ABG Intepretation and Instrument errors

Question 1

What is the formula for calculating the anion gap (AG)?

Answer 1

AG = Na⁺ – (Cl⁻ + HCO₃⁻); do not include K⁺ unless specifically asked. Normal AG = 12.

Question 2

How do you correct the anion gap for hypoalbuminaemia?

Answer 2

Corrected AG = AG + 0.25 × (40 – albumin). Only apply if albumin is provided.

Question 3

What is the delta ratio formula and what does it help identify?

Answer 3

Delta ratio = (AG – 12)/(24 – HCO₃⁻). It identifies mixed acid-base disorders.

Question 4

Interpret delta ratio < 0.4:

Answer 4

Suggests pure normal anion gap metabolic acidosis (e.g., diarrhea, RTA).

Question 5

Interpret delta ratio 0.4–0.8:

Answer 5

Suggests mixed HAGMA and NAGMA.

Question 6

Interpret delta ratio 0.8–2.0:

Answer 6

Suggests pure high anion gap metabolic acidosis.

Question 7

Interpret delta ratio > 2.0:

Answer 7

Suggests HAGMA with concurrent metabolic alkalosis or chronic respiratory acidosis.

Question 8

How do you calculate the A-a gradient?

Answer 8

A-a = (FiO₂ × 713) – (PaCO₂ × 1.25) – PaO₂

Question 9

What is a normal A-a gradient?

Answer 9

Around 10 + 1 mmHg per decade of age.

Question 10

What is the formula for PF ratio, and its significance?

Answer 10

PF ratio = PaO₂/FiO₂. <300 indicates ARDS.

Question 11

How do you identify the primary acid-base disorder?

Answer 11

Match pH with PaCO₂ and HCO₃⁻: use direction to classify as metabolic or respiratory.

Question 12

What is Winter’s formula and when is it used?

Answer 12

Used for metabolic acidosis: Expected PaCO₂ = (1.5 × HCO₃⁻) + 8 ± 2.

Question 13

What compensation is expected for metabolic alkalosis?

Answer 13

Expected PaCO₂ = (0.7 × HCO₃⁻) + 20 ± 5.

Question 14

How much does HCO₃⁻ change in acute respiratory acidosis?

Answer 14

↑1 mmol/L per 10 mmHg ↑ in PaCO₂.

Question 15

How much does HCO₃⁻ change in chronic respiratory acidosis?

Answer 15

↑4 mmol/L per 10 mmHg ↑ in PaCO₂.

Question 16

How much does HCO₃⁻ fall in acute respiratory alkalosis?

Answer 16

↓2 mmol/L per 10 mmHg ↓ in PaCO₂.

Question 17

How much does HCO₃⁻ fall in chronic respiratory alkalosis?

Answer 17

↓5 mmol/L per 10 mmHg ↓ in PaCO₂.

Question 18

What causes leukocyte larceny?

Answer 18

High WCC (e.g., leukaemia) → oxygen consumption in vitro → ↓ PaO₂.

Question 19

How does hypothermia affect ABG interpretation?

Answer 19

↓Temp = ↑ gas solubility → ↓ PaO₂ & PaCO₂. pH ↑ by 0.015/°C below 37.

Question 20

Alpha-stat vs pH-stat: what’s the difference?

Answer 20

Alpha-stat uses 37°C standard; pH-stat corrects to patient’s temperature.

Question 21

Why is pH-stat used in deep hypothermia?

Answer 21

Improves cerebral perfusion via CO₂-mediated vasodilation.

Question 22

How is pH measured in an ABG machine?

Answer 22

Glass electrode measures voltage ∝ [H⁺] concentration.

Question 23

How is PaCO₂ measured?

Answer 23

Severinghaus electrode: CO₂ diffuses into HCO₃ solution → pH change measured.

Question 24

How is PaO₂ measured?

Answer 24

Clark electrode: O₂ reduced → generates current proportional to PaO₂.

Question 25

How is HCO₃⁻ calculated?

Answer 25

Via Henderson-Hasselbalch: pH = 6.1 + log(HCO₃⁻ / (0.03 × PaCO₂)).

Question 26

What does Base Excess represent?

Answer 26

Amount of acid/base needed to titrate blood to pH 7.40 at 37°C and PaCO₂ of 40.

Question 27

What is Standard Base Excess (SBE)?

Answer 27

Base excess assuming Hb = 50 g/L to estimate extracellular fluid buffer status.

Question 28

What are the limitations of the anion gap?

Answer 28

Affected by lab errors, halide interference, hypoalbuminaemia, paraproteins, lithium.

Question 29

What can falsely increase chloride in lab tests?

Answer 29

Halides like bromide and iodide.

Question 30

What can cause a negative anion gap?

Answer 30

Lithium, magnesium, polymyxin B, severe hypoalbuminaemia, bromide toxicity.

Question 31

What ABG changes are seen in pregnancy?

Answer 31

↓ PaCO₂ (∼30 mmHg), ↑ PaO₂ (∼105 mmHg), ↓ HCO₃⁻ (∼20 mmol/L), normal or ↑ pH.

Question 32

Why does PaCO₂ drop in pregnancy?

Answer 32

Progesterone increases ventilatory drive → chronic respiratory alkalosis.

Question 33

How is O₂ delivery to the fetus maintained despite alkalosis?

Answer 33

↑ 2,3-DPG offsets pH effect → p50 remains normal.