Neurohormonal of Thermoregulation Dr. Dini Sri Damayanti,MKes

Body Temperature Shell temperature: Core temperature: Temperature closer to skin Oral temperature 36.6o-37.0oC (97.9o-98.6oF) Core temperature: Most important temperature Temperature of “core” (organs in cranial, thoracic and abdominal cavities) Rectal temperature 37.2o-37.6oC (99.0o-99.7oF)

Heat Production Exergonic reactions: Oxidation and ATP use. Most heat generated by brain, heart, liver and glands at rest. Skeletal muscles 20-30% at rest. Can increase 30-40 times during exercise.

Thermoregulatory Center Hypothalamus: Preoptic area neurons: hypothalamic thermostat: Heat-losing center Heat-promoting center Monitors temperature of blood and receives signals from peripheral thermoreceptors. Negative feedback loops

Thermoregulatory Center Heat-losing center (hipotalamus anterior): Activates heat losing mechanisms: Dilation of dermal arterioles: increase blood flow to skin. Sweating. Increased respiration through mouth. Behavioral: remove clothing. Inhibits heat-promoting center.

Thermoregulatory Center Heat-promoting center ( hipotalamus posterior ): Activates heat generating mechanisms: SNS: Vasoconstriction of dermal arterioles: decrease blood flow to skin Stimulates arrector pili muscles: hair stands on end Shivering thermogenesis: spinal reflex of alternating contractions in antagonistic muscles Nonshivering thermogenesis: Long-term mechanism stimulating thyroid hormone release T3 and T4. Inhibits heat-loss center.

Nonshivering Thermogenesis Temperature sensors are in the skin (in newborns particularly the face), the spinal cord and the hypothalamus. Temperature information is processed in the hypothalamus. Norepinephrine (NE) is released in response to cold stress. Result: Vasoconstriction and increased metabolic activity.

Nonshivering Thermogenesis Vasoconstriction also occurs in infants but the primary response is increasing heat production from brown fat metabolism. NE stimulates receptors on brown fat cells; activates lipase which releases intracellular fatty acids. Conversion of T4 to T3 inside brown fat cells. T3 cellular metabolic rate Brown fat mechanism is known as nonshivering thermogenesis

Uncoupling protein uncouples mitochondrial oxidative phosphorylation. H+ gradient heat rather than ATP Result: Large increase in heat production and O2 consumption. In case of hypoxia, temperature because of O2

Mechanisms of Heat Transfer Radiation: Infrared radiation. Conduction: Direct transfer of energy through physical contact. Convection: Heat loss to air around the human body. Evaporation: Energy change in water molecule from liquid to vapor.

Muscles of skin arteriole walls relax NEGATIVE FEEDBACK Blood temperature Muscles of skin arteriole walls relax Core body temperature >37°C Hypothalamus Sweat glands increase secretion nerves Muscles reduce activity Thermoreceptors Thermoreceptors Body loses heat Return to 37°C © 2008 Paul Billiet ODWS

Muscles of skin arteriole walls constrict NEGATIVE FEEDBACK Blood temperature Muscles of skin arteriole walls constrict Core body temperature <37°C Sweat glands decrease secretion nerves Muscles shivering Thermoreceptors Hypothalamus Thermoreceptors Body loses less heat Return to 37°C Body gains heat © 2008 Paul Billiet ODWS

Goal of Thermoregulation Maintain correct body temperature range in order to: maximize metabolic efficiency reduce oxygen use protect enzyme function reduce calorie expenditure

THERMOREGULATION IN THE NEONATE

Challenges of thermoregulation in Neonatal care Prior to delivery infants do not maintain A temperature independently Infant’s in-utero temp is generally 0.5˚C higher than mother’s temp Rapid cooling occurs after delivery

Neurologic adaptation: Thermoregulation Maintenance of body temp is a major task Skin is thin & blood vessels are close to the surface Have little subcutaneous fat to serve as barrier to heat loss Term Infants have 3x the surface to body mass of an adult Preterm infants and SGA infants have 4x the surface mass to body mass of an adult Preterm infants are especially susceptible to heat loss due to poor tone,  fat and thinner skin than term infants

Neutral Thermal Temperature A neutral thermal temperature is the body temperature at which an individual's oxygen use and energy expenditure are minimized. Minimal metabolic rate Minimal oxygen consumption

Body temperature in the newborn infant Classification of hypothermia is based on core temperature NORMAL – 36.5 to 37.3˚C (97.7 –99.2˚F) Cold Stress 36.0 to 36.4˚C (96.8 – 97.6 ˚F) Cause for concern Moderate hypothermia 32 –35.9˚C (89.6-96.6˚F) Danger, warm infant Severe hypothermia – below 32˚C (89.6 ˚F) Outlook grave, skilled care urgently needed

Neutral Thermal Environment (NTE) The air temperature surrounding the baby supports maintenance of a neutral thermal body temperature.

Thermoneutrality When the air temperature is in the correct range and the infant’s body maintains a neutral thermal temperature, we have achieved thermoneutrality

Why Are Infants At Greater Risk for Thermoregulation Problems?

Thermoregulation Risk Factors Premature SGA Neuro problems Endocrine Cardiac / respiratory problems Large open areas in the skin Sedated Infants Drug exposure

Why are they at Risk??? Brown Adipose Tissue Body surface area SQ Fat Glycogen stores Body water content Posture Hypoxia Hypoglycemia Anomalies CNS Sedation

What Do We Know? Infants have more skin surface per pound of body weight than older children or adults More skin means more radiant heat and more insensible water loss.

Risk factors for Preterm Infants less brown fat and glycogen stores decreased ability to maintain flexion increased body surface area compared to weight

What Do We Know? The majority of an infant’s thermal receptors are found in the face, neck, and shoulder area. Stimulation of these receptors will result in chilling and calorie expenditure

What Do We Know? Shivering, which is the main way in which older children and adults generate heat, is impossible or not effective in infants. Neonates and young infants generate heat by burning brown fat.

Production of Heat Metabolic Processes Voluntary Muscle Activity Peripheral Vasoconstriction Nonshivering Thermogenesis

Metabolic Process Heat Generation by Metabolic Energy Oxidative metabolism Glucose Fats Protein Metabolic Energy Brain Heart Adrenal Gland

Voluntary Muscle Activity Postural changes Restless movements Limited use to Newborn

Peripheral Vasoconstriction Reduces skin blood flow Decreases loss of heat from the body

Nonshivering Thermogenesis Metabolism of brown adipose tissue Initiated in hypothalamus Sympathetic nervous system Norepinephrine release at the site of brown fat

What is Brown Fat? Brown fat is an energy source for infants It can be found: Near Kidneys and adrenals Neck, mediastinum, scapular, and the axilla areas. Can not be replaced once used

Brown Fat In full term infants brown fat is 4 % -10% of adipose deposits. In preterm infants, brown fat will not be found until 26-30 weeks gestation, and then only in small amounts. Brown fat generally disappears 3-6 months after birth, except in cold stressed infants (where it will disappear sooner.) Hypoxia causes impairment of brown fat metabolism

So What? When the air temperature around the baby is cool, thermoreceptors in the skin are stimulated. Non- shivering thermogenesis is initiated and brown fat is burned for energy to keep the body temperature stable. This is the infant’s initial response.

What Next? Conversion of brown fat uses oxygen and glucose, therefore, the cold stressed infant will become hypoxic and hypoglycemic. Blood gas and glucose levels are affected. Growth is affected as calories are used to stay warm rather than grow.

What are the Signs and Symptoms of Thermal Instability?

Methods of heat loss Peripheral Vasodilatation Sweating  blood flow facilitates heat transfer from periphery to environment Sweating  evaporative heat loss  postnatal age increases the ability to sweat Appears first on term newborn head

Healthy Vs. Sick Neonate Healthy Newborn Brown adipose tissue Produces heat and loses heat as needed Sick or Low birth wt infants Increased energy demand Decreased energy store Vulnerable to heat stress

Hypothermia – Signs/symptoms Body cool to touch Mottling or pallor Central cyanosis Acrocyanosis Poor Feeding Abdominal distension Hypotonia Hypoglycemia gastric residuals Bradycardia Tachypnea Restlessness Shallow or Irregular Respirations Apnea Lethargy

Signs and Symptoms of Hypothermia in Infants Vasoconstriction Peripheral vasoconstriction occurs in an effort to limit heat loss via blood vessels close to the skin surface. Pallor and cool skin may be noted, due to poor peripheral perfusion

Signs and Symptoms of Hypothermia in Infants Increased Respiratory Rate Pulmonary vasoconstriction occurs secondary to metabolic acidosis. Increasing Respiratory Distress Related to decreased surfactant production, hypoxia, & acidosis

Signs and Symptoms of Hypothermia in Infants Restlessness Restlessness may be a type of behavioral thermoregulation used to generate heat through muscle movement. The first sign may be an alteration in sleep patterns. Restlessness also indicates a change in mental status as cerebral blood flow diminishes, due to vasoconstriction.

Signs and Symptoms of Hypothermia in Infants Lethargy If thermo-instability goes unrecognized, the infant will become more lethargic, as cerebral blood flow continues to diminish and hypoxemia and hypoglycemia become more pronounced.

Signs and Symptoms of Hypothermia cont. Metabolic Disturbances Metabolic acidosis Hypoxemia Hypoglycemia progress due to continued metabolism of brown fat, release of fatty acids and anaerobic metabolism (lactic acid)

Signs and Symptoms of Hypothermia Cardiac As central blood volume increases, initially the heart rate and blood pressure increase Arrhythmias May result from depressed myocardial contractility and irritability caused by hypothermia

Signs and Symptoms of Hypothermia Poor Feeding/Weight Loss Poor weight gain occurs when: calories consumed brown fat stores are used to make body heat.

Even the smallest weight loss may take days or even weeks to recover, as infants are limited in the volume of food they can eat and number of calories they can tolerate.

Consequences of Hypothermia Hypoxemia from  Oxygen consumption Hypoglycemia from  glucose metabolism Respiratory & metabolic acidosis secondary to anaerobic metabolism Inhibition of surfactant production related to  acidosis

Consequences of Hypothermia  pulmonary blood flow related to pulmonary vasoconstriction in response to  body temperature  pulmonary vascular resistance compromises the delivery of oxygen at the cell level  risk of developing PPHN in the near term, term or post term infant

Prevention of Hypothermia Hypothermia can be prevented by maintaining a neutral thermal environment and reducing heat loss. A neonate is in a neutral thermal environment when the axillary temperature remains at 36.5° - 37.3° (97.7° - 99.2° F) with minimal oxygen and calorie consumption

Prevention of Hypothermia Reduction of heat loss Consider the four ways by which the neonate experiences heat loss and intervene appropriately.

Prevention of Hypothermia Prevention of hypothermia is the best treatment but if it occurs anyway, the following is a list of what you can do to relieve the cold stress. Increase ambient air temperature Apply external heat sources Warm hat Warm blankets or diapers Chemical mattress

Prevention of Hypothermia Avoid stressing the baby Monitor skin temperature carefully and when it normalizes remove the external heat sources one at a time to prevent rebound hypothermia

Hyperthermia

Hyperthermia HYPERTHERMIA also has negative consequences for the neonate. Defined as a rectal / axillary temperature greater than 37.3°c (99.2°F)

Risk factors for Hyperthermia Excessive environmental temp Sepsis Dehydration Alterations in the hypothalamic control mechanism Birth Trauma Anomalies Drugs

Signs of Hyperthermia Tachypnea Apnea Tachycardia Flushing Hypotension Irritability Poor Feeding Skin Temp > Core Temp

Consequences of Hyperthermia  in Metabolic rate  oxygen consumption Dehydration from  insensible water loss Peripheral vasodilatation/ hypotension Fluid, electrolyte abnormalities seizures

There Are a Lot of Factors to Consider There Are a Lot of Factors to Consider. How Can I Be Sure My Patient Maintains Thermoneutrality?

It Is Important to Review and Understand the Four Methods of Heat Transfer

Possible Sources of Heat Loss

Strategies to prevent heat loss: CONVECTIVE HEAT LOSS can be prevented by: Providing warm ambient air temperature Placing infants less than 1500 grams in incubators Keeping portholes of the incubator closed Warming all inspired oxygen On open warmers keeping sides up and covering infant if possible Using Infant Servo Temperature Control

Strategies to prevent heat loss: RADIANT HEAT LOSS can be prevented by: Avoiding placement of incubators, warming tables and bassinets near cold windows, walls, air conditioners, etc.. Placing a knit hat on the infant’s head Wrapping tiny babies in saran or “bubble” wrap  environmental temperature

Strategies to prevent heat loss: CONDUCTIVE HEAT LOSS can be prevented by: Placing a warm diaper or blanket between the neonate and cold surfaces Placing infant on pre-warmed table at time of delivery Warming all objects that come in contact with the neonate Admitting infant to a pre-warmed Skin to skin contact

Strategies to prevent heat loss: EVAPORATIVE HEAT LOSS can be prevented by: Keeping the neonate and his/her environment dry. Drying the baby immediately after delivery. Placing preterm or SGA infant in occlusive wrap/bag at delivery Delay bath until temperature is stable

Interventions for at Risk Infants Pre-warmed radiant warmer bed Pre-warmed incubator Do not leave a warmer bed or incubator in the manual mode Servo mode allows the baby to control the heat output of the warmer units Heated water pad Heat lamp Warm and humidify inspired gases Occlusive wrap/bag at delivery

Interventions for at Risk Infants Open incubator portholes and doors only when necessary Blanket over incubator Cluster care

Interventions to Consider Cover thermoreceptor-rich areas, such as the head. Dry well after baths, especially the head and neck area. Dress and cover infants, when in cribs, to prevent them from chilling. Warm fluids prior to dressing changes Rewarm slowly to prevent a potential subsequent drop in blood pressure.

Rewarming the Hypothermic Infant Always be prepared to intervene Rewarm slowly (0.5˚C per hour) Monitor closely (vital signs every 15 – 30min) Core temp Skin temp will be higher than axillary Blood pressure Rewarming may lead to vasodilation - hypotension Heart rate and rhythm Bradycardia & arrhythmias common with hypothermia

Rewarming Monitor Respiratory rate and effort Oxygen saturations Increased distress Apnea Oxygen saturations Hypoxemia / desaturations Be prepared for  need for respiratory support Monitor acid/base status Blood glucose Monitor- infant at increase risk for hypoglycemia

Guidelines for Rewarming Incubator better control than warmer Set temp 1 – 1.5˚C above core temp Assess infant temp every 15-30 minutes As infants core temp reaches set temp and infant is not showing signs of deterioration increase set temp again. Continue process until temp within normal range

Signs of Deterioration during rewarming Tachycardia – due to in cardiac output Cardiac arrhythmia Hypotension Hypoxemia / Desaturations Worsening respiratory distress Worsening acidosis

Cooling the overheated neonate Extended position- not flexed Expose skin- remove clothing Keep active temp reduction methods to a minimum to prevent dramatic heat loss Monitor temperature and VS every 15 – 30minutes Be prepared to intervene

Conclusions Hypothermia in the newborn is due more to a lack of knowledge than to lack of equipment. Hypothermia is a preventable condition that has well documented impact on morbidity and mortality. Therefore, assisting the infant to maintain a normal body temperature and preventing hypothermia during stabilization is critical