The fallacy of the P crit - are there more useful alternatives?

P crit - generally defined as the P O2  below which the animal can no longer maintain a stable rate of O2 consumption (Ṁ O2 ), such that Ṁ O2  becomes dependent upon P O2  - 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 P crit is not useful for six reasons: (1) calculation of P crit usually involves selective data editing; (2) the value of P crit depends greatly on the way it is determined; (3) there is no good theoretical justification for the concept; (4) P crit is not the transition point from aerobic to anaerobic metabolism, and it disguises what is really going on; (5) P crit is not a reliable index of hypoxia tolerance; and (6) P crit carries minimal information content. Preferable alternatives are loss of equilibrium (LOE) tests for hypoxia tolerance, and experimental description of full Ṁ O2  versus P O2  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 Ṁ O2  from this profile in a mathematical fashion, promising, more informative alternatives to P crit are the regulation index and Michaelis-Menten or sigmoidal allosteric analyses.