Thermoregulation in the Preterm Infant

Preterm infants (PI) are predisposed to loss heat and have little subcutaneous tissue and a high ratio between surface and body weight and a reduced amount of glycogen deposits and brown fat (Gomella, 2005, p. 43).

In Mexico and South America in 2003 were reported 207.753 neonatal deaths, 21.4 per every 1,000 born alive in whom hypothermia was significant among infants with low birth weight and preterm infants. (Zuleta, Gomez \& Jaramillo, 2009).

PRETERM INFANT Official Mexican Norm -007-SSA2-1993 (1995) considers a preterm infant as the product of the conception from 28-37 weeks of gestation, and weighing 1000-2499 g.

THERMOREGULATION OF PRETERM INFANT Survival of the newborn increases if the excessive loss heat is prevented. To achieve this purpose the newborn should be kept in a thermal environment neutral "which is what allows an infant consume the least amount of energy to maintain normal body temperature". (Karlsen, 2006, p. 49) The heat is gained or lost by convection 37% (between a child and a fluid: air stream or during bathing), conduction 4% (between two bodies in contact with different temperatures: the fields, mattress) radiation, 43% (between two solid surfaces that are not in touch: cool walls of the incubator) and 16% evaporation (heat loss in the conversion of water from liquid to gas, skin, breathing, sweating). (Tamez \& Pantoja, 2004, pp. 29-30; Ruíz, 2007) The understanding of these forms of heat exchange provides the scientific basis for nursing interventions aimed at modifying the environment.

Response to cold stress of preterm infant: When the sensors of central and peripheral temperature detect stress for cold, they send signals to the hypothalamus. The hypothalamus activates the liberation of norepinephrine. This hormone causes increment in the metabolism, which increases the oxygen consumption and glucose utilization. The increment in the consumption of oxygen can induce hypoxemia and if it is severe, could progress to hypoxia. Peripheral vasoconstriction is limited in infant of very low birth weight in the first 48 hours of life; this increases the heat loss to level of the skin. In the preterm infant the loss heat occurs faster than its ability to produce and conserve heat. (Karlsen, 2006, p. 56) Adverse effects of cold stress in term and preterm neonate: When a term or a preterm neonate is hypothermic, the metabolism, oxygen consumption and glucose utilization increase. If the neonate, is experiencing trouble for breath, he will not be able of confront the increment in the demand of oxygen for tissues. This allows or increases the hypoxemia, which contributes to increased pulmonary vasoconstriction. The severe hypoxemia can progress to hypoxia, which leads to anaerobic metabolism. During anaerobic metabolism, the accumulation of lactic acid and blood ph drops is observed. If it is not reversed, the risk of death is high. (Jasso, 2005, p. 91; Karlsen, 2006, p. 58) The hypothermia can cause hypoglycemia, since the glucose is the primary source energy for the brain. The level of conscience of the neonate can diminish, the respiration can become slow and oxygenation will be affected. The survivors to an event of hypothermia have collateral effects as the increase of the difficulty of respiration, severe renal failure, disseminated coagulation, increase in the incidence of infection and persistence of the arterial conduit. (Karlsen, 2006, p. 58) PLACEMENT OF POLYETHYLENE BAG The program STABLE (sugar, temperature, airway, blood pressure, lab work and emotional support) for care post-resuscitation and pre-transport of sick neonates, recommended cover to premature infant with a plastic cover immediately after birth, from the neck down to reduce heat loss through evaporation and convection. (Karlsen, 2006, pp. 51, 53) The polyethylene bag, "is a cheap device, practical, simple, does not interfere with the immediate care or resuscitation (Vohra et al., 2004) and does not put in risk the integrity and security of the newborn. But it is not clear whether its effectiveness is similar with or without prior drying of the newborn, because the only study comparing these two techniques was conducted in term infants and it was not randomized (Menesses et al., 2002).

It is recommended that all apnea at birth should be treated as a secondary apnea and not delays resuscitation. (American Academy of Pediatrics and American Heart Association, 2000) At this stage every second is vital for the newborn, and from this comes the concern of knowing if the polyethylene bag is equally effective for thermoregulation of the newborn, with or without pre-drying, since it would save a few seconds for drying, and immediately beginning neonatal resuscitation.