LAUSR

ABSTRACT Pcrit – generally defined as the PO2 below which the animal can no longer maintain a stable rate of O2 consumption (ṀO2), such that ṀO2 becomes dependent upon PO2 – provides a single number into which a vast amount of experimental effort has been invested. Here, with specific reference to water-breathers, I argue that this focus on the Pcrit is not useful for six reasons: (1) calculation of Pcrit usually involves selective data editing; (2) the value of Pcrit depends greatly on the way it is determined; (3) there is no good theoretical justification for the concept; (4) Pcrit is not the transition point from aerobic to anaerobic metabolism, and it disguises what is really going on; (5) Pcrit is not a reliable index of hypoxia tolerance; and (6) Pcrit carries minimal information content. Preferable alternatives are loss of equilibrium (LOE) tests for hypoxia tolerance, and experimental description of full ṀO2 versus PO2 profiles accompanied by measurements of ventilation, lactate appearance and metabolic rate by calorimetry. If the goal is to assess the ability of the animal to regulate ṀO2 from this profile in a mathematical fashion, promising, more informative alternatives to Pcrit are the regulation index and Michaelis–Menten or sigmoidal allosteric analyses. Summary: This Commentary argues that the concept of the critical oxygen tension in water-breathers is fundamentally flawed for many reasons, and it presents the case for more useful alternatives.