Basics on blood chemistry and
ABG/VBG interpretation.
Luke Heath
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Always remove any air gap
and cap the syringe after
taking the blood.
Invert the syringe to mix the
blood with the heparin.
Process the sample within
10 minutes and invert
regularly while waiting.
Add all requested
information including FiO2,
as many values are
calculated.
Venous gasses have
differing respiratory
readings from arterial.
C6H12O6 (Glucose) + 6 O2 (Oxygen) =
6 CO2 (Carbon Dioxide) + 6 H20
(Water) + Energy (ADP to ATP)
Every living cell is performing the
above process, and will die very
quickly without oxygen. Blood must
transport oxygen and glucose to all
cells and remove the carbon dioxide
from them. Blood is the lifeline for
every cell in the body.
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Acidity is related to free hydrogen ions
in solution.
Carbon dioxide in solution is acidic.
Haemoglobins (Hb) affinity for oxygen
is related to acidity.
The body must try to maintain
homeostasis.
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Acidity is measured on the pH scale, going from 0 to
14.
Think of pH as the ‘power of Hydrogen’ (in the form of
free hydrogen ions, H+).
The scale is a negative log. Each one point represents a
ten fold increase/decrease in hydrogen ions, so small
changes in number represent large changes in H+ ion
concentrations. The lower the number, the more acidic
the solution.
Different chemicals and metabolites can result in free
hydrogen ions in solution and therefore a drop in pH
(increased acidity).
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-
(Carbon dioxide and water ↔ carbonic acid [weak acid] ↔ hydrogen
ion and bicarbonate)
The equation will balance itself (form an equilibrium),
so if there is a rise in CO2, there will be a
corresponding increase in H+ and HCO3-. Since H+ is
responsible for acidity, the more CO2 dissolved in
blood plasma, the more acid the blood becomes.
Haemoglobin has a reduced affinity for oxygen the
more acid the blood becomes. Oxygen at greater
pressure is required to overcome this decreased
affinity. This is called the Bohr effect.
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The pH of the blood is normal in the lungs, so
Hb has a high affinity for O2 and they bond.
In active cells CO2 is produced by respiration,
this creates an acid environment and O2
becomes less bonded to Hb and is released to
be used by the cells (the Bohr effect). CO2 will
now bond with Hb
When the blood reaches the lungs CO2 levels
drop, the blood becomes less acid and oxygen
levels rise helping push CO2 from Hb
allowing O2 to bond (the Haldane effect).
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The body has two methods of controlling blood
pH. Firstly, by controlling ventilation which will in
turn control the level of CO2 in the blood.
Respiratory rate is the primary control, but tidal
volumes can also increase to help change pH
rapidly.
Bicarbonate (HCO3-) is called a ‘buffer’, because
excess hydrogen ions can combine with it to
reduce their effect on pH. Non-respiratory
hydrogen ions can be removed as CO2 because of
it, which is why there is always a store of
bicarbonate in the blood.
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The second method is via the kidneys, which
are able to excrete H+ or HCO3- in the urine to
adjust pH. This renal (or metabolic) control
takes many hours or days to effect change
It will be seen in COPD patients, who will
increase their bicarbonate levels to prevent an
acid blood despite their raised CO2 levels.
However, ventilation is the primary pH
control, which is why respiratory rate is such
an important clinical observation.
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Oxygen and carbon dioxide are measured in
the blood by their partial pressure (sometimes
called ‘tension’).
The symbol is p.
The unit of measurement of this pressure is
kilopascal, abbreviated to kPa.
In normal arterial blood the partial pressure of
carbon dioxide is 4.6 – 6.4 kPa, and for oxygen
it is 11 – 14 kPa.
A venous sample will have a lower oxygen and
slightly raised carbon dioxide.
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Blood pH should be between 7.35 -7.45.
When ventilation or lung function are
compromised, CO2 levels rise in the blood
resulting in a raised pCO2 (> 6.4 kPa) and
acidic blood (< 7.35).
Small changes in pH represent substantial
changes in hydrogen ion concentrations, so a
pH < 7 is sever acidosis. It indicates that the
blood cannot function properly because Hb
will have lost its affinity to carry oxygen.
Type 1
Hypoxia (low O2) without hypercapnia (raised
CO2 ).
pO2 < 8 kPa but pCO2 normal
Type 2
Hypoxia with hypercapnia.
pO2 < 8 kPa with pCO2 >7
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Bicarbonate (HCO3-) is measured in mmol/L
(millimole per litre).
The normal range is 22 -26 mmol/L.
It helps ‘buffer’ hydrogen ions by promoting their
transfer into carbonic acid (which is less acidic
than hydrogen ions) and then CO2.
Excess bicarbonate can lead to metabolic alkalosis,
too little to metabolic acidosis. The base deficit is a
calculation of how much bicarbonate would be
required to normalise a deficiency, the base excess
how much extra has been produced.
When the pH is < 7.35 and the pCO2 is not
raised the cause of the acidosis will probably be
metabolic. Bicarbonate will be reduced (base
deficit) as it bonds with H+ to push the
equation to the left. This can be from:
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Lactic acid from non-oxygen respiration due to
high metabolic requirements (infection) or
hypoxia.
Ketoacidosis from fat metabolism in DKA.
Kidney failure.
When the pH is < 7.35 and the pCO2 is raised
the cause of the acidosis will probably be
respiratory. PO2 may be reduced or have been
partially corrected with increased FiO2.
Bicarbonate will be reduced (base deficit) as it
bonds with H+ to push the equation to the left.
This can be from:
1. Hypoventilation (CVE, opiate, obstruction)
2. Pulmonary embolism.
3. Chest trauma (mechanical failure)
When the pH is > 7.45 and the pCO2 is not low,
then the alkalosis will probably be metabolic in
nature. Bicarbonate may be raised (base
excess). It can be caused by:
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Loss of hydrogen ions (vomiting – look at the
chloride levels)
Excess bicarbonate (chronic hypoventilation)
Potassium ion exchange (hypokaleamia
response)
When a person
hyperventilates CO2 is
‘blown off’ causing blood
pH to rise (become
alkaline) resulting in a pH
> 7.45 and a pCO2 < 4.7
kPa. The pCO2 is a good
indicator of ventilatory
effectiveness.
Since there can be controlled and uncontrolled
metabolic or respiratory acidosis or alkalosis, it
follows that when an uncontrolled system creates
an acidosis a controlled system can try and correct
it. In chronic conditions the compensation will be
marked, in acute ones less so, but some
compensation will occur. However,
your body never over-compensates.
An example would be someone with COPD. An
ABG might indicate:
pH
7.35
pCO2
8.5 kPa
pO2
9.3 kPa
HCO3
34 mmol/L
The pH is normal, but it shouldn't be because the
CO2 is high. The reason is the raised bicarbonate
produced by the kidneys, which promotes
carbonic acid formation which is less acidic than
free hydrogen ions.
An example would be a septic patient. An ABG
might indicate:
pH
7.30
pCO2
2.4 kPa
pO2
10.2 kPa
HCO3
14.2 mmol/L
The pH is low, but it shouldn't be because the CO2
is low (the pH should be high). The low CO2
indicates hyperventilation. The reduced
bicarbonate shows it has been used to ‘mop up’
excess hydrogen ions to remove them as CO2.
Look at your patient! You will know if they are
chronic COPD or have been acutely vomiting.
Your body never over-compensates, whatever
the direction of the pH, the process that
corresponds with that will be the primary
cause, all else will be compensation.
Compensation can be partial or fully achieved
(ie. pH will be normal with abnormal numbers)
What are the oxygen levels?
Cells die from lack of oxygen – it is the primary
concern.
Is the pH normal?
If not, is it too high (alkali) or too low (acidic)
Does the pCO2 fit with the pH?
An acidic pH should have a high CO2 and vice versa.
Does the HCO3- fit with the pH?
An acidic pH should have a low HCO3- and vice versa.
Is the lactate raised?
A high lactate indicates an unmet metabolic demand
and suggests a metabolic cause.
What is the likely primary cause and what is the
compensation?
ctHb. (Calculated total haemoglobin)
The level of haemoglobin in the blood, given in
grams per litre. Normal range between 120 170, lower in women.
FO2Hb (Fraction of oxygenated haemoglobin)
The amount of haemoglobin bound with
oxygen (not the same as pO2) given as %.
Hctc (Haematocrit)
The proportion, by volume, of the blood that
consists of red blood cells given as a %.
cK+ (Calculated potassium)
Given in mmol/L. Normal range 3.4 – 4.5
cNa+ (Calculated sodium)
Given in mmol/L. Normal range 136 – 146
cCa2+ (Calcualted ionised calcium)
Given in mmol/L. Normal range 1.2 – 1.4
cCl- (Calculated chloride)
Given in mmol/L. Normal range 98 – 106
mOsm (Osmolality)
Given in mmol/kg. Normal range 275 -295
cLac (calculated lactate)
Given in mmol/L. Normal range 0.5 – 1.6
cBase (calculated base deficit/excess)
Given in mmol/L. Normal range -2.4 - +2.2
cHCO3- (calculated bicarbonate)
Given in mmol/L. Normal range 22 -26
Blood gasses contain a large amount of
information, that is what makes them so useful.
Learning the values takes time, however,
always check the basics such as pH, pO2, pCO2
Hb, electrolytes and base excess when you
have performed a test, or get the patients
doctor to sign that they have checked the
results slip. It would be pointless, and possibly
negligent, to perform a test that monitors the
life chemistry of a person who is unwell, and
not have those results reviewed by a competent
person.