INTERPRETING ARTERIAL
BLOOD GASES
With thanks to Allison Keegan
Advanced Critical Care Practitioner
NMGH

Learning Outcomes
•Revise the normal physiological control of blood
gases
•Utilise a systematic approach to blood gas analysis
•Apply the above to clinical scenarios & discuss
common treatments

Why do we measure arterial blood
gases?
•Assess oxygenation: How much oxygen is being
delivered to the organs/tissues? Is the pO2
appropriate given the amount of oxygen the patient
is on?
•Determine acid-base: What is the pH? Is this driven
by a respiratory problem or a metabolic one? (Or
both?)
•Tells us how unwell the patient is and aids with
diagnosis & treatment decisions

Arterial Blood Gas Sampling
Sample sites:
•ARTERIAL STAB – WARD AREAS
•IT HURTS! ONLY DO IT IF NECESSARY!
•Arterial cannula - Critical Care
•Capillary - Paeds/Cardio-resp/some chest wards
• WEAR GLOVES WHEN TAKING & TRANSPORTING SAMPLES

Oxygen Dissociation Curve
This helps us
understand how
blood carries and
releases oxygen.
It plots the
proportion of
oxyhaemoglobin
(haemoglobin that
is oxygen-
saturated) vs
oxygen tension.
(more info on the oxygen
dissociation curve can be
found back in the webpage)

Normal Physiological Control of pH balance
Changes in arterial pCO2 & pO2 are detected by:
CENTRAL CHEMORECEPTORS
• In medulla
• Most responsive to increased arterial pCO2
• CO2 diffuses into CSF where it becomes hydrated and forms
carbonic acid
• Receptors detect decrease in pH of CSF
PERIPHERAL CHEMORECEPTORS
• In aorta and carotid arteries
• Most responsive to decreased arterial pO2

Normal Physiological Control of pH
•Intake & production of hydrogen ions & their
excretion must be controlled to maintain
homeostasis. Normal pH of blood is 7.35- 7.45
•Blood pH maintained by:
• Respiratory system
• Renal system
• Buffer systems
• Bicarbonate
• Phosphate
• proteins (Haemoglobin)

Acids & Bases
•Acid – Substance that can donate a hydrogen ion to a
solution
•Base – Substance that can accept a H+ ion, thereby
removing it from a solution. Main base in blood is
bicarbonate
Two types of acids
• Volatile acids – move between liquid & gaseous states and are
removed by the lungs eg carbonic acid is broken down to CO2
& H2O and removed by the lungs
• Non-volatile acids – cannot change into a gaseous form & can
only be excreted by metabolic processes eg Lactic acid and
Ketoacids

Compensation Mechanisms to maintain pH
1. Respiratory system
• Fast response - should be
immediate
• The respiratory system assists
in maintaining pH balance by
either retaining or excreting
carbon dioxide (CO2)
• PaCO2 – dissolves into carbonic
acid in presence of water
H2O + CO2  H2CO3  H+ + HCO3
• Carbon dioxide is a ‘volatile’
acid - it moves easily between
its gaseous and liquid form -
easier to excrete
2. Renal/Metabolic system
The kidneys can respond to imbalances by:
• Eliminating H+ or HCO3
• Retaining H+ or HCO3
NB
Slower response - approx.2-4 days
Non-volatile acids cannot change into a
gaseous state & can only be excreted by
metabolic processes
e.g. Lactic acid, ketones
3. Buffering systems
Buffers act like chemical sponges either
soaking up or releasing H+
This is a fast response
There are 3 main buffers:–
Bicarbonate
Haemoglobin and other proteins
Inorganic phosphates & other buffers
(cellular)

Normal Values for Arterial Blood Gases
pH 7.35 – 7.45
PaO2 10 – 13 kPa
PaCO2 4.7 – 6 kPa
HCO3 22 – 26 mEq/L
Base Excess +2 /-2
NB On air

A Systematic Approach to ABG Analysis
•pO2 Is the patient hypoxic?
•pH Is this normal, acidotic or alkalotic?
•PaCO2 Is the respiratory system driving the
derangement in pH?
•HCO3 & Is the metabolic system driving the
BE derangement in pH?
•Any compensation? Partial or complete?

Assessment of oxygenation
What is the PaO2?
• Healthy PaO2 = 10 kPa on air (21%)
•  PaO2 should normally be approx. 10kPa less than the % of
inspired oxygen concentration. E.g. a patient on 50% oxygen is
expected to have a PaO2 of approximately 40kPa.

Respiratory Failure
Hypoxic patients can display two forms of respiratory failure:
Type 1 respiratory failure: hypoxaemia with normocapnia
(normal PaCO2)
Type 2 respiratory failure: hypoxaemia with hypercapnia
(PaCO2 > 6.0kPa)
This usually causes a respiratory acidosis. In patients with
chronic respiratory conditions type 2 respiratory failure may
be accompanied by partial or complete metabolic
compensation

CO2 as a driver of an abnormal pH
pH CO2 HCO3-
Respiratory
acidosis
  Normal
Respiratory
alkalosis
  Normal
Respiratory
acidosis with
metabolic
compensation
 /   
Respiratory
alkalosis with
metabolic
compensation
 /   
The PaCO2 will help rule in
or out the respiratory
system as a cause of an
abnormal pH.
Excess CO2 causes a
decrease in pH in the
blood causing acidosis.
Low levels of CO2 will make
the blood more alkalotic.
Does the CO2 level on the
ABG fit with the pH?

Metabolic system as the driver of an abnormal
pH
pH CO2 HCO3-
Metabolic acidosis   Normal
Metabolic alkalosis   Normal
Metabolic acidosis
with respiratory
compensation
  
Metabolic alkalosis
with respiratory
compensation   
HCO3- is a base that “mops
up” acids. A high HCO3-
raises the pH causing
alkalosis, a low HCO3-
decreases the pH causing
acidosis.
Base excess (BE) is a surrogate
marker for metabolic acidosis
or alkalosis. BE lowers in
metabolic acidosis and
increases in metabolic
alkalosis.
Does the HCO3- level on the
ABG fit with the pH?

Degree of Compensation
• Examine pH, PaCO2 and HCO3
• None: pH abnormal with no evidence of respiratory or metabolic
compensation
- may indicate acute onset of illness
• Partial: pH is still abnormal but either the respiratory or metabolic
system is working to correct the imbalance
• Complete: pH normal because the system not responsible for
primary problem has compensated for the imbalance
- usually indicates chronic disease process

Other considerations
• Mixed acidosis & alkalosis: the CO2 and HCO3– will be moving in
opposite directions and will both contribute to the pH abnormality
• Anion Gap: In metabolic acidosis this equation helps determine how
much of the acidosis is caused by unmeasured anions (e.g. lactate,
ketones, uric acid, toxins)

The following are some scenarios for you to review and work
through.
The answers are back in the webpage but do try to work the
answers through yourself before looking at them!

• 19 yr old male deposited outside A&E by his mates!
• Signs of drug abuse
• RR 7bpm & shallow
• Only responding to painful stimuli
• ABG taken on non-rebreathe mask - 85% oxygen
pH 7.20
PaO2 20 kPa
PaCO2 10 kPa
HCO3 25 mmol/L
BE -1.4 mmol/L
Scenario 1

Scenario 2
• 33 year old IDDM
• Been unwell for 3 days - dysuria, frequency, pyrexial
• Last 24 hrs - sweaty, nauseous, not eating
• ABG taken on 60% oxygen via face mask
pH 7.02
PaO2 33 kPa
PaCO2 3 kPa
HCO3 12 mmol/L
BE -10 mmol/L

Scenario 3
• 16 year old girl admitted with abdominal pain
• Been arguing with boyfriend on ward
• RR 34 bpm
• ABG taken on room air
pH 7.5
PaO2 13 kPa
PaCO2 3.3 kPa
HCO3 24 mmol/L
BE +2 mmol/L

Scenario 4
• 65 yr old man admitted to MAU
• 3 day history of SOB, chesty cough, malaise
• Smokes 20 cigs/day, ”always been chesty”
• Cyanosed, RR 40bpm
• ABG taken on 24% venturi mask
pH 7.15
PaO2 8 kPa
PaCO2 11 kPa
HCO3 24 mmol/L
BE -2

Scenario 5
• 65 yr old man admitted to MAU
• 3 day history of SOB, chesty cough, malaise
• Smokes 40 cigs/day, ”always been chesty”
• Slightly cyanosed, RR 35bpm
• ABG taken on 24% venturi mask
pH 7.45
PaO2 10 kPa
PaCO2 10 kPa
HCO3 32 mmol/L
BE 0

Scenario 6
• 4hrs later - still in MEU - RR now 40 bpm
• Been confused & agitated but now settled & asleep!
• ABG taken on 24% venturi mask
pH 7.20
PaO2 7 kPa
PaCO2 13 kPa
HCO3 32 mmol/L
BE -2

Scenario 7
• 45 yr old lady admitted to medical ward 2 days ago
• Known ALD with cirrhosis, asthma, INR 2.1
• Increasing 02 requirements – now on 60%, RR35
•Planned for discharge home today – also low BP
pH 7.15
PaO2 6 kPa
PaCO2 3 kPa
HCO3 16 mmol/L
BE -7 mmol/L

Scenario 8
• 65 yr male admitted with constipation/abdo pain
• Pyrexial, looks awful , RR 30, distended abdomen
• ABG taken on 35% oxygen via face mask
pH 7.10
PaO2 10 kPa
PaCO2 2 kPa
HCO3 14 mmol/L
BE -12 mmol/L
Lactate 7

Scenario 9
• 42 yr old man admitted following OD of Amitryptilline
• Has had a seizure in the ambulance
• Now unconscious but breathing spontaneously
• RR 7 bpm, awaiting anaesthetist
• ABG taken on non-rebreathe mask
pH 6.9
PaO2 40 kPa
PaCO2 9 kPa
HCO3 10 mmol/L
BE -20

Scenario 10
• 90 yr old lady post-arrest
• Intubated during the arrest, unresponsive
• Hypotensive, arrhythmic, twitching
pH 6.9
PaO2 8 kPa
PaCO2 8 kPa
HCO3 13 mmol/L
BE -12

Scenario 11
• 60 yr old man on surgical ward
• He had repair of ruptured AAA 3 days ago
• Been oliguric for 24hrs despite an adequate BP
• ABG taken on 35% humidified oxygen mask
pH 7.20
PaO2 14 kPa
PaCO2 4 kPa
HCO3 18 mmol/L
BE -8

Scenario 12
• 77 yr old lady post-op ureteric stent replacement
• RR 35 bpm, BP 80/40, T 39, HR 120 AF
• ABG taken on 35% oxygen
pH 7.15
PaO2 10 kPa
PaCO2 3 kPa
HCO3 15 mmol/L
BE -10