ABGs

PROCEDURE

•Sample of Arterial blood

•Stab

•Arterial line (“A” line)

• Usually medical staff or nursing staff

•Blood gas machine

•Correct temperature

•Analysis

PURPOSE OF ABGS

▪Determine the significance of respiratory or metabolic
compromise

▪Monitor deterioration / improvement

▪Identify respiratory failure

▪Guide treatment

▪Oxygen therapy

▪Ventilatory support

▪Outcome measure

▪e.g. following physiotherapy treatment to aid clinical
reasoning and treatment selection

Blood Gas Measurements

Normal Range

pH

Measure of level of acidity or
alkalinity of blood

7.35-7.45

PaCO2 Partial pressure of carbon dioxide
in arterial blood plasma

4.7 – 6 kPa
(35-45mmHg)

PaO2 Partial pressure of oxygen in
arterial blood plasma

10.7 – 13.3 kPa
(80-100mmHg)

SaO2 Saturation level of Hb in arterial
blood (measures how fully saturated
each unit of Hb is, given as a %)

95-100%

HCO3

-

Amount of bicarbonate (base) in the blood

22-26mmol l-1

BE
(base excess)

Base excess is a simpler way of
expressing bicarbonate levels &
gives the amount of HCO3

- above

or below the average value of 24mmoll-1

-2 to +2

PROCESS OF INTERPRETING ABGS

1.

Check pH

2.

Check PaCO2

3.

Check HCO3

-

4.

Check for compensation

5.

Check oxygenation

6.

Check for respiratory failure

1. CHECK PH

•pH is a measure of H+ ions in the blood

•Indicates an acid or alkali state

•pH is determined by the levels of CO2 (respiratory system)
& HCO3 (renal system)

•Decide whether pH is within normal limits

•If there is an acidosis pH below 7.35

•If there is an alkalosis pH above 7.45

2. CHECK PACO2

•If there is an acidosis or alkalosis

•Decide whether this is respiratory or metabolic

•Identify hypercapnia = PaCO2 above 6kPa

•Identify hypocapnia = PaCO2 below 4.7kPa

•Respiratory acidosis is when hypercapnia causes the blood to become more acidic

•pH = 7.20 (lower than normal)

•PaCO2 = 8.3kPa (higher than normal)

•PaO2 = 8.4kPa

•Respiratory alkalosis is when hypocapnia causes the blood to become more alkalotic (low paCO2 & high pH)

•pH = 7.49

•PaCO2 = 4.2kPa

•PaO2 = 15.0kPa

PACO2 – CLINICAL
REASONING

What effect would
the following have?

RR

PaCO2

7bpm

40bpm

3. CHECK HCO3- (OR BE)

•Identify whether the patient has a high or low bicarbonate or Base Excess

•Higher values (above 26mmol l-1 or BE > +2) makes blood
more alkaline – metabolic alkalosis

•pH = 7.5

•HCO3

- = 30mmol l-1

•BE = +8

•Lower values (below 22mmol l-1 or BE < -2) makes blood
more acidic – metabolic acidosis

•pH = 7.28

•HCO3

- = 14 mmol l-1

•BE = -8

Buffering Systems

The body has different ways of maintaining a normal acid-base balance:

● Buffering systems

● Respiratory system

● Renal system

4. CHECK FOR COMPENSATION

H+ +HCO3 <=> H2CO3 <=> H2O + CO2

Acid

Henderson –Hasslebach Equation

Base

2

4

6

8

10

12

Arterial PaCO2 (kPa)

7.3

7.2

7.4

7.5

7.6

7.1

7.7

7.0

Normal range

Arterial
pH

Acidosis

Alkalosis

Respiratory Alkalosis

Metabolic Alkalosis

Metabolic Acidosis

Chronic Respiratory

Acidosis

Acute Respiratory
Acidosis

RELATIONSHIP
TABLE

Primary
condition

pH

PaCO2

bicarbonate

Respiratory
acidosis

↓

↑

(↑)

Respiratory
alkalosis

↑

↓

(↓)

Metabolic
acidosis

↓

(↓)

↓

Metabolic
alkalosis

↑

(↑)

↑

COMPENSATION
EXAMPLES

Fully compensated
respiratory acidosis

pH = 7.36

PaCO2 = 8.4

HCO3

- = 28

BE = +4

Partially compensated
metabolic acidosis

pH = 7.32

PaCO2= 4.2

HCO3

- =16

BE = -6

PLEASE NOTE…..

It may be impossible to distinguish
between compensated respiratory
acidosis & compensated metabolic
alkalosis, or between compensated
respiratory alkalosis and
compensated metabolic acidosis
without looking at other clinical
findings or a series of ABGs.

5. CHECK OXYGENATION

•HAS NO PLACE IN THE INTERPRETATION OF ACID-BASE
STATUS AND NO EFFECT ON pH

•PaO2 reflects the ability of the lungs to allow the transfer of O2 from the environment to the circulating blood

•A reduced PaO2 regardless of FiO2 (how much oxygen the
patient is receiving) is called hypoxaemia

•Check if a reading has been taken while the patient is
receiving oxygen therapy, and if so document how much

•E.g. a PaO2 of 10.7kPa is normal – but would not be
considered normal if the patient is receiving 60% FiO2

•Oxygen therapy should be considered for patients with significant or worsening hypoxaemia

6. CHECK FOR

RESPIRATORY FAILURE

•Identify whether the patient is in respiratory failure

•Is it type I or type II

•Type I respiratory failure occurs when PaO2 is low and PaCO2 is normal or slightly reduced

•Type II respiratory failure occurs when PaO2 is low and PaCO2 is high

•Acute or Chronic respiratory failure

RESPIRATORY FAILURE

• pO2 < 8 kPa

• Type I

• Normal/low pCO2
• V/Q mismatch/diffusion limitation
• Ventilation able to compensate

• Type II

• ↑ PCO2
• ↓ pH if acute
• Ventilatory failure
• Needs controlled O2 ± mechanical ventilation

ACUTE RESPIRATORY DISEASE

When respiratory failure is due to acute respiratory disease, there may be little or no compensation from raised HCO3- levels because it takes time (days) for adequate compensation to occur.

Therefore pH is likely to be low due to respiratory acidosis

CHRONIC RESPIRATORY DISEASE

When respiratory failure is due to chronic respiratory disease there may be compensation from raised HCO3- levels.

Therefore the pH may be normal, or only slightly lower than normal due to compensated respiratory acidosis.

Check the current ABG results against any previous or usual
results as this will help to confirm the diagnosis of acute or
chronic failure

CONCEPT OF
HYPOXIC DRIVE

Case One

A 65 year old man enters the A & E department short of
breath. He has no chest pain but a productive cough. His
ABGs are reflected below. What do these show? Can you
suggest a possible cause?

pH 7.29
PaO2

8.2

PaCO2 8.4
HCO3- 24
BE 1

Case Two

A 16 year old girl is about to go theatre for removal of her tonsils.
She has no previous medical history but is extremely anxious.
Her ABGs immediately before her operation are shown below.
What do they show and what is the possible explanation?

pH 7.49
PaO2 10kPa

PaCO2 3.9kPa
HCO3 -22 mmol/l

BE -2

Case Three

An 80 year old ex-miner presents with these ABGs. What do they
show? Would you be concerned about these results? What is the
reason for your decision?

pH 7.39
PaO2 9.9 kPa

PaCO2 7.2 kPa
HCO3 - 32 mmol/l

BE 6

Case Four

A 25 year old man with no past medical history was admitted with a history of frequent urination, excessive thirst and nausea for the past 3 days. He was drowsy but coherent and his breathing was notably deep.

pH 7.1
PaO217kPa

PaCO2 2.5 kPa
HCO3 - 17 mmol/l

BE - 6