How newborn mammals cope with hypoxia.

The most immediate response to acute hypoxia in newborn mammals is hyperventilation, like in the adult. However. hyperventilation is often achieved by a reduction in metabolic rate (hypometabolism), rather than by an increase in ventilation (hyperpnea). This response is a regulated phenomenon largely based on inhibition of thermogenesis in all its forms, shivering, non-shivering and behavioural, with a resetting of the thermocontrol at a lower value of body temperature (Tb). Forcing Tb to the normoxic value in an hypoxic newborn can therefore provoke responses that are disadvantageous to the general strategy against hypoxia. The small or absent hyperpnea in the hypoxic newborn is the expected response to the decrease in metabolic rate; therefore, it should not be necessarily regarded as an expression of inadequate ventilatory control. However, during hypoxia the low metabolic rate can enhance the relative efficacy of inputs inhibitory on breathing, and this could be a mechanism contributing to ventilatory irregularities and apneas. The advantages of the hypometabolic strategy are numerous, and are at the basis of the extraordinary ability of newborn mammals to survive periods of severe hypoxia. The disadvantages become apparent with chronic hypoxia, because the reduced growth of tissues and organs may be incompatible with survival, or could lead to long-lasting structural and functional alterations.