Blood Gas Analysis

Blood gas analysis is readily available and provides useful information about the acid-base status of a patient. The following is a simplified approach to interpreting arterial blood gas analysis. A more comprehensive resource is found at http://www.anaesthesiamcq.com/AcidBaseBook/ABindex.php

A normal pH is 7.35 to 7.45.

• Acidaemia: low pH < 7.35
• Alkalaemia: high pH > 7.45

The venous pH is ~ 0.05 lower than arterial pH.

The PCO₂ is a marker of ventilation. A normal PCO₂ is 35-45mmHg (or 4.7-6.0 kPa).

• A high PCO₂ is > 45 mmHg and implies hypoventilation.
• A low PCO₂ is < 35 mmHg and implies hyperventilation.

A venous PCO₂ is ~5 mmHg higher than an arterial PCO₂.

Respiratory acidosis (PCO₂ > 45 mmHg) is the result of hypoventilation, causes include:

• Opioid intoxication
• COPD

Respiratory alkalosis (PCO₂ < 35 mmHg) is the result of hyperventilation, causes include:

• Hypoxia
• Anxiety

The bicarbonate is a marker for the metabolic acid-base status of a patient. A normal HCO₃⁻ is 22-26 mmol/L, but we tend to use 24 mmol/L for calculations.

• A low HCO₃⁻ (< 24 mmol/L) implies a metabolic acidosis.
• A raised HCO₃⁻ (> 24 mmol/L) implies a metabolic alkalosis.

The base excess gives similar information with a normal BE being −3 to +3. With a low base excess (BE less than −3) implying a metabolic acidosis and a raised base excess (BE greater than +3) implying a metabolic alkalosis.

Both the lungs and kidneys adapt to compensate for acid-base disturbances in an attempt to bring the pH closer to normal. The adequacy of this compensation should be assessed.

A quick rule is that the PCO₂ should roughly equal the last two digits of the pH value. This only works within a pH range of 7.1-7.6.

A better rule is that:

• In metabolic acidosis, expected PCO₂ = 1.5 [HCO₃⁻] + 8
• In metabolic alkalosis, expected PCO₂ = 0.7 [HCO₃⁻] + 20

Renal metabolic compensation occurs quickly via intracellular buffering, and more slowly via the kidney, where under normal conditions, HCO₃⁻ is absorbed and H⁺ is secreted in varying amounts.

The following rules can determine the adequacy of metabolic derangement:

In respiratory acidosis:

• Acutely, for every rise in 10 mmHg of PCO₂ the HCO₃⁻ rises by 1 mmol/L
• Chronically, for every rise in 10 mmHg of PCO₂ the HCO₃⁻ rises by 4 mmol/L

In respiratory alkalosis:

• Acutely, for every fall in 10 mmHg of PCO₂ the HCO₃⁻ falls by 2 mmol/L
• Chronically, for every fall in 10mmHg of PCO₂ the HCO₃⁻ falls by 5 mmol/L

The Anion Gap is the difference between measured cations and measured anions. It reflects any unmeasured anions. The normal value is < 12 mmol/L.

It is calculated by the equation Na⁺ − (Cl⁻ + HCO₃⁻)

When you have a metabolic acidosis, you need to measure this value, as it helps determine what sort of metabolic acidosis exists.

A high anion gap metabolic acidosis (HAGMA) occurs when AG is > 12

The causes of HAGMA can be grouped into

• Lactic acidosis
• Ketoacidosis
• Renal failure
• Toxins (eg toxic alcohols)

Some prefer difficult to remember mnemonics such as CAT MUDPILES

• Carbon monoxide, cyanide
• Alcohol, alcohol ketoacidosis
• Toluene
• Metformin, methanol
• Uraemia
• Diabetic ketoacidosis
• Paracetamol, propylene glycol, pyroglutamic acid
• Iron, isoniazid
• Lactic acidosis
• Ethylene glycol
• Salicylates

A normal anion gap metabolic acidosis (NAGMA) occurs when AG ≤ 12

A NAGMA is due to either bicarbonate loss or chloride gain.

There is another difficult to remember mnemonic (USED CARP)

• Ureterostomy
• Small bowel fistula
• Extra chloride
• Diarrhoea
• Carbonic anhydrase inhibitors
• Adrenal insufficiency
• Renal tubular acidosis
• Pancreatic fistula

• PO₂ denotes oxygenation of the blood, a PO₂ < 60mmHg is concerning for hypoxia.
• Lactate is often quantified, with normal concentrations < 2mmol/L.
• Electrolytes such as sodium (Na⁺), potassium (K⁺) and chloride (Cl⁻) are usually reported on a blood gas
• COHb quantifies the percentage of circulating carboxyhaemoglobin. Smoking can be associated with levels up to 10%.
• MetHb quantifies the percentage of circulating methaemoglobin.