Workshop Slides - Arterial Blood Gas Analysis - PCSICU.com

RC (UK)

Objectives :
• To know the meaning of common terms used
in arterial blood gas interpretation .
• To know the normal ranges for arterial blood gas values
• To have a systematic approach to arterial blood gas
interpretation .
• To be able to interpret simple arterial blood gas analyses
in the context of cardiac arrest or impending cardiac arrest .
• To know some of the common causes of arterial blood gas
abnormalities and what to do to correct them .
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Why we do Arterial Blood Gas Analysis?
• Oxygenation
Represented by PaO2
• Ventilation
Represented by Pa Co2
• Acid Base Status
Represented by pH, HCO3
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Terminology
Acid :
H + Donor
Base : H + acceptor
pH :
- ve log to the base 10 of H + activity
Acidemia :
Alkalemia :
pH < 7.35
pH > 7.45
acidosis
alkalosis
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HENDERSSEN - HASSELBALCH EQUATION
pH = pka + log
B ( HCO 3 )
A { H 2 CO 3 ( CO 2 ) }
pH
∞
B ( HCO 3 )
A { H 2 CO 3 ( CO 2 ) }
pH
∞
HCO 3
PaCO 2
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Interpretion of ABG
1
Acid - base status
Oxygenation
PaO2= FiO2 X 5
PaCO 2 pH HCO 3
40 4 7.4 0.04 24 2
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Compensatory organs
Lungs
Kidneys
Liver
INTESTINE
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Compensatory mechanisms :.
Respiratory acidosis
pH
∞
HCO 3
PaCO2
Respiratory Alkalosis
pH
∞
HCO 3
PaCO2
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Compensatory mechanisms :.
Metabolic Acidosis
pH
∞
HCO 3
PaCO2
Metabolic Alkalosis
pH ∞
HCO 3
PaCO2
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5-Step approach to arterial blood gas interpretation
1. Assess oxygenation
Is the patient hypoxic?
Is there a significant alveolar-arterial gradient?
2. Determine status of the pH or H + concentration
pH > 7.45 (H + < 35 nmol l -1 ) – alkalaemia
< 7.35 (H + > 45 nmol l -1 ) – acidaemia
3. Determine respiratory component
PaCO 2 > 6.0 kPa (45 mmHg) – respiratory acidosis
< 4.7 kPa (35 mmHg) – respiratory alkalosis
4. Determine metabolic component
HCO 3
-
< 22 mmol l -1 – metabolic acidosis
> 26 mmol l -1 – metabolic alkalosis
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5-Step approach to arterial blood gas interpretation
(continued)
5. Combine the information from 2, 3 and 4 (Final Diagnosis)
Acidosis (PH decreases ) Alkalosis (PH increases )
Respiratory CO 2 CO 2
Metabolic HCO 3
-
or base excess HCO 3- or base excess
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1 -Interpret the PaO 2
In the light of the inspired O 2
(F i
O 2
)
Ranges of PaO2 on Room Air
• Normal
: 97 mm Hg
• Acceptable Range : > 80 mm Hg
• Hypoxemia
: < 80 mm Hg
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Interpret the PaO 2
In the light of the inspired O 2
(F i
O 2
)
Ranges of PaO2 on Supplemental O2
FiO2
PaO2 = FiO2X500
Predicted PaO2 mm Hg
0.3 150
0.4 200
0.5 250
0.8 400
1.0 500
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2- Determine the pH
•pH > 7.45 – Alkalaemia
•pH < 7.35 – Acidaemia
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3-Determine the Respiratory Component
of pH change
• PaCO 2
: > 6.0 kPa (45 mmHg )
Respiratory Acidosis (or respiratory compensation for a
metabolic alkalosis)
• PaCO2 : < 4.7 kPa (35 mmHg)
Respiratory Alkalosis (or respiratory compensation for a
metabolic acidosis)
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4- Determine the Metabolic Component
of pH change
HCO 3
-
> 26 mmols l -1 –
Metabolic Alkalosis (or renal compensation for a
respiratory acidosis)
HCO3- < 22 mmols l-1 –
Metabolic Acidosis (or renal compensation for a
respiratory alkalosis)
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5- Determine which is the Primary Disturbance
• Is there any metabolic or respiratory compensation .
• If the direction of change in the PaCO 2
mirrors that of
the pH, then the primary condition is a respiratory
acidosis or alkalosis .
• If the direction of change in the PaCO 2
does not mirror
the pH, it is compensation for a metabolic acidosis or
alkalosis.
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Summary of changes in pH, PaCO 2 and HCO 3
-
in acid-base disorders
Acid-base disorder
pH
PaCO 2
HCO 3
-
Respiratory acidosis
N
Metabolic acidosis
N
Respiratory alkalosis
N
Metabolic alkalosis
N
Respiratory acidosis with renal
compensation
*
Metabolic acidosis with
respiratory compensation
*
* If the compensation is virtually complete the pH may be in the normal range –
over compensation does not occur.
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Summary of changes in pH, PaCO 2 and HCO 3
-
in acid-base disorders (continued)
Acid-base disorder
pH
PaCO 2
HCO 3
-
Respiratory alkalosis with renal
compensation
*
Metabolic alkalosis with respiratory
compensation
*
Mixed metabolic and respiratory
acidosis
Mixed metabolic and respiratory
alkalosis
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Summary
• Consider the clinical condition.
• Assess oxygenation .
• Assess Acid Base Status:
- Acidosis or Alkalosis .
• Respiratory Vs Metabolic .
• Compensated Vs non compensated .
• Correct:
• Clinical condition .
• Oxygenation .
• Acid Base status .
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RC (UK)