Commentaries on Viewpoint: Time for a new metric for hypoxic dose?

TO THE EDITOR: The proposal by our well-respected colleagues (2) to introduce a new metric—incorporating the altitude elevation and the total exposure duration, termed “kilometer hours”—for better describing the “hypoxic dose” is decidedly a step forward. By only quantifying the “external” stress, this metric presents several limitations: It suggests a linear relationship between altitude elevation and saturation decrease [but the Fick curve is curvilinear (3)] or that it applies to all athletes irrespectively of their training background [but elite endurance athletes suffer the largest decrease in V̇O2max (1)], altitude experience [but elite athletes who have had previous hypoxic exposure better adapt to hypoxic condition (4)], or type of hypoxia [but hypobaric vs. normobaric hypoxia induces larger desaturation (5)]. The large intersubject variability in the physiological responses to a given “hypoxic dose” implies that the magnitude of the stimulus rather than the altitude elevation should instead be considered. We therefore propose a new metric based on the sustained duration at a given arterial saturation level. Hence, desaturation levels in normoxia (exercise-induced arterial hypoxemia) or in hypoxia (3) predict the decrement in V̇O2max in hypoxia and therefore the ̇amplitude of the “hypoxic stimulus.” This metric termed “saturation hours” is defined as %·h (98/s 1) h 100, where s is the saturation value (in %) and h the time (in hours) sustained at any second level. Practically, with the development of new sport gears incorporating the oximeter inside the textile, this metric will readily be measured without any disturbances to individuals.