# NURS 131 Brown Fat, Heat Loss, and Cold Stess

jasmine's version from 2016-05-04 04:57

## Section 1

Cold stress in newborns A very serious condition that occurs when the body can no longer maintain a normal temperature; Decreased body temperature (hypothermia); life and death challenge for many newborns, especially premature infants and babies who are small for their gestational age
Some results of cold stress Serious cold-related illnesses and injuries, permanent tissue damage or death
4 ways heat loss can occur Radiation; Convection; Conduction; Evaporation
Radiation Peripheral vasoconstriction facilitates heat loss; Occurs when the relatively large surface area of the infant is exposed to surrounding objects with a lower than body temperature; The greater the temperature gradient between surface of the infant and the surrounding environment, the greater is the degree of heat loss (radiant heat loss); Electromagnetic waves; Approximately 45-60 percent of heat loss from the body occurs by this
Convection Transfer of heat from the body to the surrounding air, a process by which the air temperature can deliver or take heat away from the body; For example, if the infant’s temperature is 36 degree Celsius (96.8 degree Fahrenheit), and the temperature of the operating room is 34 degree Celsius (93.2 degree Fahrenheit), the infant will be losing heat to the air in the room even though he/she is lying on a warm water mattress; Approximately 35 percent of the infant’s heat loss is through this
Conduction Transfer of heat directly from molecule to molecule; Only occurs when there is direct physical contact of the skin to a solid surface; The temperature of the object in relation to the infant’s own body temperature will determine if heat will be lost or gained by this; An infant placed on a warm (higher than body temperature) water mattress will not lose heat by this to the mattress
Evaporation A change of state from a liquid to a gas or vapor; Molecules in liquid are drawn to the center of this liquid; Some of the more energetic molecules overcome this force, escape form the liquid and change to a vaporous state. When this happens, the energy remaining in the liquid will fall, as is reflected by a reduction in the temperature of this liquid; Evaporative losses are dependent upon the surface area, the velocity of air movements over this surface, and the relative humidity of the surrounding environment; For example, evaporative loss in the lungs depends upon the difference in absolute humidity of the inspired and expired air and upon the amount of sir breathed; In infants, evaporative loss from the skin in minimal due to immature sweating mechanisms

## Section 2

Non-shivering thermogenesis Primary heat production in infants; It is not shivering that produces heat. It is the metabolism of brown fat, which increases body temperature when the thermal receptors in the skin detect a skin temperature of 35 to 36 degree Celsius (95-96.8 degree Fahrenheit)
Brown fat Adipose tissue or brown adipose tissue, a special kind of highly vascular fat found in newborns; It contains an ample supply of blood vessels, which cause the brown color; Primarily located in the back of the neck, in the axillae, around the kidneys, adrenals, sternum, between the scapulae, and along abdominal aorta
Without brown fat to metabolize No heat production will counteract the cold stress, hence the infant is at risk to serious complications
Preterm infants and brown fat May be born before the stores of brown fat have accumulated; Aside from the said case, intrauterine growth restriction also deplete brown fat stores before birth occurs
Newborns exposed to prolonged cold stress May have insufficient brown fat stores as a large amount of brown fat is consumed for heat production in this situation; Thus, these infants will NOT be able to raise their body temperature if they are subjected to further episodes of cold stress
Heat production in shivering The rhythmical contraction of muscle fibers, which produces a rise in body heat
Processes involved in non-shivering thermogenesis (chemical method) 1) Non-shivering thermogenesis begins when the thermal receptors in the skin detect a skin temperature of 35 to 36 degrees Celsius (95 to 96.8 degrees Fahrenheit). 2) The thermal receptors stimulation is then transmitted to the hypothalamus thermal center. 3) In response to the hypothalamic stimulation, norepinephrine is released in brown fat. 4) Presence of norepinephrine in the brown fat initiates its metabolism. 5) As brown fat is metabolized, it generates more heat than other fats. 6) Thus, blood passing through the brown fats is warmed and carries heat to the systemic circulation or to the rest of the body
Newborn and adult differences in heat production Shivering vs non-shivering thermogenesis
Other newborn differences 1) A lack of subcutaneous fat; 2) A larger surface area to body weight ratio; and 3) The maximal tissue insulation resulting from vasoconstriction is considerably less than adults (due to their small size)
Newborn must be maintained in an environment Which is neither too cold nor too hot, a so-called neutral thermal environment
Neutral thermal environment Allows control of body temperature by vasomotor adjustments, with minimal changes in metabolic rate and oxygen consumption
3 different approaches thermal neutrality can be measured 1) Body temperature (rectal temperature between 36.5-37.5 degree Celsius [97.7-99.5 degree Fahrenheit] associated with minimal thermal stress); 2) Temperature gradients (measured between infant’s skin temperature and the environment) A rectal/environment gradient (difference) of 1.5° C (34.7 F) for a 1 kg infant who is vasodilated, premature and one day old appears to be associated with minimal stress. For a three-week old who is vasoconstricted, a rectal/environment temperature gradient of 4.8° C (40.64 F) is still considered in the neutral thermal zone; and 3) Environmental temperature
In order to achieve a neutral thermal state The infant must be above the critical temperature for cold inducing metabolic stress, but below a temperature associated with evaporative heat loss and increased metabolic rate. The older the infant, the greater is the resting rate of heat production relative to surface area
Physiologic changes of an infant challenged by cold stress 1) Peripheral vasoconstriction resulting in maximal tissue insulation; 2) an obligatory rise in metabolic rate; 3) sympathetic response in which norepinephrine release will increase the metabolic rate leading to increased oxygen consumption; 4) metabolic acidosis, which is the result of two functions, a) increased metabolic rate, and b) persistent vasoconstriction, causing a reduction in tissue perfusion and oxygenation; and, 5) pulmonary vasoconstriction which decreases pulmonary perfusion
The major mechanism in the newborn's defense against cold stress is its ability to Release norepinephrine and that the maturity of the infant parallels thermal stability. Thus, the larger the infant, the greater is the thermal stability