ABG - Arterial blood gas - interpreting results

Arterial blood gas (ABG) analysis is an essential part of diagnosing and managing a patients oxygenation status and acidbase balance. The usefulness of this diagnostic tool is dependent on being able to correctly interpret the results. Disorders of acidbase balance can create complications in many disease states, and occasionally the abnormality may be so severe so as to become a life-threatening risk factor. A thorough understanding of acidbase balance is mandatory for any physician, and intensivist, and the anesthesiologist is no exception.

The three widely used approaches to acidbase physiology are the HCO3- (in the context of pCO2), standard base excess (SBE), and strong ion difference (SID). It has been more than 20 years since the Stewarts concept of SID was introduced, which is defined as the absolute difference between completely dissociated anions and cations. According to the principle of electrical neutrality, this difference is balanced by the weak acids and CO2. The SID is defined in terms of weak acids and CO2 subsequently has been re-designated as effective SID (SIDe) which is identical to buffer base. Similarly, Stewarts original term for total weak acid concentration (ATOT) is now defined as the dissociated (A-) plus undissociated (AH) weak acid forms. This is familiarly known as anion gap (AG), when normal concentration is actually caused by A-. Thus all the three methods yield virtually identical results when they are used to quantify acidbase status of a given blood sample.

Why is it Necessary to Order an ABG Analysis

The utilization of an ABG analysis becomes necessary in view of the following advantages:

  • Aids in establishing diagnosis.
  • Guides treatment plan.
  • Aids in ventilator management.
  • Improvement in acid/base management; allows for optimal function of medications.
  • Acid/base status may alter electrolyte levels critical to a patients status.

Accurate results for an ABG depend on the proper manner of collecting, handling, and analyzing the specimen. Clinically important errors may occur at any of the above steps, but ABG measurements are particularly vulnerable to preanalytic errors. The most common problems that are encountered include nonarterial samples, air bubbles in the sample, inadequate or excessive anticoagulant in the sample, and delayed analysis of a noncooled sample.