Blood gas analysis and pulse oximetry

Question 1

What are the five key pieces of information in an arterial blood sample?

Answer 1

• pH
• PaCO2
• Bicarbonate
• Base excess
• PaO2

Question 2

What does pH measure in blood?

Answer 2

The acidity or alkalinity of the blood, determined by hydrogen ion concentration

A lower pH indicates higher acidity.

Question 3

What is the normal pH range for arterial blood?

Answer 3

7.35–7.45

Question 4

What is acidaemia?

Answer 4

A condition where blood pH is below 7.35 due to increased hydrogen ion concentration

Question 5

What is alkalemia?

Answer 5

A condition where blood pH is above 7.45 due to decreased hydrogen ion concentration

Question 6

What are the primary buffers in the body?

Answer 6

• Proteins
• Phosphate
• Haemoglobin
• Plasma proteins
• Bicarbonate

Question 7

What is the normal PaCO2 range?

Answer 7

4.7–6.0 kPa

Question 8

What is respiratory acidosis?

Answer 8

Acidaemia caused by a problem with the respiratory system leading to increased PaCO2

Question 9

What is respiratory alkalosis?

Answer 9

Alkalaemia caused by increased alveolar ventilation reducing PaCO2

Question 10

What is bicarbonate’s role in blood?

Answer 10

It acts as a buffer and neutralizes hydrogen ions to prevent a decrease in pH

Question 11

What is the normal bicarbonate concentration?

Answer 11

22–26 mmol L-1

Question 12

What is base excess?

Answer 12

A measure of the amount of excess acid or base in the blood due to metabolic derangement

Question 13

What does a base excess greater than +2 mmol L-1 indicate?

Answer 13

Metabolic alkalosis

Question 14

What does a base excess less than -2 mmol L-1 indicate?

Answer 14

Metabolic acidosis

Question 15

What happens in a patient with COPD regarding acid-base balance?

Answer 15

Respiratory acidosis occurs with compensatory metabolic alkalosis

Question 16

What is the primary mechanism for adjusting [H+] in the body?

Answer 16

The lungs regulate PaCO2

Question 17

What is the effect of increased respiratory rate in diabetic ketoacidosis?

Answer 17

Compensatory respiratory alkalosis occurs as CO2 is removed

Question 18

What is the concentration of oxygen in inspired air?

Answer 18

21% or a partial pressure of 21 kPa

Question 19

What is the normal concentration of oxygen in inspired air?

Answer 19

21% or a partial pressure of 21 kPa

This concentration decreases as air passes through the respiratory tract due to water vapor and carbon dioxide addition.

Question 20

What is the normal range for PaO2 in a healthy individual breathing air?

Answer 20

Higher than 11 kPa

This is approximately 10 kPa lower than the inspired partial pressure of oxygen.

Question 21

What indicates hypoxaemia in a patient receiving supplemental oxygen?

Answer 21

A difference of greater than 10 between the inspired oxygen concentration and PaO2

This suggests a defect in oxygenation.

Question 22

What pH value indicates acidaemia?

Answer 22

pH < 7.35

This indicates an acidotic state in the patient.

Question 23

What indicates alkalemia in blood gas analysis?

Answer 23

pH > 7.45

This indicates an alkaline state in the patient.

Question 24

What does an increased PaCO2 (> 6.0 kPa) indicate when pH is < 7.35?

Answer 24

Respiratory acidosis

This may account for part of the derangement in acid-base balance.

Question 25

What does a reduced PaCO2 (< 4.7 kPa) indicate when pH is > 7.45?

Answer 25

Respiratory alkalosis ## Footnote This is often seen in patients with high mechanical ventilation rates.

Question 26

What indicates a metabolic acidosis when pH is < 7.35?

Answer 26

Base deficit (< -2 mmol L-1) and/or bicarbonate reduced (< 22 mmol L-1) ## Footnote This indicates the presence of an acidotic state.

Question 27

What indicates a metabolic alkalosis when pH is > 7.45?

Answer 27

Base excess (> +2 mmol L-1) and/or bicarbonate increased (> 26 mmol L-1) ## Footnote This suggests a primary metabolic alkalosis.

Question 28

What indicates a primary respiratory acidosis in compensation?

Answer 28

pH < 7.40, PaCO2 > 6.0 kPa, base excess > +2 mmol L-1 ## Footnote This indicates a compensatory metabolic alkalosis.

Question 29

What indicates a primary metabolic acidosis in compensation?

Answer 29

pH < 7.40, PaCO2 < 4.7 kPa, base excess < -2 mmol L-1 ## Footnote This indicates a compensatory respiratory alkalosis.

Question 30

What is the primary disturbance in a patient with chronically raised PaCO2?

Answer 30

Respiratory acidosis ## Footnote This condition may be compensated by metabolic alkalosis.

Question 31

What is the typical blood glucose concentration in a patient with DKA?

Answer 31

23 mmol L-1

Question 32

What does a high arterial lactate concentration indicate post-cardiac arrest?

Answer 32

Lactic acidosis due to inadequate tissue oxygenation

Question 33

What is the advantage of using venous blood gases (VBGs) in assessing acutely unwell patients?

Answer 33

Monitor metabolic disturbances without respiratory compromise

Question 34

What can alter VBG values when taking a peripheral venous sample?

Answer 34

Use of a tourniquet

Question 35

What does pulse oximetry measure?

Answer 35

Arterial blood oxygen saturation (SpO2)

Question 36

What can cause falsely low saturation readings on a pulse oximeter?

Answer 36

Presence of other haemoglobins, surgical dyes, nail varnish, high ambient light

Question 37

What are the limitations of pulse oximetry?

Answer 37

Does not measure PaCO2 or tissue oxygenation, unreliable below 70% SpO2

Question 38

What is the target oxygen saturation for patients with Type II respiratory failure?

Answer 38

88–92%

Question 39

What is the target oxygen saturation for patients with myocardial infarction or acute coronary syndrome?

Answer 39

94–98% or 88–92% if at risk of hypercapnic respiratory failure