Arterial blood gas

“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
SaO2 Oxygen Saturation

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 below 6.8
will interfere with cellular functioning, and if uncorrected, will
lead to death.

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 imbalance
starts to occur within 1-3 minutes

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.

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.
>95%

ACID BASE DISORDER
Res. Acidosis
• is defined as a pH less than 7.35 with a
paco2 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 respiratory
acidosis.

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(lung
collapse), 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
• is defined as a pH more than 7.45 with a
paco2 less than 35 mmHg
CAUSES:
• Psychological responses, anxiety or fear.
• Pain
• Increased metabolic demands such as fever,
sepsis, pregnancy or thyrotoxicosis(increased
thyroid hormone).
• 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
(muscular) spasms of the arms and legs.

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

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

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

Management
• Treat the cause
• Hypoxia of any tissue bed will produce metabolic
acidosis 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, 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.

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

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

• Step 3
Assess pao2 < 80 mm Hg - Hypoxemia
For a resp. disturbance : acute, chronic
The differentiation between Acute & Chronic,
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 excess base

BASE EXCESS
• Is a calculated value estimates the
metabolic component of an acid based
abnormality.
• It is defined as the amount of strong acid
or base added to blood to restore plasma
pH to 7.40 at a Paco2 40 mmHg.
• Positive value: Base Excess(increases in met. Alkalosis)
• Negative value: Base Deficit(decreases in met. Acidosis)

Formula
• With the base excess is -10 in a 50kg
person with metabolic acidosis mM of
Hco3 needed for correction is:
= 0.3 X body weight X BE
= 0.3 X 50 X10 = 150 mM

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 94 SaO2 99 pH 7.49 PaCO2 40 HCO3 30
5. PaO2 95 SaO2 99 pH 7.31 PaCO2 38 HCO3 15
6. PaO2 65 SaO2 89 pH 7.30 PaCO2 50 HCO3 24
7. 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. Metabolic alkalosis
5. Metabolic acidosis
6. Respiratory acidosis
7. Metabolic alkalosis

Any Questions?