Week 5 - Arterial Blood Gases

Question 1

What does the A-a gradient measure?

Answer 1

It tells us the difference between Alveolar (A) and Arterial (a) oxygen tensions

Question 2

What does a large A-a gradient indicate?

Answer 2

A problem with gas exchange
- Either a large V/Q mismatch and/or diffusion problem

Question 3

What is the adequacy of alveolar ventilation?

Answer 3

Hyperventilation PaCO₂ < 35mmHg (large)

Between 35-45 = diffusion problem

Hypoventilation PaCO₂ > 45mmHg (V/Q mismatch)

Question 4

What are the normal values for pH, PaCO₂, HCO₃⁻ and PaO₂?

Answer 4

pH: 7.38-7.42
PaCO₂: 38-42mmHg
HCO₃⁻ : 22-26
PaO₂: 80-100mmHg

Question 5

What is an acid?

Answer 5

A substance that releases H⁺

Question 6

What is base?

Answer 6

A substance that absorbs/accepts H⁺

Question 7

What does this equation represent?
H₂O + CO₂ ⇌ H₂CO₃ (carbonic acid) ⇌ H⁺ + HCO₃⁻

Answer 7

Represents the process of carbon dioxide dissolving in water to form carbonic acid, which can then dissociate into hydrogen ions and bicarbonate ions

Question 8

Which system is CO₂ controlled by?

Answer 8

Respiratory system
↑CO₂ –> ↑H⁺ = ↓pH (acidosis)
↓CO₂ –> ↓H⁺ = ↑pH (alkalosis)

Question 9

Which system is the levels of bicarbonate (HCO₃) controlled by?

Answer 9

Renal system
↑HCO₃⁻ –> ↓H⁺ = ↑pH (alkalosis)
↓HCO₃⁻ –> ↑H⁺ = ↓pH (acidosis)

Question 10

How to calculate ABGs?

Answer 10

1. Look at pH and determine if it is acidosis or alkalosis
2. Decide if it is metabolic/respiratory acidosis/alkalosis
3. Determine whether compensation is occurring

Question 11

What is the normal A-a gradient while breathing room air?

Answer 11

<10mmHg

Question 12

What is the normal neutral FiO₂?

Answer 12

0.21
21%

Question 13

What is the equation for PₐO₂ and how do you find A-a gradient?

Answer 13

PₐO₂ = (FiO₂ x 713) - (PₐCO₂ / 0.8) = x
A-a = x - PaO

0.8 - respiratory quotient

Question 14

What is the problem if PaCO₂ is normal?

Answer 14

Must be gas exchange (V/Q mismatch)

Question 15

What are the causes of respiratory acidosis and respiratory alkalosis?

Answer 15

Respiratory acidosis: reduced alveolar ventilation/alveolar hypoventilation

Respiratory alkalosis: increased alveolar ventilation/alveolar hyperventilation

Question 16

What are the causes of metabolic acidosis and metabolic alkalosis?

Answer 16

Metabolic acidosis: Excess H⁺ production, renal failure, bicarbonate loss (diarrhoea)

Metabolic alkalosis: vomiting (loss of acid from stomach), ingesting bicarbonate

Question 17

What are some management/interventions for <35mmHg CO₂?

Answer 17

Overventilated as a result of mechanical ventilation. Reduce tidal volume, RR or FiO₂ to allow CO₂ to correct and progress towards getting off ventilator

Question 18

What are some management/interventions for >45mmHg CO₂?

Answer 18

Improve alveolar ventilation through positioning, deep breathing exercises with inspiratory hold, SOOB/mobilisation, incentive spirometry