Lecture 28: Acid-base homeostasis 2

Question 1

How do we determine the acid-base status of a patient?

Answer 1

Gases (pCO2, pO2)
Metabolites (Glucose, Lactate)
Electrolytes (Sodium, Potassium, Chloride, Calcium)
pH
Actual bicarbonate
Co-oximetry (Total Hb, O2 saturation, OxyHb, COHb, MetHb)
Derived parameters (Base excess, Standard bicarbonate, Anion gap)

Question 2

Give 3 derived parameters used in investigating acid-base homeostasis:

Answer 2

1) standard bicarbonate

2) base excess

3) anion gap

Question 3

What is standard bicarbonate?

Answer 3

a calculated hypothetical value that predicts the expected bicarbonate concentration in a blood sample if the pCO2 is normal

Question 4

What assumption does the calculation of standard bicarbonate make?

Answer 4

there are no contributions from any respiratory disturbance

Question 5

What does an abnormal standard bicarbonate tell us?

Answer 5

there is a metabolic component to the disorder

Question 6

What is base excess?

Answer 6

a calculated parameter telling us the amount of acid or alkali required to titrate blood pH to 7.40

Question 7

What is anion gap?

Answer 7

the difference between the sum of measured anions and cations

Question 8

What does an increased anion gap indicate?

Answer 8

there are significant amounts of unmeasured anions present e.g ketones, lactate, salicylate

Question 9

How does having air bubbles in blood gas samples affect readings? (3)

Answer 9

1) pO2 is affected

2) pH increases

3) pCO2 decreases

Question 10

Does respiratory acidosis present with high or low pCO2?

Answer 10

high

Question 11

Does respiratory alkalosis present with high or low pCO2?

Answer 11

Low

Question 12

Does metabolic acidosis present with high or low HCO3-?

Answer 12

Low

Question 13

Does metabolic alkalosis present with high or low HCO3-?

Answer 13

high

Question 14

Give 2 key general symptoms of acidosis:

Answer 14

1) shortness of breath

2) coughing

Question 15

Give 2 key general symptoms of alkalosis:

Answer 15

1) hand tremor

2) numbness or tingling in the face, hands or feat

Question 16

Does metabolic acidosis present with high or low pCO2?

Answer 16

normal pCO2 but low if there is compensation

Question 17

Does metabolic alkalosis present with high or low pCO2?

Answer 17

normal pCO2 if there is compensation

Question 18

Give three causes of metabolic acidosis:

Answer 18

1) increased acid formation (ketoacidosis, lactic acidosis, poisoning)

2) decreased acid excretion (uraemia/ renal failure, renal tubular acidosis type 1)

3) loss of bicarbonate (diarrhoea, fistula, renal tubular acidosis type 2)

Question 19

What two causes of metabolic acidosis are associated with an increased anion gap?

Answer 19

1) increased acid formation

2) decreased acid excretion

Question 20

Give two compensation mechanisms used in metabolic acidosis:

Answer 20

1) hyperventilation to blow off CO2

2) increased rate of bicarbonate regeneration

Question 21

Give 4 symptoms of metabolic acidosis:

Answer 21

1) nausea

2) vomiting

3) anorexia

4) Kussmaul breathing

Question 22

What is Kussmaul breathing?

Answer 22

rapid, deep, laboured breathing

Question 23

What drug is used to manage metabolic acidosis?

Answer 23

sodium bicarbonate (if pH is under 7)

Question 24

Give 3 causes of metabolic alkalosis:

Answer 24

1) loss of H+ through vomiting

2) administration of bicarbonate (iatrogenic)

3) potassium depletion

Question 25

How can potassium depletion cause metabolic alkalosis? (2)

Answer 25

1) in the kidneys, excretion of H+ takes place to spare K+ at aldosterone-controlled renal transporters 2) in red blood cells, K+ is transported out of the cell and H+ is transported in to maintain electroneutrality, decreasing [H+]

Question 26

Give one way that the body responds (compensates) to metabolic alkalosis:

Answer 26

reduced respiratory rate to retain CO2

Question 27

How is metabolic alkalosis managed?

Answer 27

IV fluids (normal saline)

Question 28

Give two causes respiratory acidosis?

Answer 28

1) defective respiratory function (COPD, asthma, pneumothorax) 2) defective control of respiration either by CNS depression or CNS disease

Question 29

How do the kidneys respond to respiratory alkalosis?

Answer 29

they maximise bicarbonate regeneration and maximise H+ excretion

Question 30

Give 3 causes of respiratory alkalosis:

Answer 30

1) pulmonary issues (PEs, pneumonia, asthma) 2) central issues (head injury, stroke, anxiety hyperventilation) 3) iatrogenic causes (excessive mechanical ventilation)

Question 31

How do the kidneys compensate during respiratory alkalosis?

Answer 31

they decrease renal regeneration of bicarbonate

Question 32

Give a key symptom of SEVERE respiratory alkalosis:

Answer 32

increased protein binding of Ca2+ leads to hypocalcaemia (headaches, lethargy, delirium, seizures)

Question 33

Give 2 management techniques for respiratory alkalosis:

Answer 33

1) paper bag rebreathing 2) sedation and mechanical ventilation

Question 34

Give the three crucial physiological pathways the kidney facilitates in acid-base homeostasis:

Answer 34

1) reabsorption of bicarbonate in the proximal tubule 2) excretion of H+ in the distal tubule 3) regeneration of bicarbonate in the distal tubule