How To Interpret ABG?
Basic Acid-Base Terminology
| Clinical terminology | Criteria |
| Normal pH | 7.4 (7.35-7.45) |
| Acidaemia | pH< 7.35 |
| Alkalaemia | pH>7.45 |
| Normal PaCO2 | 40 (35-45)mm of Hg |
| Respiratory acidosis (failure) | PaCO2>45 mm of Hg |
| Respiratory alkalosis (hyperventilation) | PaCO2<35 mm of Hg |
| Normal HCO3 | 24(22-26)mEq/L |
| Metabolic acidosis | HCO3<22 mEq/L & low pH |
| Metabolic alkalosis | HCO3 >26mEq/L & high pH |
Basics of acid base disorders & compensation
If the initial disturbance affects HCo3
➤ Metabolic acidosis (fall in bicarbonate)
➤ Metabolic alkalosis (rise in bicarbonate)
If the initial disturbance affects PaCO2
➤ Respiratory acidosis (Rise in PaCO2)
➤ Respiratory alkalosis (Fall in PaCO2)
(When you see “metabolic” think of HCO3 & when you see “respiratory” think of PaCO2)
Charecteristics of Primary Acid-base Disorders
| BASIC CHARECTER | pH | H+ | PRIMARY CHANGE | SECONDARY CHANGE |
| METABOLIC ACIDOSIS | LOW | HIGH | HCO3 LOW | PaCO2 decreased |
| METABOLIC ALKALOSIS | HIGH | LOW | HCO3 HIGH | PaCO2 increased |
| RESPIRATORY ACIDOSIS | LOW | HIGH | PaCO2 HIGH | HCO3 increased |
| RESPIRATORY ALKALOSIS | HIGH | LOW | PaCO2 LOW | HCO3 decreased |
Compensation in acid base disorders
WHAT IS COMPENSATION & HOW IT OCCURS?
The body’s response to neutralize the effect of the initial insult on pH homeostasis is called compensation
WHAT IS THE “SAME DIRECTION RULE”?
Compensatory changes are in the same direction as the primary changes.Decreased HCO3 leads to decreased PaCO2 & increased HCO3 leads to increased PaCO2.
Prediction Of Compensation For Simple Acid Base Disorders
| DISORDER | EXPECTED COMPENSATION |
| METABOLIC ACIDOSIS | PaCO2=(1.5*HCO3)+8 |
| METABOLIC ALKALOSIS (rise in HCO3) |
PaCO2=HCO3+15 |
RESPIRATORY ACIDOSIS
|
(rise in PaCO2)
|
RESPIRATORY ALKALOSIS
|
(Fall in PaCO2)
|
Diagnosis of Acid Base Disturbances
Step by step analysis
➤ Step1: Is there an acid base disorder?
➤ Step2: Is there acidosis or alkalosis?
➤ Step3: What is the primary acid base disorder?
➤ Step4: Calculate the expected compensation.
➤ Step5: Determine the presence of mixed acid base disorder
➤ Step6: Clinical correlation & to establish etiological diagnosis