Free Pulmonary Course on Arterial Blood Gas Analysis
by Andrew M. Luks, MD

• Introduction
• Acid-Base Status
  • Terms and Normal Values
  • How the Data Are Presented
  • Before you get started
  • A Step-wise Approach to Interpretation
    • Step 1: Examine the pH and compare it to the normal range
    • Step 2: Determine the primary process that led to the change in the pH
    • Step 3: Calculate the serum anion gap (SAG)
    • Step 4: Identify the compensatory process (if one is present)
    • Step 5: Determine if a Mixed Acid-Base Disorder is Present
  • Generating Differential Diagnoses
    • Elevated Anion Gap Metabolic Acidosis
    • Normal Anion Gap Metabolic Acidosis (non-gap acidosis)
    • Metabolic Alkalosis
    • Respiratory Acidosis
    • Respiratory Alkalosis
  • Use Old Arterial Blood Gas Results to Your Advantage
• Oxygenation
  • Assessing the Cause of Hypoxemia
  • Assessing the Adequacy of Gas Exchange
    • The P/F Ratio
    • The AaO₂ Difference
  • Special Situations To Be Aware of In Evaluating the PaO2 on an ABG

Introduction

Arterial blood gases play an important role in the work-up and management of critically ill patients and patients with a variety of pulmonary complaints and disorders. For example, they are used to guide the adjustment of ventilator parameters on mechanically ventilated patients and are also a standard part of the work-up of patients who present with unexplained hypoxemia or dyspnea. It is, therefore, important that students and physicians be able to interpret the results of arterial blood gas sampling, determine the patient's acid-base status and assess the adequacy of oxygenation.

This primer describes a clinical approach to interpreting arterial blood gases. It will outline a step-wise approach to interpreting the acid-base status and generating differential diagnoses for the observed problems. It will then address the proper means for assessing the adequacy of oxygenation and determining the etiology of any observed abnormalities.

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Acid-Base Status

Terms and Normal Values

Before reviewing the assessment of acid-base status, it is helpful to review the normal values for the main acid-base parameters and some basic terminology.

The normal values for acid-base parameters are as follows:

• pH: 7.38 - 7.42
• PaCO2: 36 - 44 mmHg
• Bicarbonate: 22 – 26 mmol/L

Be aware that the normal ranges for these parameters will vary slightly from laboratory to laboratory.

The following terminology is applied to acid-base interpretation:

• Acidemia: refers to a low blood pH (< 7.38). Patients with a low pH, are said to be "acidemic."
• Alkalemia: refers to a high blood pH (> 7.42). Patients with a high pH are said to be "alkalemic."
• Acidosis: refers to any process that, if left unchecked, will lead to acidemia. This can occur through one of two mechanisms.
  • A respiratory acidosis is present when the PCO2 is high (> 44)
  • A metabolic acidosis is present when the HCO3- is low (< 22)
• Alkalosis: refers to any process that if left unchecked will lead to alkalemia. This can occur through one of two mechanisms.
  • A respiratory alkalosis is present when the PCO2 is low (< 36
  • A metabolic alkalosis is present when the HCO3- is high (> 26)

It is important to keep these terms straight in your mind and in your communications with others. It is common for people to refer to the patient with a low pH, for example, and say they are "acidotic." Similarly, they often refer to the patient with a high pH as "alkalotic." This is incorrect terminology.

When you are referring to the patient and their pH, the correct terminology is as follows:

• The patient with a low pH has "acidemia" or is "acidemic."
• The patient with a high pH has "alkalemia" or is "alkalemic."

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Next: How the Data Are Presented