INTERPRET THE ABG REPORTS . FOR
THE DIAGNOSIS OF ACIDOSIS AND
ALKALOSIS
LEARNING OBJECTIVES
At the end of lecture, students must able to know:
•
•
•
Buffers and buffer system
Interpretation of ABG’s.
Acid base disorders.
Normal Hydrogen Ion Concentration
and pH of Blood
 The blood H+ ion concentration is normally
maintained within tight limits around a
normal value of about 40 nEq/L
pH= log 1/H= -log [H]
Normal pH=7.35 – 7.45
pH
7.7
7.5
7.4
7.3
7.1
7.0
6.8
Defenses Against Changes in Hydrogen Ion
Concentration in the Body
• The Chemical Buffer Systems
– Bicarbonate Buffer System
– Phosphate Buffer System
– Protein Buffer System
[H+]
20
31
40
50
80
100
160
• The Physiological Buffer Systems
– The Respiratory Mechanism
– The Renal Mechanism.
Buffer Systems
Chemical Buffer Systems
Bicarbonate Buffer System
NaOH + H2CO3
strong base
HCl + Na HCO3
Strong acid
H2O + Na HCO3
weak base
NaCl + H2CO3
weak acid
Defenses Against Changes in Hydrogen Ion Concentration in
the Body
•
•
The Chemical Buffer Systems
– Bicarbonate Buffer System
– Phosphate Buffer System
– Protein Buffer System
The Physiological Buffer Systems
– The Respiratory Mechanism
– The Renal Mechanism
Buffer Systems
•
A buffer system is a combination of two compounds that minimizes pH changes
when acid or base is added to a solution
•
One substance yield H+ ion when pH is increased the other binds with H+ ion
when pH is decreased
Chemical Buffer Systems
•
•
•
•
•
•
Combination of weak acid and weak base
Binds to H+ as H+ concentration rises
Releases H+ as H+ concentration falls
Can restore normal pH almost immediately
Buffering accomplished by converting:
• Strong acid  Weak acid
• Strong base  Weak base
Three major chemical buffer systems
• Bicarbonate system
• Phosphate system
• Protein system
•
Combination of weak acid and weak base
•
Binds to H+ as H+ concentration rises
•
Releases H+ as H+ concentration falls
•
Can restore normal pH almost immediately
•
Buffering accomplished by converting:
•
Strong acid  Weak acid
•
•
Strong base  Weak base
Three major chemical buffer systems
•
Bicarbonate system
•
Phosphate system
•
Protein system
Bicarbonate Buffer System

The most powerful extracellular buffer in the body

Weak acid - Carbonic acid (H2CO3)

Weak base - Bicarbonate ion (HCO3-)

CO2 + H20  H2CO3  H+ + HCO3-

Works along with respiratory and urinary system (these systems remove CO2 or
HCO3- )
NaOH + H2CO3
HCO3
strong base
HCl + Na HCO3
H2CO3
Strong acid
H2O
+
Na
weak base
NaCl
weak acid
+
Respiratory mechanisms to regulate pH
Renal mechanisms to regulate pH
•
•
Compensates for Metabolic Acidosis & Alkalosis
Consider CO2 as an acid because:
CO2 + H20 ↔ H2CO3 ↔ H+ + HCO3-
•
•
•
•
Metabolic Acidosis
CO2 eliminated
Metabolic Alkalosis
CO2 retained
Compensates for Respiratory Acidosis & Alkalosis
Consider HCO3 as a base
•
•
Respiratory Acidosis
Respiratory Alkalosis
HCO3 retained
HCO3 eliminated
Diagnosing Acid Base Disorders
Blood Gas Report
•
•
•
•
Acid-Base Information
pH
PCO2
HCO3 [calculated vs measured]
•
•
•
Oxygenation Information
PO2 [oxygen tension]
SO2 [oxygen saturation]
NORMAL VALUES
•
•
•
•
•
pH:
7.35 – 7.45
pCO2 :
35 – 44 mm Hg
HCO3 :
22 – 26 mEq/L
pO2 : 75 – 100 mm Hg
SO2 : 92 – 98 %
pH = 6.1 + log HCO3
0.3X CO2
What is Compensation?
•
•
•
•
•
•
The pH is governed by the ratio of pCO2/HCO3[H+]= 24 (pCO2/HCO3) Henderson formula
The “partner” should travel in the same direction as the primary disturbance; e.g.,
If HC03- falls, then the pCO2 should fall also; this is called compensation
YOU CANNOT OVER-COMPENSATE
The kidneys compensate for changes in CO2
The lungs compensate for changes in HCO3-
Acid Base Disorders:
•
Acidosis – (pH < 7.35)
• Metabolic
• Respiratory
•
Alkalosis - (pH > 7.45)
• Metabolic
• Respiratory
•
Simple (Acid-Base) Disorders are those in which there is a single primary
etiological acid-base disorder.
•
Mixed (acid-Base) Disorders are those in which two or more primary etiological
disorders are present simultaneously
Acid-Base Disorder
Primary Change
Compensatory Change
Respiratory acidosis PCO2 up
Respiratory alkalosis PCO2 down
Metabolic acidosis
HCO3 down
Metabolic alkalosis HCO3 up
HCO3 up
HCO3 down
PCO2 down
PCO2 up
Analysis of Acid-Base Disorders
Thank You