A pilot assessment of alpha-stat vs pH-stat arterial blood gas analysis after cardiac arrest

Purpose: Resuscitated cardiac arrest (CA) patients typically receive therapeutic hypothermia, but arterial blood gases (ABGs) are often assessed after adjustment to 37°C (alpha-stat) instead of actual body temperature (pH-stat). We sought to compare alpha-stat and pH-stat assessment of Pao₂ and Paco₂ in such patients. Materials and methods: Using ABG data obtained during the first 24 hours of intensive care unit admission, we determined the impact of measured alpha vs calculated pH-stat on Pao₂ and Paco₂ on patient classification and outcomes for CA patients. Results: We assessed 1013 ABGs from 120 CA patients with a median age of patients 66 years (interquartile range, 50-76). Median alpha-stat Pao₂ changed from 122 (95-156) to 107 (82-143) mm Hg with pH-stat and median Paco₂ from 39 (34-46) to 35 (30-41) mm Hg (both P < .001). Using the categories of hyperoxemia, normoxemia, and hypoxemia, pH-stat estimation of Pao₂ reclassified approximately 20% of patients. Using the categories of hypercapnia, normocapnia, and hypocapnia, pH stat estimation of Paco₂ reclassified approximately 40% of patients. The mortality of patients in different Pao₂ and Paco₂ categories was similar for pH-stat and alpha-stat. Conclusions: Using the pH-stat method, fewer resuscitated CA patients admitted to intensive care unit were classified as hyperoxemic or hypercapnic compared with alpha-stat. These findings suggest an impact of ABG assessment methodology on Pao₂, Paco₂, and patient classification but not on associated outcomes.