Arterial Blood Gas Analysis

Introduction

• ABG analysis provides a rapid assessment of oxygenation, ventilation, and acid–base status in perioperative and critical care environments.
• Modern analyzers report a combination of directly measured and calculated values, each derived under standardized assumptions.¹
• Most ABG analyzers heat all blood samples to 37°C, regardless of patient temperature.
• The only measured components on an ABG are pH, PaCO₂, and PaO₂.
  • All other values, including bicarbonate, base excess, and calculated oxygen saturation, are derived mathematically.
• Understanding what the analyzer measures, what it calculates, and how temperature variations impact gas solubility and acid-base chemistry is essential for informed interpretation.
• Temperature corrections can significantly affect clinical decision-making in hypothermia, cardiopulmonary bypass (CPB), neonatal care, targeted temperature management, and prolonged low-flow states.

Physicochemical Principles of Temperature and Blood Gases

• As temperature decreases, the solubility of gases increases, resulting in greater dissolution of O₂ and CO₂ in plasma and a lower partial pressure of both gases.
  • As a result, at temperatures lower than 37°C, the reported PaO₂ and PaCO₂ on an ABG are lower than the actual values in the patient.¹
  • Conversely, at temperatures above 37°C, the reported PaO₂ and PaCO₂ exceed the patient’s actual values.
• As the temperature decreases, the neutral point of water rises above a pH of 7.0
  • As a result, pH increases by approximately 0.015 units for every 1°C decrease in temperature, and ABG may appear falsely alkalotic.¹

Alpha-Stat versus pH-Stat

• Because temperature affects the physicochemical properties of ABG readings, values must be interpreted in context or corrected by the provider to support accurate clinical assessments.
• Alpha-stat and pH-stat represent two distinct strategies for managing ABG values during CPB, differing primarily in how PaCO₂ is targeted relative to patient temperature.
• Alpha-stat management uses uncorrected values at 37°C.
  • Clinical management targets “normal” homeostatic values, such as a pH of 7.4 at 37°C and a PaCO2 of 40 mmHg at 37°C.²
  • Alpha-stat maintains the patient’s intrinsic cerebral autoregulation and maintains enzymatic function.
• pH-Stat ABG management uses temperature-corrected blood gases
  • CO₂ is added to maintain a pH of ~7.4 and a PaCO₂ of ~40 mmHg at the patient’s actual body temperature.
  • pH-stat is superior at maintaining cerebral perfusion, especially in neonates.³

Measured and Calculated Values

• Most analyzers measure the following using temperature-controlled electrodes1:
  • pH – via a glass electrode
  • PaCO₂ – via Severinghaus electrode (CO₂-sensitive pH change)
  • PaO₂ – via Clark polarographic electrode
• Some analyzers may additionally measure1:
  • Hemoglobin concentration
  • Oxyhemoglobin, carboxyhemoglobin, methemoglobin (co-oximetry)
  • Electrolytes
  • Lactate
• These measured values are corrected to 37°C, regardless of the patient’s temperature.
• Calculated (Derived) Values:
  • Bicarbonate (HCO₃^–): Calculated using the Henderson–Hasselbalch equation
  • Base excess (BE): Represents the metabolic contribution to acid–base balance
  • Oxygen saturation (SaO₂) – SaO₂ is calculated using the patient’s measured PaO₂ and standard hemoglobin dissociation curve, assuming normal hemoglobin structure, temperature, pH, and PCO₂

This becomes unreliable in specific situations:

• • Dyshemoglobinemias
  • Hypothermia
  • Severe acidosis/alkalosis
  • Fetal hemoglobin
  • Abnormal 2,3-DPG levels

Measurement Errors

There are several potential sources of error in ABG analysis at different stages of the analysis.

Preanalytical Phase

• Samples should be analyzed within 15 minutes, as ongoing metabolism by red blood cells can affect the concentration of gases and other analytes.⁴
  • Cooling the sample slows glycolysis but increases the permeability of plastics, allowing gas to diffuse through the syringe.⁴
• The type of syringe used can affect the accuracy of PO₂ measurement.
  • Glass syringes are more accurate for PO₂ compared to plastic, but they are similar for pH and PCO₂.⁴
• Inadequate mixing of the sample with the anticoagulant in the syringe can cause red blood cell sedimentation.⁴
• Air bubbles in the syringe or introduced during sample processing can raise PaO₂ and lower PaCO₂.⁴

Analytical Phase

• Adequate calibration of the analyzer is necessary for accurate data.
• Aberrant hemoglobins cause inaccurate O₂ saturation if co-oximetry is not used