Circumstances When Arterial Blood Gas Analysis Can Lead Us Astray.

In this issue of RESPIRATORY CARE, Van de Louw et al1 describe the magnitude of spurious hypoxemia as suggested by arterial blood gases (ABGs) obtained from subjects with acute leukemia and hyperleukocytosis. Spurious hypoxemia, commonly known as leukocyte larceny, is just one of several erroneous values or discrepancies with pulse oximetry and the bedside assessment (or bioassay) that may occur when obtaining or interpreting ABGs. What follows is a short description of these other common pitfalls, including pre-analytic issues such as syringe material, sample volume, and non-blood contaminating solutions, as well as more clinically oriented aspects such as barometric pressure, temperature, and hemoglobin ligands. Rapid recognition of these inaccuracies or confounders can minimize mistaken conclusions and improve patient care. It is paramount to eliminate erroneous values when assessing a patient’s oxygenation, ventilatory, or acid/base status, and several important pre-analytic steps must be followed in order to obtain a valid, interpretable ABG. The first is to verify that the values make sense and fit the HendersonHasselbalch equation, pH pKa log(HCO3 /CO2), for acid/base values and that PaO2 does not yield negative or irrational alveolar PaO2 differences as calculated by the alveolar gas equation, PaO2 (Patm PH20) FIO2 (PaCO2/R). Admittedly, using the Henderson-Hasselbalch equation can be tedious; however, there is a modified version of the Henderson-Hasselbalch equation that can be quite useful: 1) calculate the hydrogen ion concentration using the equation [H ] 24 PCO2/HCO3 , with PCO2 given in mm Hg, H as nanomolar, and HCO3 as millimolar; 2) use the calculated [H ] to determine what the pH should be, since pH is a function of hydrogen ion concentration ([H ] of 25 nM correlates to pH of 7.60, [H ] of 40 nM correlates to 7.40, [H ] of 63 nM correlates to 7.20, etc.); 3) compare the calculated pH with the measured pH, and if there is a discrepancy there is probably a measurement or transcription error. Performing these simple calculations before interpreting a blood gas can help to detect mistaken entries or erroneous values and avoid the interpretation of confusing results.