ThermoRegulation and the Neonate Heat Loss RSTH 421 Perinatal and Pediatric Respiratory Care and Treatment

metabolic response to cold stress

Consequences of Cold Stress      As the body temperature decreases, the baby becomes less active, lethargic, hypotonic, sucks poorly and their cry becomes weaker.  Respiration becomes shallow and slow and the heart-beat decreases.  Sclerema – hardening of skin with redness – develops mainly on the back and the limbs.  The face can also become bright red.  As the condition progresses it causes profound changes in body metabolism resulting in impaired cardiac function, hemorrhage (especially pulmonary), jaundice and death.  (WHO, 1994)

Importance of Thermoregulation A.    Newborns have disadvantages in the maintenance of body temperature. B.    Thermoregulation influences oxygen consumption. C.    Thermoregulation is important to the success of resuscitation D.    Respiratory Care Practitioners administer therapeutic modalities that influence thermoregulation in the newborn.

Disadvantages in Newborn A. Decreased subcutaneous fat stores/thin skin (transepitermal water/heat loss) B. Increased surface area to mass ratio C. / speed of resuscitation, metabolism rate, effect of medications. D. Weight gain/loss

Disadvantages in Newborn A. Decreased subcutaneous fat stores/thin skin (transepitermal water/heat loss) B. Increased surface area to mass ratio C. / speed of resuscitation, metabolism rate, effect of medications. D. Weight gain/loss

Thermoregulatory System (3) 1.     A sensory system for monitoring temperature (skin) Sensors detect changes in environmental temperature and try to maintain a constant internal temperature. 2.     A central regulatory system to keep core temperature constant (hypothalamus) Altered by asphyxia, hypoglycemia, birth trauma, intracranial hemorrhage and prematurity 3.     Method of regulating heat production and heat loss

Thermogenesis 1. Heat production 2.     Shivering and Nonshivering thermogenesis a. Shivering does not occur until temperature is very low b. Nonshivering thermogenesis   Major mechanism of heat production in the newborn

Metabolism of Brown Fat 2-6% of the neonates body weight First appears at 26-30 weeks gestation Will enlarge up to the 3d-5th post-natal week and disappear some weeks after birth (unless significant cold stress occurs).

Brown Fat

Metabolism of Brown Fat Located in the face, shoulders (diamond shaped mass), neck, around blood vessels, axillae (large masses), deep masses: in the anterior mediastinum, vertebral column, and around the kidneys and adrenals. Skin sensors, especially in the face (trigeninal nerve) are activated by cold, which induces sympathetic release of norepinephrine.

Metabolism of Brown Fat

Metabolism of Brown Fat Different than adipose tissue as it contains small lipid inclusions dispersed throughout its cytoplasm (white fat only has one large fat globule). Named due to it’s copious blood supply, dense cellular contents and profusion of nerve endings that are densely innervated by sympathetic nerves The other type of adipose CT found in vertebrates is "brown fat," called such because of its color in gross specimens. Brown fat is brown because it's very heavily vascularized and because its cells contain thousands of mitochondria, which lend an overall brown cast to the fresh specimen. Brown fat is thermogenic tissue: its only function is to generate heat. Energy released from the fat molecules is given off as heat, which is subsequently transferred by the liquid phase of the blood to warm other parts of the body. In non-rodents brown most easily found in young animals where it provides a supplementary source of body heat to organisms without a fully developed thermoregulation capacity. This sketch of an isolated brown adipocyte schematically depicts the smaller and multiple fat droplets, and the scattered mitochondria, which are actually far more numerous than is represented here.

Metabolism of Brown Fat Metabolized during cold stress by releasing norepinephrine (NE). NE stimulates brow fat metabolism. Activating lipase – triglycerides – hydrolysis = Glycerol and nonesterified fatty acids (NEFA) Oxidation of NEFA in brow fat causes a reaction that produces heat. Increases oxygen consumption by 60%,  glucose consumption. Brown fat thermogenesis is a disadvantage in the hypoxic, hypoglycemic patient.

Mechanisms of Heat Loss and Gain Environmental considerations Effects and consequences of Cold Stress A.    Definition of Cold Stress B.    Definition of Hypothermia C.    Constriction of cutaneous vessels - anaerobic metabolism - metobolic acidosis - pulmonary vasoconstriction - hypoxemia. D.    Increased metabolism leads to increased calorie usage and less weight gain. E.     Cold stress can lead to hypothermia. F.     Increased metabolism can also lead to hypoglycemia.

Hyperthermia Causes 1. Iatrogenic 2. Sepsis 3. Trauma from birth 4.      Dehydration Effects

Neutral Thermal Environment Definition The environmental teperature at which VO2 is minimal and body core temperature is normal. Normally between 30-35 C, but is narrowest at the lowest gestational ages. Widest in the term infant. Dependent on Weight, Age, Clothing.

Temperature Gradients External temperature gradient Difference in temperature between the body surface(skin) and environmental temperature Internal temperature gradient Difference in temperature between body surface(skin) and core (rectal) temperature. Should not be more than one degree Centigrade with core being warmer than skin.

Radiant Heat Loss

Convection Heat Loss

Evaporative Heat Loss

Conductive Heat Loss Heat loss due to direct contact with cold or cooler objects, i.e., cold bed, mattress, touching the cool/cold walls of incubator, etc.