This document provides information on interpreting arterial blood gases (ABG), including: - Definitions of key terms like pH, PaCO2, HCO3, and the normal ranges for these values. - A 3-step process for interpreting ABGs: 1) identify acid-base disturbance, 2) look at PaCO2 and HCO3 levels, 3) check for compensatory mechanisms. - Explanations and examples of the four main acid-base disorders: respiratory acidosis, respiratory alkalosis, metabolic acidosis, and metabolic alkalosis. Compensatory responses aim to return pH to normal levels. - Causes and identifying features of each disorder type, such

1. INTERPRETATION OF
ARTERIAL BLOOD GASES (ABG)
Dr. Nidhi Ahya ( M.P.T Cardio-Pulmonary)
Assistant Professor, Cardio-Vascular & Respiratory PT
DVVPF College of Physiotherapy,
Ahmednagar 414111

2. CONTENTS
 Introduction to ABG
 Background Definitions & Terminology
 Normal Acid-Base Status
 Normal Values
 Steps in Interpretation of ABG
 Acid-Base Disorders and Compensatory Mechanisms
 Respiratory Acidosis
 Respiratory Alkalosis
 Metabolic Acidosis
 Metabolic Alkalosis
 Examples
4/12/20182 Dr.Nidhi Ahya

3. Introduction to ABG
 Analysis of arterial blood sample provides
precise measurement of acid-base balance in
the body and of the lungs ability to oxygenate
the blood and remove excess carbon dioxide
 Assessment of a patient with respiratory
dysfunction is incomplete without an ABG
report
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4. Arterial Vs Venous Blood
 Assess respiratory function
 Oxygen & Carbon dioxide
levels determined primarily
by lungs
 Does not reflect respiratory
function
 Exposed to peripheral
vascular beds where gas
exchange with tissues alters
oxygen &carbon dioxide
levels
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5. Arterial Blood Sampling
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 Arteriotomy- Invasive procedure
 Review patients chart for clinical
laboratory tests- PT/INR and
APTT
 Arterial blood sample obtained
from- Radial, Dorsalis Pedis,
Brachial or femoral arteries
 Radial Artery is the preferred
 accessible
 easy to stabilize post-puncture

6. Modified Allen’s Test
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 To evaluate the degree of collateral circulation to the
hand before puncture of radial artery

7. Obtaining Arterial Blood Sample
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 Requirements-
 Anticoagulant (liquid sodium)
 Sterile glass or low diffusibility plastic syringe
 Short bivel 20-22 gauge needle
 Isopropyl alchohol
 Sterile gauze squares
 Ice water bath

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 Sample must be obtained without being exposed to
the environment
 Air bubbles if any, should be removed, sample stored
in ice water bath to inhibit continued metabolism
 Samples at room temperature to be analyzed within
10-15 minutes

9. Background Definitions & Terminology
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 pH : The negative logarithm to the base 10 of the
hydrogen ion concentration in mol/L
ph = 1
log 10 (H+ )ion concentration
 Neutral pH : It is the pH at which there are equal no. of
H+ ions and OH– ions
The body preserves neutrality inside our cells
maintaining the blood at pH 7.4

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 Henderson-Hasselbalch equation:
pH œ OH- ion concentration
H+
ion concentration
 An increase in the pH indicates a proportionate
decrease in the [H+] and a decrease in the pH
indicates a proportionate increase in the [H+].

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 Acidosis and alkalosis :
Acidosis is an abnormality which tends to produce
an acidic pH i.e < 7.45
Alkalosis is the opposite. It produces an alkaline pH
i.e
pH > 7.45
7.00 7.25 7.45 7.67 7.85

12. Normal Acid-Base Status
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 Normal metabolism of proteins and nucleotides
generates about 100 mmol H+ per day in the form of
sulphuric and phosphoric acids.
 By comparison, hydration of CO2 to form H2CO3
generates 12,500 mmol H+ per day.
 The hydrogen ion environment is tightly controlled
by buffer systems
Minutes: CO2 excretion by the lungs
Hours to days: renal excretion of H+, reabsorption of
HCO3

13. Normal Values
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 pH - Measure of acid/base balance & ventilation
7.35-7.45
 PaCO2 - Measure of ventilation
35-45mmHg
 PaO2 - Measure of oxygenation or gas exchange
85-100mmHg
 HCO3 - Measure of metabolic disturbance
22-28 mEqm/l
 Base Excess - Measure of metabolic disturbance
-2 to +2

14. Steps in Interpretation of ABG
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 Step 1: Identify the pH. Is it within normal range?
pH >7.45 Alkalosis, pH < 7.35 Acidosis
Alkalosis- Respiratory or Metabolic
Acidosis- Respiratory or Metabolic
To confirm
 Step 2: Look at PaCO2 and HCO3
Increase in PaCO2 and decrease in HCO3 contributes to
acidosis
Decrease in PaCO2 and increase in HCO3 contributes to
alkalosis

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 Step 3: If pH is normal, but PaCO2 and HCO3 are not in
the ideal range- Compensatory mechanisms are active.
For metabolic abnormality, respiratory system
compensates, and for respiratory abnormality, metabolic
system acts.
RULE: Compensation always happens in the same
direction as the pH
i.e. To increase pH, HCO3 should increase
To decrease pH, HCO3 should decrease.
Alkolotic pH should decrease to become normal
Acidotic pH should increase to become normal

16. Interpretation of Acid Base Disorders
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 pH- 7.22
 PaCo2 - 56mmHg
 PaO2 -78 mmHg
 HCO3 - 23 mEqm/lit
Respiratory Acidosis

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 Respiratory Acidosis:
 Abnormality in which there is primary reduction in
alveolar ventilation relative to rate of CO2 production
 PaCO2 levels are above normal
 Respiratory acidosis is seen in-
 Severe diffuse airway obstruction
 Massive pulmonary edema
 Drug overdose
 Neuromusculoar disease
 Identifying the expected change in HCO3 is useful to
determine the level of compensation
 HCO3 increases 1mEq/L for each 10-15mmHg rise in
PaCO2

18. Interpretation of Acid Base Disorders
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 pH- 7.54
 PaCo2 - 29mmHg
 PaO2 -85 mmHg
 HCO3 - 23 mEqm/lit
Respiratory Alkalosis

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 Respiratory Alkalosis:
 Abnormality in which there is primary increase in
alveolar ventilation relative to rate of CO2 production
 PaCO2 levels are below expected level indicating
Hyperventilation
 Respiratory alkalosis is seen in-
 Pain
 Anxiety
 Fever
 Kidneys compensate by excreting HCO3
 Normal HCO3 with low PaCO2 and increased pH is a
uncompensated respiratory alkalosis.

20. Interpretation of Acid Base Disorders
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 pH- 7.28
 PaCo2 - 39mmHg
 PaO2 -89 mmHg
 HCO3 - 19 mEqm/lit
Metabolic Acidosis

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 Metabolic Acidosis:
 It is identified when the plasma HCO3 or base excess
falls below normal
 Occurs when increased load of H+ ions is present or body
is unable to excrete acids
 Loss of HCO3- Diarrhea, renal disease
 Increase in metabolic acid production- Ketoacidosis,
Lactic acidosis or ingestion of certain toxins like methanol
 A reduction in PaCO2 via hyperventilation compensates
for metabolic acidosis
 Kussmaul’s respiration is obvious sign of metabolic
acidosis

22. Interpretation of Acid Base Disorders
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 pH- 7.50
 PaCo2 - 42mmHg
 PaO2 -89 mmHg
 HCO3 - 33 mEqm/lit
Metabolic Alkalosis

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 Metabolic alkalosis:
 It is identifies by an above normal elevation of
plasma HCO3
 Occurs whenever HCO3
- ions accumulate in the
blood or when abnormal no. of H+ ions are lost from
plasma
 Common causes-
 Hypokalemia
 Persistant vomiting
 Diuretic Therapy
 Hypoventilation and an elevation of PaCO2
compensate for metabolic alkalosis

24. Summary
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 Introduction to ABG
 Background Definitions & Terminology
 Normal Acid-Base Status
 Normal Values
 Steps in Interpretation of ABG
 Acid-Base Disorders and Compensatory Mechanisms
 Examples

25. QUESTION
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 WRITE THE DEFINATION AND TERMINOLOGY OF
THE ABG ANALYSIS. 5MARKS

26. Thank You…
Solve the examples on the board....Before a Bye
Bye….!!!
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