Acid-base abnormality

Question 1

Define partial pressure

Answer 1

Contribution of one individual gas
within a gas mixture (such as air) to the total pressure

When gas dissolves in liquid (e.g. blood), the amount dissolved depends on the partial pressure.

Question 2

Difference between PO2 and PaO2

Answer 2

PO2 = partial pressure of O2
PaO2 = partial pressure of O2 in arterial blood

Question 3

How is PaCO2 controlled

Answer 3

PaCO2 is controlled by ventilation (respiratory centre in brainstem adjusts breathing rate via accessory muscles) and the level of ventilation is adjusted to maintain PaCO2 within tight limits.

THEREFORE high PaCO2 is always a sign of poor alveolar ventilation

Question 4

Hypoxic drive: practical example

Answer 4

Give 28% Venturi mask O2 to COPD patients in T2RF

Question 5

SO2 and SaO2

Answer 5

SO2 = O2 saturation in (any) blood
SaO2 = O2 saturation in arterial blood

Question 6

Pulse oxymetry

Answer 6

• less accurate with saturations below 75%
• unreliable when peripheral perfusion is poor
• No PaCO2, so not a replacement for ABG in ventilatory impairment

Question 7

What determines oxygen concentration in blood

Answer 7

SaO2 and Hb (not PO2)

Question 8

If SaO2 and Hb determine O2 concentration in blood, what is the role of PO2

Answer 8

driving force for O2 molecules to bind to Hb: as such, it regulates the So2
Look at oxyhaemeglobin curve

Question 9

3 factors that determine PaO2

Answer 9

1. Alveolar ventilation
2. Ventilation/perfusion (V/Q)
3. Concentration of oxygen in inspired air (FiO2)

Question 10

Why is alveolar ventilation important

Answer 10

Both oxygenation and CO2 elimination depend on alveolar
ventilation: impaired ventilation causes PaO2 to fall and PaCO2 to
rise.

Question 11

What is V/Q mismatch

Answer 11

Not all blood flowing through the lung meets well-ventilated alveoli and not all ventilated alveoli are perfused with blood

V/ Q mismatch allows poorly oxygenated blood to re-enter the
arterial circulation, thus lowering PaO2 and SaO2.

Question 12

Useful rule re FiO2 and PaO2

Answer 12

difference between Fio2 and Pao2 (in kPa) should not normally be greater than 10

Question 13

Define hypoxia

Answer 13

Cells not getting enough O2 (either due to hypoxaemia or ischaemia)

Question 14

Define hypoxaemia

Answer 14

Not enough O2 in blood

Question 15

Define impaired oxygenation

Answer 15

hypoxaemia resulting from reduced
transfer of O2 from lungs to the bloodstream.

Question 16

Causes of T1RF (usually V/Q mismatch)

Answer 16

Pneumonia
Acute asthma
Pulmonary embolism
Acute respiratory distress syndrome
Pneumothorax
Fibrosing alveolitis
Pulmonary oedema
COPD

Question 17

Causes of T2RF (usually inadequate alveolar ventilation)

Answer 17

COPD
Opiate/benzodiazepine toxicity
Exhaustion
Inhaled foreign body
Flail chest injury
Neuromuscular disorders
Kyphoscoliosis
Obstructive sleep apnoea

Question 18

Compensatory mechanisms in respiratory vs metabolic

Answer 18

Respiratory takes hours
Metabolic takes days

Question 19

Dominant symptom in metabolic acidosis

Answer 19

Hyperventilation (Kussmaul breathing - fast & deep breaths)

Question 20

Metabolic acidosis with raised anion gap

Answer 20

Ingestion of an exogenous acid or increased production of an
endogenous acid (increased H+)

M/C: Lactic acidosis

Others:
Ketoacidosis
Renal failure (accumulation of sulphate, phosphate, urate)
Poisoning (aspirin, methanol, ethylene glycol)
Massive rhabdomyolysis

Question 21

Metabolic acidosis without raised anion gap

Answer 21

Excessive HCO3 loss, either renal (e.g. renal tubular acidosis) or GI (Diarrhoea = HCO3 loss)

Kidneys retain Cl- when HCO3 is lost i.e. hyperchloraemic metabolic acidosis

Other examples:
Adrenal insufficiency
Ammonium chloride ingestion
Urinary diversion (e.g. ureterosigmoidostomy)
Drugs (e.g. acetazolamide)

Question 22

Lactic acidosis

Answer 22

Low HCO3 and lactate>4
Anaerobic metabolism as a result of ischaemia or profound hypoxaemia/shock

Question 23

DKA

Answer 23

1. A high anion gap metabolic acidosis
2. hyperglycaemia
3. The presence of ketones (detectable in blood or urine)

Question 24

Metabolic alkalosis

Answer 24

Sustained vomiting or diuretics

M/C: Loss of gastric secretion (vomiting, NG suction)
Potassium depletion (e.g. diuretics)

Others:
Cushing syndrome
Conn syndrome (primary hyperaldosteronism)
Chloride-rich diarrhoea (e.g. villous adenoma)
Excessive administration of sodium bicarbonate

Question 25

Kidney response to
1. Loss of Cl-
2. Loss of Na+ and K+
3. Result

Answer 25

1. Loss of Cl- –> increase in HCO3 reabsorption
2. Loss of Na+ and K+ –> increase in Na+ and K+ reabsorption at the expense of H+
3. Metabolic alkalosis

Question 26

Why is respiratory acidosis almost always uncompensated

Answer 26

metabolic compensatory responses take days to develop, acute
respiratory acidosis is almost always uncompensated, leading rapidly to
profound and dangerous acidaemia