Arterial Blood Gases

What is the pH?

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.


What is the pCO2?

The pCO2 is a marker of ventilation. A normal pCO2 is 35-45mmHg.

• A high pCO2 is > 45 mmHg and implies hypoventilation
• A low pCO2 is < 35 mmHg and implies hyperventilation

A venous CO2 is ~ 5mmHg higher than an arterial pCO2.

Respiratory acidosis (pCO2 > 45) is the result of hypoventilation, causes include:

• Opioid intoxication
• COPD

Respiratory alkalosis (pCO2 < 35) is the result of hyperventilation, causes include:

• Hypoxia
• Anxiety

What is the bicarbonate?

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

• a low HCO3 (< 24) implies a metabolic acidosis
• a raised HCO3 (> 24) 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 more than 3) implying a metabolic alkalosis.

Is there any compensation?

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. Respiratory compensation

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, pCO2 = 1.5 [HCO3] + 8
• in metabolic alkalosis, pCO2 = 0.7 [HCO3] + 20

Metabolic compensation

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 10mmHg of pCO2 the HCO3 rises by 1mmol/L
• Chronically, for every rise in 10mmHg of CO2 the HCO3 rises by 4mmol/L

In respiratory alkalosis

• Acutely, for every fall in 10 mmHg of CO2 the HCO3 falls by 2 mmol/L
• Chronically, for every fall in 10mmHg of CO2 the HCO3 falls by 5 mmol/L

The anion Gap is the measured cations minus the measured anions and reflects any unmeasured anions. The normal value is < 12.

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

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• 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.