When and why is an ABG required?

Published on 09/04/2015 by admin

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Last modified 22/04/2025

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1.7 When and why is an ABG required?

1 To establish a diagnosis

The ABG is integral to the diagnosis of respiratory failure and primary hyperventilation. It also identifies the presence of metabolic acidosis and alkalosis.

2 To assess illness severity

The four ABG values shown in the table below, in addition to overall clinical assessment, may help identify critically unwell patients requiring urgent intervention.

PaO2< 8 kPa

Below 8 kPa, falls in PaO2 produce a marked reduction in SaO2 (the ‘steep’ part of the O2 dissociation curve) and hence O2 content of blood

Rising PaCO2

Since renal compensation occurs over days to weeks, acute rises in PaCO2 produce a corresponding drop in pH. In respiratory distress, rising PaCO2 often signifies exhaustion and is an ominous sign. Patients require urgent reversal of the process, leading to ventilatory failure or assisted ventilation

BE < –10/HCO3< 15

This value is included in several severity scoring systems and, when due to lactic acidosis, indicates severe hypoxia at cellular level

H+ > 55/pH < 7.25

A significant decrease in pH outside the normal range indicates that compensatory mechanisms have been overwhelmed and is a medical emergency

3 To guide and monitor treatment

Regular ABG monitoring can help provide early warning of deterioration and judge the effectiveness of therapeutic interventions. It is essential for titrating O2 therapy in patients with chronic type 2 respiratory failure and for optimising ventilator settings.

Box 1.7.1 Clinical scenarios in which an ABG is useful

Establishing diagnosis and assessing illness severity

Suspected hypercapnia (↑PaCO2)

Drowsiness, flapping tremor, bounding pulses
Clinical deterioration in patient with chronic type 2 respiratory impairment or predisposing condition (e.g. chronic obstructive pulmonary disease)

Suspected severe hypoxaemia

Very low or unrecordable O2 saturation; cyanosis

Severe, prolonged or worsening respiratory distress

Smoke inhalation (carboxyhaemoglobin level)

Hyperventilation (confirm ↓PaCO2, check for underlying metabolic acidosis)

Acute deterioration in consciousness

Any severely unwell patient*

Pulse oximetry unreliable or suspicious result

As part of a recognised illness severity scoring system (e.g. Glasgow criteria in pancreatitis)

Guiding treatment and monitoring response

Mechanically ventilated patients

Patients receiving non-invasive assisted ventilation

Patients with respiratory failure

Patients with chronic hypercapnia receiving O2

Critically ill patients undergoing surgery

Candidates for long-term oxygen therapy

* Including, but not restricted to, shock, sepsis, burns, major trauma, acute abdomen, poisoning, cardiac/renal/hepatic failure, diabetic ketoacidosis.

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