blood gases

What does an ABG measure?

pH, pO₂, pCO₂, HCO₃⁻, base excess, lactate

Normal pH range?

7.35–7.45

Normal pCO₂ range?

4.7–6.0 kPa

Normal HCO₃⁻ range?

22–26 mmol/L

Normal pO₂ range?

10.6–13.3 kPa

Normal base excess?

−2 to +2

Normal lactate?

<2 mmol/L

Why do ABG results matter in acute patients?

Results are instant and guide urgent management

Does pO₂ alone determine acid–base status?

No, but it determines oxygenation

Which values relate to acid–base?

pH, CO₂, HCO₃⁻, base excess

Which value tells you respiratory contribution to pH?

pCO₂

Which value tells you metabolic contribution to pH?

HCO₃⁻

Henderson–Hasselbalch equation?

pH = 6.1 + log10( HCO₃⁻ / (0.03 × pCO₂) )

What does the equation describe?

Relationship between pH, bicarbonate, and CO₂

What does CO₂ act as in acid–base chemistry?

An acid (forms carbonic acid)

What does HCO₃⁻ act as?

A base (buffer)

What determines pH most strongly?

The ratio HCO₃⁻ / CO₂

If CO₂ increases, what happens to pH?

pH decreases (respiratory acidosis)

If CO₂ decreases, what happens to pH?

pH increases (respiratory alkalosis)

If HCO₃⁻ increases, what happens to pH?

pH increases (metabolic alkalosis)

If HCO₃⁻ decreases, what happens to pH?

pH decreases (metabolic acidosis)

Which system adjusts CO₂?

Lungs (fast, minutes)

Which system adjusts HCO₃⁻?

Kidneys (slow, hours–days)

What is carbonic acid proportional to?

pCO₂ × 0.03

Why is HCO₃⁻ not directly measured?

It is derived using H–H equation

What is pKa in this system?

A fixed constant (6.1)

Actual Base Excess definition?

Mmol of strong acid required to return blood to pH 7.4 assuming normal pCO₂

Standard Base Excess definition?

Base excess corrected for haemoglobin buffering in extracellular fluid

What does negative base excess indicate?

Base deficit → metabolic acidosis

What does high base excess indicate?

Metabolic alkalosis

Why use base excess?

More accurate measure of metabolic contribution than HCO₃⁻

Actual bicarbonate definition?

Machine-derived bicarbonate at measured conditions

Standard bicarbonate definition?

HCO₃⁻ adjusted to standard pCO₂ of 5.3 kPa at 37°C

Why standardize bicarbonate?

To remove respiratory influence

First step of ABG interpretation?

Assess pH (acidosis vs alkalosis)

Second step?

Check pCO₂ (respiratory component)

Third step?

Check HCO₃⁻ (metabolic component)

How to identify respiratory acidosis?

High CO₂, low pH

How to identify respiratory alkalosis?

Low CO₂, high pH

How to identify metabolic acidosis?

Low HCO₃⁻, low pH

How to identify metabolic alkalosis?

High HCO₃⁻, high pH

What is compensation?

A secondary process trying to normalize pH

Fastest compensatory system?

Respiratory (minutes)

Slowest compensatory system?

Metabolic (kidneys, hours–days)

Can compensation overshoot?

No, never

Full compensation—when does pH normalise?

Chronic respiratory disorders only

Partially compensated metabolic acidosis example?

DKA with low CO₂

Partially compensated respiratory acidosis example?

COPD with high HCO₃⁻

What does the Davenport diagram show?

Graphical relationship between pH, HCO₃⁻, and pCO₂

What does each isobar represent?

A line of constant pCO₂

Why is Davenport diagram useful?

Shows physiologic responses to acid–base disorders

Where is metabolic acidosis located on the diagram?

Low pH, low HCO₃⁻ region

Where is respiratory acidosis located?

Low pH, high HCO₃⁻ (with high CO₂)

Purpose of the anion gap?

Detect unmeasured anions causing metabolic acidosis

Anion gap formula (standard)?

AG = Na⁺ − (Cl⁻ + HCO₃⁻)

Why does the anion gap exist?

Blood must remain electrically neutral

What are unmeasured anions?

Proteins, phosphates, sulphates, organic acids

Normal AG range?

4–16 mmol/L (local reference)

High AG metabolic acidosis cause?

Addition of organic acids

Examples of high AG causes?

DKA, lactic acidosis, renal failure, toxins

Normal AG metabolic acidosis cause?

Loss of bicarbonate

Examples of normal AG acidosis

Diarrhoea, renal tubular acidosis

Why does chloride rise in normal AG acidosis?

To replace lost bicarbonate (hyperchloraemia)

Example of AG calculation

AG = 133 − (108 + 19) = 6

Interpretation of AG = 6 (normal)?

Loss of bicarbonate (not organic acid accumulation)

Why is potassium sometimes added?

Some formulas include K⁺ as a measured cation

What does high AG indicate physiologically?

Accumulation of unmeasured anions

Why does diarrhoea cause normal AG acidosis?

Loss of bicarbonate from stool

A–a gradient definition?

Alveolar–arterial oxygen difference

Purpose?

Semi-quantitative measure of gas exchange

What does A–a gradient detect?

V/Q mismatch, shunt, diffusion impairment

What increases A–a gradient?

PE, pneumonia, fibrosis, shunt

What keeps A–a gradient normal?

Hypoventilation without V/Q mismatch

What three variables needed to calculate A–a?

FiO₂, pCO₂, pO₂

What equation concept?

PAO₂ = FiO₂ × (PiO₂ − (pCO₂ / R))

What reduces PiO₂?

Water vapour pressure

Why is arterial CO₂ used in calculation?

pACO₂ approximates pCO₂

Why is FiO₂ from nasal cannula unreliable?

Depends on respiratory rate, tidal volume

Why are Venturi masks better?

Deliver fixed, predictable FiO₂

What does a large A–a gradient indicate?

Problem with oxygen transfer

What is FiO₂?

Fraction of inspired oxygen

Room air FiO₂?

21%

Why is nasal cannula FiO₂ variable?

Depends on patient’s breathing pattern

What happens with high RR/large tidal volume on nasal O₂?

Fraction of O₂ delivered decreases

Why can overoxygenation be harmful?

Causes CO₂ retention, oxidative injury

Why must oxygen be prescribed?

It is a drug with risks

What do Venturi masks use?

Bernoulli principle

Advantage of Venturi masks?

Fixed FiO₂ regardless of breathing pattern

Why is FiO₂ estimation “imprecise”?

True inspired oxygen depends on mixing with room air

What increases FiO₂ delivered by nasal cannula?

Slower RR, smaller tidal volume

P/F ratio formula?

PaO₂ / FiO₂

What is it used for?

Assessing ARDS severity

Why is it quick but limited?

Insensitive to subtle gas exchange problems

What decreases P/F ratio?

Severe V/Q mismatch, ARDS

Normal P/F ratio?

300

Type 1 respiratory failure definition?

Hypoxia with normal/low CO₂

Cause of Type 1 failure?

Gas exchange failure

Examples of Type 1 causes?

PE, pneumonia, fibrosis

Treatment for Type 1?

CPAP to keep alveoli open

Type 2 respiratory failure definition?

Hypoxia + hypercapnia