Arterial blood gas
By
Mrs. Amala Rajan
Reader
Medical Nursing Dept

“Life is a struggle, not against
sin, not against the Money
Power, not against malicious
animal magnetism, but against
hydrogen ions."
H.L. MENCKEN

What is an ABG
Arterial Blood Gas
Drawn from artery- radial, brachial, femoral
It is an invasive procedure.
Caution must be taken with patient on
anticoagulants.
Arterial blood gas analysis is an essential part
of diagnosing and managing the patient’s
oxygenation status, ventilation failure and acid
base balance.

What Is An ABG?
pH [H+
]
PCO2 Partial pressure
CO2
PO2 Partial pressure
O2
HCO3 Bicarbonate
BE Base excess

Acid/Base Balance
 The pH is a measurement of the acidity or
alkalinity of the blood.
 It is inversely proportional to the no. of (H+) in
the blood.
 The normal pH range is 7.35-7.45.
Changes in body system functions that occur in
an acidic state decreases the force of cardiac
contractions, decreases the vascular response to
catecholamines, and a diminished response to the
effects and actions of certain medications.
An alkalotic state interferes with tissue
oxygenation and normal neurological and
muscular functioning.
Significant changes in the blood pH above 7.8 or

H2O + CO2  H2CO3  HCO3
+ H+
Acid/Base Relationship

There are two buffers that work in
pairs
H2CO3 NaHCO3
Carbonic acid base bicarbonate
These buffers are linked to the
respiratory and renal compensatory
system
Buffers

The Respiratory buffer
response
• The blood pH will change
acc.to the level of H2CO3
present.
• This triggers the lungs to
either increase or decrease
the rate and depth of
ventilation
• Activation of the lungs to
compensate for an

The Renal Buffer
Response
• The kidneys excrete or retain
bicarbonate(HCO3-).
• If blood pH decreases, the
kidneys will compensate by
retaining HCO3
• Renal system may take from
hours to days to correct the
imbalance.

ACID BASE DISORDER
Res. Acidosis
• is defined as a pH less than 7.35
with a pCO2 greater than 45
mmHg.
• Acidosis –accumulation of co2,
combines with water in the body
to produce carbonic acid, thus
lowering the pH of the blood.
• Any condition that results in
hypoventilation can cause

Causes
1. Central nervous system depression r/t
medications such as narcotics, sedatives,
or anesthesia.
2. Impaired muscle function r/t spinal cord
injury, neuromuscular diseases, or
neuromuscular blocking drugs.
3. Pulmonary disorders such as atelectasis,
pneumonia, pneumothorax, pulmonary
edema or bronchial obstruction
4. Massive pulmonary embolus
5. Hypoventilation due to pain chest wall
injury, or abdominal pain.

Signs & symptoms of Respiratory
Acidosis
• Respiratory : Dyspnoea, respiratory
distress and/or shallow respiration.
• Nervous: Headache, restlessness and
confusion. If co2 level extremely high
drowsiness and unresponsiveness
may be noted.
• CVS: Tacycardia and dysrhythmias

Management
• Increase the ventilation.
• Causes can be treated rapidly include
pneumothorax, pain and CNS
depression r/t medication.
• If the cause can not be readily
resolved, mechanical ventilation.

Respiratory alkalosis
• Psychological responses, anxiety or fear.
• Pain
• Increased metabolic demands such as fever,
sepsis, pregnancy or thyrotoxicosis.
• Medications such as respiratory stimulants.
• Central nervous system lesions

Signs & symptoms
• CNS: Light Headedness, numbness,
tingling, confusion, inability to
concentrate and blurred vision.
• Dysrhythmias and palpitations
• Dry mouth, diaphoresis and tetanic
spasms of the arms and legs.

Management
• Resolve the underlying problem
• Monitor for respiratory muscle
fatigue
• When the respiratory muscle
become exhausted, acute
respiratory failure may ensue

Metabolic Acidosis
• Bicarbonate less than 22mEq/L with a
pH of less than 7.35.
• Renal failure
• Diabetic ketoacidosis
• Anaerobic metabolism
• Starvation
• Salicylate intoxication

Sign & symptoms
• CNS: Headache, confusion and
restlessness progressing to lethargy,
then stupor or coma.
• CVS: Dysrhythmias
• Kussmaul’s respirations
• Warm, flushed skin as well as nausea
and vomiting

Management
• Treat the cause
• Hypoxia of any tissue bed will produce
metabolic acids as a result of anaerobic
metabolism even if the pao2 is normal
• Restore tissue perfusion to the hypoxic
tissues
• The use of bicarbonate is indicated for
known bicarbonate - responsive acidosis
such as seen with renal failure

Metabolic alkalosis
• Bicarbonate more than 26m Eq /L with a pH
more than 7.45
• Excess of base /loss of acid can cause
• Ingestion of excess antacids, excess use of
bicarbonate, or use of lactate in dialysis.
• Protracted vomiting, gastric
suction,hypchoremia,excess use of
diuretics, or high levels of aldesterone.

Signs/symptoms
• CNS: Dizziness, lethargy
disorientation, siezures & coma.
• M/S: weakness, muscle twitching,
muscle cramps and tetany.
• Nausea, vomiting and respiratory
depression.
• It is difficult to treat.

COMPONENTS OF THE
ABG
pH: Measurement of acidity or alkalinity, based on the hydrogen (H+)
7.35 – 7.45
Pao2 The partial pressure oxygen that is dissolved in arterial
blood.
80-100 mm Hg.
PCO2: The amount of carbon dioxide dissolved in arterial blood.
35– 45 mmHg
HCO3
: The calculated value of the amount of bicarbonate in the blood
22 – 26 mmol/L
B.E:
The base excess indicates the amount of excess or insufficient
level of bicarbonate. -2 to +2mEq/L
(A negative base excess indicates a base deficit in blood)
SaO2:The arterial oxygen saturation.

Stepwise approach to ABG
• Step 1: Acidemic or Alkalemic?
• Step 2: Is the primary disturbance respiratory or
metabolic?
• Step 3. Asses to Pa O2. A value below 80mm Hg
indicates Hypoxemia. For a respiratory
disturbance, determine whether it is acute or
chronic.
• Step 4. For a metabolic acidosis, determine
whether an anion gap is present.
• Step 5. Assess the normal compensation by the
respiratory system for a metabolic disturbance

STEPS TO AN ABG
INTERPRETATION
• Step:1
• Assess the pH –acidotic/alkalotic
• If above 7.5 – alkalotic
• If below 7.35 – acidotic

Contd…..
• Step 2:
• Assess the paCO2 level.
• pH decreases below 7.35, the paCO2
should rise.
• If pH rises above 7.45 paCO2 should
fall.
• If pH and paCO2 moves in opposite
direction – primary respiratory
problem.

contd
• Step:2
• Assess HCO3 value
• If pH increases the HCO3 should also
increase
• If pH decreases HCO3 should also
decrease
• They are moving in the same direction
• primary problem is metabolic

• Step 3
Assess pao2 < 80 mm Hg - Hypoxemia
For a resp. disturbance : acute, chronic
The differentiation between A/C &
CHR.respiratory disorders is based on whether
there is associated acidemia / alkalemia.
If the change in paco2 is associated with the
change in pH, the disorder is acute.
In chronic process the compensatory process
brings the pH to within the clinically acceptable
range ( 7.30 – 7.50)

• J is a 45 years old female admitted with the severe
attack of asthma. She has been experiencing
increasing shortness of breath since admission three
hours ago. Her arterial blood gas result is as follows:
• pH : 7.22
• paCO2 : 55
• HCO3 : 25
• Follow the steps
• pH is low – acidosis
• paCO2 is high – in the opposite direction of the pH.
• Hco3 is Normal.
• Respiratory Acidosis
• Need to improve ventilation by oxygen therapy,
mechanical ventilation, pulmonary toilet or by
administering bronchodilators.

• EXAMPLE 2: Mr. D is a 55 years old
admitted with recurring bowel
obstruction has been experiencing
intractable vomiting for the last
several hours. His ABG is:
• pH : 7.5
• paCO2 :42
• HCO3: 33
• Metabolic alkalosis
• Management: IV fluids,
measures to reduce the

Respiratory
acidosis
pH PaCo2 HC03
normal
Respiratory
Alkalosis
normal
Metabolic
Acidosis
normal
Metabolic
Alkalosis
normal

BASE EXCESS
• Is a calculated value estimates the
metabolic component of an acid based
abnormality.
• It is an estimate of the amount of
strong acid or base needed to correct
the met. component of an acid base
disorder (restore plasma pH to 7.40at
a Paco240 mmHg)

Formula
• With the base excess is -10 in a 50kg
person with metabolic acidosis mM of
Hco3needed for correction is:
= 0.3 X body weight X BE
= 0.3 X 50 X10 = 150 mM

Anion GAP
Step 4
• Calculation of AG is useful approach to
analyse metabolic acidosis
AG = (Na+ + K+) – (cl- + Hco3-)
• * A change in the pH of 0.08 for each 10
mm Hg indicates an ACUTE condition.
* A change in the pH of 0.03 for each 10
mm Hg indicates a CHRONIC condition.

• K etoacidosis
• U remia
• S epsis
• S alicylate & other drugs
• M ethanol
• A lcohol (Ethanol)
• L actic acidosis
• E thylene glycol
REMEMBER

COMPENSATION
• Step 5
• A patient can be uncompensated or
partially compensated or fully
compensated
• pH remains outside the normal range
• pH has returned within normal range-
fully compensated though other values
may be still abnormal
• Be aware that neither the system has
the ability to overcompensate

Step 5 cont…
Determine if there is a compensatory
mechanism working to try to correct the
pH.
ie: if have primary respiratory acidosis will
have increased PaCO2 and decreased pH.
Compensation occurs when the kidneys
retain HCO3.
ABG Interpretation

Assess the PaCO2
• In an uncompensated state – when the pH
and paCO2 moves in the same direction: the
primary problem is metabolic.
• The decreasing paco2 indicates that the
lungs acting as a buffer response (blowing
of the excess CO2)
• If evidence of compensation is present but
the pH has not been corrected to within the
normal range, this would be described as
metabolic disorder with the partial
respiratory compensation.

Assess the HCO3
• The pH and the HCO3 moving in the
opposite directions, we would
conclude that the primary
disorder is respiratory and the
kidneys acting as a buffer
response: are compensating by
retaining HCO3 to return the pH to
normal range.

Example 3
• Mrs. H is admitted, he is kidney
dialysis patient who has missed his
last 2 appointments at the dialysis
centre his ABG results:
• pH : 7.32
• paCo2 : 32
• HCO3 : 18
• Pao2 : 88
• Partially compensated metabolic
Acidosis

Example 4
• Mr. K with COPD.His ABG is:
• pH : 7.35
• PaCO2 : 48
• HCO3 : 28
• PaO2 : 90
• Fully compensated
Respiratory Acidosis

Example 5
• Mr. S is a 53 year old man presented to
ED with the following ABG.
• pH : 7.51
• PaCO2 : 50
• HCO3 : 40
• Pao2 : 40 (21%O2)
• He has metabolic alkalosis
• Acute respiratory alkalosis
(acute hyperventilation).

FULLY COMPENSATED
pH paco2 Hco3
Resp.Acidosis Normal
but<7.40
Resp.Alkalosis Normal
but>7.40
Met. Acidosis Normal
but<7.40
Met. Alkalosis Normal
but>7.40

Partially compensated
pH paco2 Hco3
Res.Acidosis
Res.Alkalosis
Met. Acidosis
Met.Alkalosis

~ PaCO2 – pH Relationship
80 7.20
60 7.30
40 7.40
30 7.50
20 7.60

Precautions
 Excessive Heparin Decreases bicarbonate
and PaCO2
 Large Air bubbles not expelled from sample
PaO2 rises, PaCO2 may fall slightly.
 Fever or Hypothermia, Hyperventilation or
breath holding (Due to anxiety) may lead to
erroneous lab results
 Care must be taken to prevent bleeding

2SD NORMAL CL.ACCEPTABLE
• PH 7.35 – 7.45 7.30 – 7.50
• PCO2 35 – 45 30 – 50
• PO2 97 >80
(ON 21% O2)
(ON VENTILATOR) 60 – 90
• HCO3 24 - 28

Take Home Message:
Valuable information can be gained from an
ABG as to the patients physiologic condition
Remember that ABG analysis if only part of the patient
assessment.
Be systematic with your analysis, start with ABC’s as always
and look for hypoxia (which you can usually treat quickly),
then follow the four steps.
A quick assessment of patient oxygenation can be achieved
with a pulse oximeter which measures SaO2.

It’s not magic understanding
ABG’s, it just takes a little
practice!

Practice ABG’s
1. PaO2 90 SaO2 95 pH 7.48 PaCO2 32 HCO3 24
2. PaO2 60 SaO2 90 pH 7.32 PaCO2 48 HCO3 25
3. PaO2 95 SaO2 100 pH 7.30 PaCO2 40 HCO3 18
4. PaO2 87 SaO2 94 pH 7.38 PaCO2 48 HCO3 28
5. PaO2 94 SaO2 99 pH 7.49 PaCO2 40 HCO3 30
6. PaO2 62 SaO2 91 pH 7.35 PaCO2 48 HCO3 27
7. PaO2 93 SaO2 97 pH 7.45 PaCO2 47 HCO3 29
8. PaO2 95 SaO2 99 pH 7.31 PaCO2 38 HCO3 15
9. PaO2 65 SaO2 89 pH 7.30 PaCO2 50 HCO3 24
10. PaO2 110 SaO2 100 pH 7.48 PaCO2 40 HCO3 30

What is going on?

Answers to Practice ABG’s
1. Respiratory alkalosis
2. Respiratory acidosis
3. Metabolic acidosis
4. Compensated Respiratory acidosis
5. Metabolic alkalosis
6. Compensated Respiratory acidosis
7. Compensated Metabolic alkalosis
8. Metabolic acidosis
9. Respiratory acidosis
10.Metabolic alkalosis

Any Questions?