A Normal Arterial Blood Gas (ABG)

A Normal Arterial Blood Gas (ABG)

What is an ABG?

For analysis a small sample of arterial blood (approximately 2ml) taken via an intermittent ‘stab’ into an artery (usually at the wrist, see above); or from an arterial sampling device (arterial line) situated in an artery (if on intensive care).

Arterial blood gases (ABGs) are usually only done when in hospital, if you are very unwell.

It measures the oxygen and carbon dioxide levels in your blood as well your blood’s pH (acid/alkali) balance; and some other things (see below).

These are the normal ranges of the various parts of an ABG test.

Normal ABG ranges

• pH = 7.35-7.45. Below 7.35 is acidosis (too much acid in the blood), above 7.45 is alkalosis (too much alkali in the blood); <7.3 needs action today
• pO2 (oxygen) = 11-13 kPa (75-100 mmHg). Below 10 kPa is hypoxia (low oxygen level in blood)
• pCO2 (carbon dioxide) = 4.5-6 kPa (35-45 mmHg). Above 6 kPa is hypercapnia (high carbon dioxide in blood)
• HCO3 (bicarbonate) = 22-26 mmol/L. Below 22 is acidosis, above 26 is alkalosis
• Base excess (BE) = -2 to +2 mmol/L
• Lactate = 0.5-1.0 mmol/L
• Oxygen (O2) saturation = 94-98%. Below 90% needs action today.

Venous blood gases (VBG)

Venous blood gases are also sometimes done in emergency situations.

This is because they are quicker, less painful for the patient and have fewer complications.

They are a good way of rapidly assessing acid-base balance (via bicarbonate level), haemoglobin and electrolytes/glucose (see below).

In this way VBGs are especially helpful in (say) the recovery phase of diabetic ketoacidosis (DKA); when you need daily assessment of acid/base balance but don’t want to do an invasive test like ABGs every day.

These are the normal values of a VBG.

• pH = 7.31-7.41
• pO2 = 4.0-5.3
• pCO2 = 5.5-6.8
• HCO3 = 23-29
• O2 saturation  = 75%.

Note 1. Clearly pO2, pCO2 and O2 saturation vary significantly between arterial and venous samples
Note 2. The blood gas analyser (for ABG and VBG) also measures other key substances:

• Haemoglobin
• Electrolytes (sodium (Na, potassium (K+), calcium (Ca2+), chloride (Cl-))
• Glucose.

Hence VBG are a quick way of assessing those parameters. Some analysers also measure urea and creatinine.

Interpretation of ABGs

• pH < 7.35 – blood is acidic
• pH > 7.45 – blood is alkaline.

Note. The pH scale ranges from 1–14, where 1 is the strongest acid, 14 is the strongest alkali and 7 is neutral. So, in fact, all these levels are slightly alkaline. It’s complicated!

In a patient with acidosis (pH <7.35):

• pCO2 > 6 kPa – respiratory acidosis (usually due to cardiorespiratory failure, e.g. exacerbation of COPD or acute heart failure)
• HCO3 < 22 mmol/L – metabolic acidosis (usually due to acute kidney injury (AKI), diabetic ketoacidosis (DKA), lactic acidosis or salicylate overdose)
• Base excess – more negative than -2 mmol/L – indicates a metabolic acidosis (negative base excess).

In a patient with alkalosis (pH > 7.45):

• pCO2 < 4.5 kPa – respiratory alkalosis (less common)
• HCO3 > 26 mmol/L – metabolic alkalosis (less common)
• Base excess – more positive than +2 mmol/L – indicates a metabolic alkalosis (positive base excess).

Lactate > 2.0 mmol/L is hyperlactataemia – and requires action today; levels above 4.0 mmol/L are severe and need action now.

It is a bit more complicated than this, as the following case represents.

ABGs in a typical stable patient with COPD (at home)

• pH: 7.36 (7.35-7.45)
• pO2: 8.0 (10–13). Patients with chronic lung disease can be stable at this level of hypoxia
• pCO2: 7.6 (4.5–6.0). Patients with chronic lung disease can be stable at this level of hypercapnia
• HCO3: 31 (22-26)
• BE: +5 (-2 to +2).

Interpretation

•  This is a compensated respiratory acidosis (mild).
  • This does not represent acute pathology
  • Rather it reflects a compensation for a chronic respiratory acidosis secondary to chronic obstructive pulmonary disease (COPD).
  • Note this is an acidosis, not an acidaemia (pH normal, but only due to compensatory mechanisms: the high bicarbonate).

Summary

We have described a normal ABG blood gas, and how its is interpreted. We hope it has been helpful.

Other Resource

Interpretation of ABGs

Review article: Castro, 2024