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a path 3 > CHEMPATH: Acid-base handling > Flashcards

CHEMPATH: Acid-base handling Flashcards

1
Q
  1. What is the normal H+ concentration?
  2. What is normal pH?
  3. What is the definition of pH?
A
  1. [H+] 35-45 nmol/l in ECF
  2. pH 7.35 -7.46
  3. pH = log 1/ [H+]
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2
Q

How is H+ produced? How much? Where is it excereted?

A

By metabolism of protein, carbohydrates and fats which make CO2, water and H+.

Around 50-100mmol/day of H+ is made

Excretion mainly by kidney

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3
Q

Why does addition of H+ ions not cause increase in overall H+ concentration in the body?

A

Buffering systems are at play

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4
Q

Which buffering systems are used to regulate H+ cocentration? What is the disadvantage of these systems?

A
  1. Bicarbonate H++HCO3- H2CO3
    • (ECF, Glomerular Filtrate)
  2. Haemoglobin H+ + Hb- HHb
    1. (Red Cells)
  3. Phosphate H++HPO4- H2PO4
    1. (Renal tubular Fluid / Intracellular)
  4. Also Protein and Bone

Disadvantage: eventually kidneys need to excrete the H+ and regenerate bicarbonate as buffering is only effective in the short term

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5
Q

How is bicarbonate replenished?

A

Reabsorption in the proximal tubule - virtually all bicarbonate is reabsorbed

Regeneration through carbonic acid i.e. water and CO2, although H+ is produced as a biproduct and this needs to be excreted through exchange with Na+

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6
Q

How is CO2 excreted? How much is produced each day?

A

NB: also produced by metabolism of proteins, carbohydrates, fats –> CO2, water and H+.

20,000-25,000 mmol/day of CO2 produced

Excreted by the lungs through expiration

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7
Q

What is a buffer?

A

A buffer is a weak acid with a base that can be used to mop up extra H+ ions (allows excess H+)

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8
Q

What is the acid-base equation?

A

CO2 + H2O H2CO3 H+ + HCO3-

NB: all parts are interlinked, if one side changes then the other changes too.

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9
Q

What is the buffering system used in blood?

A

Haemoglobin

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10
Q

What do blood gas machines measure? What is not measured but rather calculated?

A

Blood gases measure partial pressure of O2 and CO2 and [H+].

Bicarbonate is calculated by the equation below (which uses k coefficient which relates to the solubility of CO2 in blood)

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11
Q

What is the primary abnormality in metabolic acidosis? Name 3 causes of metabolic acidosis.

A

Primary abnormality is increased H+ (decreased pH) with decreased bicarbonate

Causes of metabolic acidosis:

  1. Increased H+ production e.g. diabetic ketoacidosis
  2. Decreased H+ excretion e.g. Renal tubular acidosis
  3. Bicarbonate loss e.g. intestinal fistula
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12
Q

How is metabolic acidosis compensated?

A

Respiratory centre is stimulated

Reduction of CO2 occurs by increased respiration

Picture: If you did not have respiratory compensation you could go up to about pH6.9. However, with respiratory compensation the rise should be limited to about pH7.1

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13
Q

What is the primary abnormality in acute respiratory acidosis? What are 3 causes?

A

Primary abnormality is increased CO2 producing increased H+ (decreased pH) and a slight increase in bicarbonate (2-4 mmol/L).

May be due to:

  1. Decreased Ventilation
  2. Poor Lung Perfusion
  3. Impaired Gas Exchange
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14
Q

How is chronic respiratory acidosis compensated?

A

Increased renal excretion of H+ combined with generation of bicarbonate

H+ may return to normal but pCO2 and bicarbonate will be elevated

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15
Q

How is metabolic alkalosis compensated?

A

There is inhibition of the respiratory centre –> rise in pCO2 –> H+ returns to normal

NB: this is limited as pCO2 is required to stimulate respiratory drive

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16
Q

What is the primary abnormality in metabolic alkalosis? What are 3 causes?

A

Primary abnormality is decreased H+ (increased pH) with increased bicarbonate

Due to:

  • H+ loss (e.g. pyloric stenosis)
  • Hypokalaemia - K is one of the components in the Na+K+ATPase pump used to excrete H+
  • Ingestion of bicarbonate
17
Q

What are the causes of respiratory alkalosis? What abnormalities occur?

A

Primarily there is a fall in CO2 so H+ ions are used up so both bicarbonate and H+ will become low.

Causes:

  • Voluntary or anxiety related
  • Artificial ventilation
  • Stimulation of respiratory centre
18
Q

How is chronic respiratory alkalosis compensated?

A

Decreased renal excretion of H+ and less bicarbonate generation

H+ may return to near normal but pCO2 and bicarbonate will remain low. This will not occur in acute conditions.

19
Q

What is the name of this diagram?

A

Fenley acid-base nomogram

20
Q

Describe the utility of each of these in acid-base assessment of a patient:

  • H+/pH
  • pCO2
  • pO2
  • bicarbonate
A
  • H+/pH - presence of acidosis/alkalosis
  • pCO2 - respiratory disturbance
  • pO2 - no direct effect on acid-base but indicates respiratory function
  • bicarbonate - predominantly present in metabolic disturbance but also sometimes respiratory
21
Q

Give examples of the bicarbonate levels which would be seen in each of these conditions:

A
  • 4 - metabolic acidosis
  • 18 - respiratory alkalosis
  • 24 - normal
  • 28 - acute respiratory acidosis
  • 43 - chronic respiratory acidosis
  • 55 - metabolic alkalosis
22
Q

Case 1..?

  1. Normal
  2. Metabolic Acidosis
  3. Metabolic Alkalosis
  4. Acute Respiratory Acidosis
  5. Chronic Respiratory Acidosis
  6. Respiratory Alkalosis
  7. Mixed metabolic acidosis and respiratory acidosis
  8. Mixed metabolic alkalosis and respiratory alkalosis
A

Severe metabolic acidosis with partial (complete is impossible) respiratory compensation

23
Q

Case 2: 64yo female with 3 week history of intermittent vomiting, abdominal pain, weight loss, O/E: Dehydrated, Jaundiced, Hypotensive, Oliguric…

  1. Normal
  2. Metabolic Acidosis
  3. Metabolic Alkalosis
  4. Acute Respiratory Acidosis
  5. Chronic Respiratory Acidosis
  6. Respiratory Alkalosis
  7. Mixed metabolic acidosis and respiratory acidosis
  8. Mixed metabolic alkalosis and respiratory alkalosis
A
  1. Metabolic Alkalosis with respiratory compensation
  • Underlying problem is pyloric stenosis –> loss of HCl in vomit produces a metabolic alkalosis (Low H+, high bicarbonate)
  • Loss of fluid produces dehydration –> raised urea, creatinine and total protein
  • Dehydration stimulates RAAS mechanism
  • Low potassium as it is lost in vomit and urine
24
Q

Case 3:

  1. Normal
  2. Metabolic Acidosis
  3. Metabolic Alkalosis
  4. Acute Respiratory Acidosis
  5. Chronic Respiratory Acidosis
  6. Respiratory Alkalosis
  7. Mixed metabolic acidosis and respiratory acidosis
  8. Mixed metabolic alkalosis and respiratory alkalosis
  9. Mixed metabolic acidosis and respiratory alkalosis
  10. I haven’t a clue!
A

Respiratory alkalosis - if it were chronic there would be more change in the bicarbonate

25
Q

Case 4:

  1. Normal
  2. Metabolic Acidosis
  3. Respiratory Acidosis
  4. Metabolic Alkalosis
  5. Respiratory Alkalosis
  6. Mixed metabolic acidosis and respiratory acidosis
  7. Mixed metabolic alkalosis and respiratory alkalosis
  8. Mixed metabolic alkalosis and respiratory acidosis
  9. Mixed metabolic acidosis and respiratory alkalosis
  10. I haven’t a clue!
A

8.

Respiratory Acidosis

  • ? Due to COPD
  • ? compensation for metabolic alkalosis

Metabolic Alkalosis

  • ? due to hypokalaemia
  • ? compensation for respiratory acidosis
26
Q

Case 5:

  1. Normal
  2. Metabolic Acidosis
  3. Respiratory Acidosis
  4. Metabolic Alkalosis
  5. Respiratory Alkalosis
  6. Mixed metabolic acidosis and respiratory acidosis
  7. Mixed metabolic alkalosis and respiratory alkalosis
  8. Mixed metabolic alkalosis and respiratory acidosis
  9. Mixed metabolic acidosis and respiratory alkalosis
  10. I haven’t a clue!
A

9.

  • Classically associated with aspirin = respiratory alkalosis and metabolic acidosis (low bicarbonate)
  • Aspirin –> stimulates respiratory centre + decreases H+ ion excretion in the kidney
  • Change in pH limited as she has two conditions which are acting in opposite directions on the pH
27
Q

Case 6:

  1. Normal
  2. Metabolic Acidosis
  3. Respiratory Acidosis
  4. Metabolic Alkalosis
  5. Respiratory Alkalosis
  6. Mixed metabolic acidosis and respiratory acidosis
  7. Mixed metabolic alkalosis and respiratory alkalosis
  8. Mixed metabolic alkalosis and respiratory acidosis
  9. Mixed metabolic acidosis and respiratory alkalosis
  10. I haven’t a clue!
A

6.

  • Bicarbonate is very low so there must be metabolic acidosis
  • CO2 level is not enough to give that low pH
  • Must be on oxygen as otherwise O2 would not be this high

a path 3 (89 decks)

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