The fallacy of the P_crit - are there more useful alternatives?

P_crit - generally defined as the P_O  below which the animal can no longer maintain a stable rate of O₂ consumption (Ṁ_O ), such that Ṁ_O  becomes dependent upon P_O  - 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_O  versus P_O  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_O  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.