Chapter 14: Blood Gases and Acid-Base Disorders

INTRODUCTION

BLOOD GAS BASICS

Blood gases provide information concerning the oxygenation, ventilatory, and acid-base status of the patient. Blood gas results are usually given as pH, PO₂, PCO₂, $[HCO3−]$, base excess or deficit (base difference), and O₂ saturation. This test gives information on acid-base homeostasis (pH, PCO₂, $HCO3−$, and base difference) and on blood oxygenation (PO₂, O₂ saturation). Arterial blood gases (ABGs) are most commonly measured; venous, mixed venous, and capillary blood gases are less often measured. Indications for blood gas determinations are as follows:

• To determine a patient’s ventilator status (PaCO₂), acid-base balance (pH and PaCO₂), and oxygenation and O₂-carrying capacity (PaO₂ and O₂Hb)

• To quantitate the response to therapeutic interventions (e.g., supplemental O₂ administration, mechanical ventilation) or diagnostic evaluation (e.g., exercise desaturation)

• Monitoring the severity and progression of documented disease processes (e.g., COPD)

NORMAL BLOOD GAS VALUES

Normal values for blood gas analysis are given in Table 14-1 below, and capillary blood gases are discussed in a following section. Mixed venous blood gases are reviewed in Chapter 28, Pulmonary Artery Catheters (Swan–Ganz Catheter) and Additional Functions and Measurement Capabilities of PA Catheters. The bicarbonate concentration ($[HCO3−]$) from the blood gas is a calculated value and should not be used in interpretation of blood gases; the $[HCO3−]$ from a concurrent chemistry panel should be used. Note: The $HCO3−$ values on the chemistry panel and those calculated from the blood gases should be about the same. A major discrepancy (>10% difference) means one or more of the three values is in error (pH, PCO₂, or $[HCO3−]$) or the blood gas and the chemistry panel were not collected at the same time. ABGs and chemistry panels $[HCO3−]$ should be obtained at the same time for the most accurate interpretation.