Comments on Teboul and Scheeren: understanding the Haldane effect

Dear Editor,

We read with interest the article recently published in Intensive Care Medicine by Teboul and Scheeren [1]. In this article, the authors describe the effects of oxygen extraction on the relationship between pCO₂ and CO₂ content as an explanation of altered interrelation between changes in flow and v–a pCO₂ gradients. We and others strove to understand and quantify this effect using data from critically ill patients and animal models many years ago, resulting in several publications not mentioned in the article by Teboul and Scheeren [2–4]. Teboul and Scheeren also failed to address the effect of hemoglobin on the pCO₂/CO₂ content relationship (CO₂ binding capacity decreases in anemia), which is relevant at extreme values not uncommon in the critically ill [2, 4]. Finally, a recent article highlighted the effects of increasing FiO₂ in shock states on venous–arterial pCO₂ gradients and the ratio between pCO₂ gradient and arterial–venous O₂ content difference (both increasing) [5]. We believe that much of the Haldane effect is already understood—but too often overlooked.