H2CO3 H2O + CO2

Excess CO2 produced by reduced ventilation pushes reaction LEFT, so more H+ and HCO3- is produced
Chronically - CO2 remains raised (due to reduced ventilation), and HCO3- remains raised to maintain physiological pH

Question 8

Describe the change in the acid-base equilibrium in a metabolic alkalosis

Answer 8

Equilibrium =
HCO3- + H+ H2CO3 H2O + CO2

Pathology = decreased H+ / increased HCO3-
Either way - need to regenerate H+
Therefore, reaction moves LEFT
To do this: resp rate decreases (to increase CO2)

Question 9

Describe the acute change in the acid-base equilibrium in a respiratory alkalosis

Answer 9

Equilibrium =
HCO3- + H+ <> H2CO3 <> H2O + CO2
Hyperventilation –> reduced CO2
Reaction moves RIGHT to restore CO2

Question 10

What are the possible causes of metabolic alkalosis?

Answer 10

H+ production: DKA

H+ loss: pyloric stenosis, hypokalaemia

HCO3- excess: lots of antacids

Question 11

Describe the chronic change in the acid-base equilibrium in a respiratory alkalosis

Answer 11

Equilibrium =
HCO3- + H+ H2CO3 H2O + CO2

Acutely, reaction moves RIGHT to restore CO2 (so you get low H+ and HCO3-)

Chronically, kidneys compensate by reducing H+ excretion - so H+ returns to normal, but HCO3- and CO2 remain low

Question 12

What happens to H+, HCO3-, CO2, pH in partially compensated metabolic acidosis?

Answer 12

• High H+
• Low HCO3-
• **low pCO2**
• → pH change might be very small (only slightly acidotic)
• Note: H+ will increase lots and pH very low if respiratory failure (can’t offload CO2)

Question 13

Causes of respiratory acidosis + examples?

Answer 13

Decreased ventilation- acute exacerbation of asthma

Poor lung perfusion - cardiac arrest

Impaired gas exchange - emphysema (decreased SA in alveoli) & foreign body

Question 14

What is seen in chronic respiratory acidosis?

Answer 14

Normally in a few days kidneys compensate via excretion of H+ and bicarb generation

Initially bicarb increase is only a little however in chronic resp acidosis it becomes high (meaning H+ excretion isnt as necessary)

H+ may return to normal but pCO2 and bicarbonate will remain elevated (e.g. COPD, emphysema) - not seen in acute events (eg. Renal tubular acidosis)

Question 15

How can hypokalaemia cause alkalosis and vice versa in cells?

Answer 15

Hypokalaemia -> Alkalosis:

• In cells, Na/K pump used to bring K into cells
• As less K moves in less Na goes out
• Na gets moved out of cells via H/Na exchanger meaning H+ moves in from ECF causing alkalosis

Alkalosis -> Kypokalaemia:

• Basically the opposite, not enough H+ in ECF to move Na out hence only route out into ECF is via K/Na exchanger
• As Na leaves into ECF more K is bought into ICF = hypokalaemia

Question 16

How can hypokalaemia cause alkalosis and vice versa in the kidneys?

Answer 16

Hypokalemia -> Alkalosis:

• Due to low K+ kidneys need to reabsorb from the lumen
• This uses K+/H+ exchanger leading to H+ excretion

Alkalosis -> Hypokalaemia:

• Due to low H+, more needs to be reabsorbed from lumen
• This uses K+/H+ exchanger leading to K+ excretion

Question 17

What happens to H+, HCO3-, CO2, pH in metabolic alkalosis?

Answer 17

• H+ low
• pH high
• HCO3- high
• normal Co2

Question 18

What happens to H+, HCO3-, CO2, pH in compensated metabolic alkalosis?

Answer 18

• H+ low but higher than before
• Normal pH
• HCO3- high
• Slightly raised Co2

Question 19

Causes of respiratory alkalosis?

Answer 19

This is when pH is high and H+ is low

Hyperventilation:

• Voluntarily - anxiety
• Artifical - CPAP
• Stimulation of resp centre (drugs)
• CNS disease
• PE

Question 20

Chronic respiratory alkalosis - what happens to H+, HCO3- and CO2?

Answer 20

If resp alkalosis is prolonged, it will result in decreased renal H+ excretion and decreased bicarbonate generation

Hence:

• H+ may return to normal
• pCO2 and HCO3- remain low

Question 21

What acid base imbalance may you see in a COPD patient w/ concurrent diabetes mellitus?

Answer 21

Mixed metabolic and respiratory acidosis

metabolic = due to ketones (can also be uraemia)

respiratory = resp failure (impaired gas exchange)

Question 22

What acid base imbalance is seen in a patient w/ aspirin overdose and why?

Answer 22

TRICK QUESTION - Usually limited change in pH due to changes in opposite directions

Respiratory alkalosis - stimulation of resp centre

Metabolic acidosis - effect on H+ excretion in kidneys -> reduced bicarb reabsorption

Question 23

Acid-base summary?

Answer 23

• H+/pH will tell us whether it is an acidosis or an alkalosis
• Then look at the pCO2 - this will tell you whether it is a respiratory disturbance (and whether it is a primary disturbance causing the pH imbalance or whether it is compensatory)
• pO2 gives an indication of respiratory function and tissue oxygenation
• NOTE: Bicarbonate predominantly reflects metabolic disturbances but is also affected by respiratory disturbances

Question 24

Remember to try cases on notion:

Answer 24

Okay!