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 O-2 consumption ((M)over dot(O2)), such that (M)over dot(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 (M)over dot(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 (M) over dot(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.