Anoxia Tolerance to Oxygen Necessity: Paradigm Shift in the Physiology of Survival of Apneic Deep Hypothermia in Neonatal Rodents

Abstract

Neonatal small rodents stop breathing at body temperatures of 0–6ºC, yet they often tolerate such hypothermia for several hours and recover spontaneously when rewarmed. The prevailing paradigm of deep-hypothermia tolerance for half a century has been that the heart stops beating; oxygen uptake and delivery from the environment stop; tissues are tolerant of anoxia; and this anoxia tolerance is a key to survival. I here report that the heart does not stop beating in neonates that survive hours of hypothermia, even in species earlier reported to undergo cardiac arrest. Furthermore, as was reported in 1955 but ignored, neonates consume oxygen steadily at low rates while apneic, and this oxygen consumption is essential for survival. Thus, the old paradigm must be succeeded by a new one: Although tissues may become hypoxic and benefit from hypoxia tolerance during apneic deep hypothermia, oxygen is taken up steadily across the lungs; the circulatory system likely transports oxygen; and certain vital tissues require oxygen influx if a neonate is to survive.