prevention of hypothermia.ppt

Editor's Notes

• #4: Maintaining a neutral thermal environment is one of the key physiologic challenges that a newborn must face after delivery Hypothermia is one of the commonest cause of morbidity and mortality of newborns In early 1900s Realized warm environment was essential in the care of low birth weight and preterm newborns because they couldn’t maintain their own body temprature Last decades Improved care of newborns in developed world because of awareness of importance of creating warm enviroment Still common problem in developing countries, this situatation results more from lack of knowledge rather than lack equipements because in most parts of developing country, there is little understanding of the thermal needs of newborn babies and of the extent and significance of neonatal hypothermia Thermal care is central to reducing morbidity and mortality in newborns.
• #5: The average “normal” axillary temperature is considered to be 37°C (Leduc & Woods, 2013). The Canadian Paediatric Society recommends taking temperature via the axillary route to screen low risk newborns from birth to 2 years (Leduc & Woods, 2013). There is a lack of evidence on what constitutes the “normal” temperature range for a newborn. The American Academy of Pediatrics (AAP) and the American College of Obstetricians and Gynecologists (ACOG) (1997) and the World Health Organization (WHO) (1997; 2003) define normal axillary temperatures to be between 36.5°C and 37.5°C. The Acute Care of at-Risk Newborns Neonatal Society (ACoRN) define normal axillary temperature to be between 36.3°C-37.2°C (ACoRN, 2012). Hypothermia Hypothermia occurs when the newborn’s axillary temperature drops below 36.3°C (ACoRN, 2012) or below 36.5°C (AAP/ACOG, 1997; WHO, 1997). The following characteristics put newborns at a greater risk of heat loss: · A large surface area-to-body mass ratio · Decreased subcutaneous fat · Greater body water content · Immature skin leading to increased evaporative water and heat losses · Poorly developed metabolic mechanism for responding to thermal stress (e.g. no shivering) · Altered skin blood-flow (e.g. peripheral cyanosis) (Aylott, 2006; Blackburn, 2007; Galligan, 2006; Hackman, 2001; WHO, 1997) PHYSIOLOGY OF THERMOREGULATION Normal body temperature is maintained constant by a balance of heat loss and heat gain with the assistance of an efficient thermoregulatory mechanism. Extreme environmental temperature variations, however, can overcome this effective thermoregulatory function and lead to heat- or cold-related illnesses. Body temperature consists of core and shell temperatures. Rectal, esophageal, bladder, and oral temperatures represent core temperature, whereas axillary and skin temperatures represent shell temperature. Core temperature determines the risk of injury to various organs in the body. Air temperature, air movement, thermal radiation, sweating, skin blood flow, and temperature of underlying tissue all influence shell temperature (1). Thermoreceptors for the shell reside in the skin. Core thermoreceptors exist in the cortex, hypothalamus, midbrain, medulla, spinal cord, and deep abdominal structures in addition to the skin (2,3). On sensing a temperature change, these receptors transmit afferent impulses via the lateral spinothalamic tract to the central thermostat located in the preoptic/anterior hypothalamus, which maintains the temperature set point (4). Thermoregulation is initiated when sensed temperature is different from the set point. The conditions associated with failed thermoregulatory mechanisms that lead to hyperthermia or hypothermia will be discussed in this chapter Heat Gain Warm-blooded animals have the capacity to raise their body temperature above their environmental temperature, which occurs when endogenous or exogenous heat gain exceeds heat loss. Heat is generated in the human body from basal metabolism, physical activity, food consumption, metabolic activity, emotional change, hormonal effects, and certain medications that typically raise body metabolism. The body may also acquire heat passively when the environmental temperature exceeds body temperature. Heat Loss Heat is lost from the body via conduction, convection, radiation, and evaporation. In most situations, humans produce more heat than necessary and dissipate the excess heat into the environment. Conduction is heat loss by the transfer of heat from a warmer to a cooler object when the two objects are in direct contact. The amount of heat loss depends on the contact area and the temperature difference between the body and the other surface. Typically, only 3% of http://obgynebooks.com P.500 body heat is lost by conduction; however, conduction may be a major source of heat loss in wet clothing or immersion incidents because of the excellent conductive properties of water. Convection is heat loss by the movement of air or fluid that circulates around the skin. More heat is carried away from the body in windy conditions, as the movement of air rapidly removes the insulating layer of warmer air normally around the body surface. Approximately 12%-15% of body heat is lost by convection. Radiation is heat loss due to infrared heat emission to surrounding air. Heat loss occurs primarily from the head and noninsulated areas of the body and usually occurs rapidly. Radiation can account for 55%-65% of heat loss. Evaporation is heat loss by the change of water from a liquid (sweat) to a gas state via the skin or respiration. Evaporation normally accounts for 25% of heat loss, but depends on surface area, temperature difference, and humidity. Evaporative heat loss is highest in cold, dry, and windy conditions
• #6: Thermoneutral zone: The range of ambient temperature required for the infant (for each gestational age and weight) to keep a normal body temperature (core body temperature from 36.5°C to 37.5°C) and a minimal basal metabolic rate Extreme environmental temperature variations, however, can overcome this effective thermoregulatory function and lead to heat- or cold-related illnesses
• #8: Physical process of converting liquid to vapor Accounts major cause of heat loss…50% Heat loss occurs from energy needed to vaporize liquids from skin,lung,mucosa and serosa Depends on The exposed body surface area Relative humidity of the ambient air the speed of the wind
• #9: Radiation Energy transmitted by waves through the medium Heat loss to the nearby cold objects with out physical contact Major source of heat loss in most surgical patients Depends on The T differences The body exposed to the environment The skin blood flow Radiation: when the newborn is near cool objects, walls, tables, cabinets, without actually being in contact with them. The transfer of heat between solid surfaces that are not touching. Factors that affect heat change due to radiation are temperature gradient between the two surfaces, surface area of the solid surfaces and distance between solid surfaces. This is the greatest source of heat loss after birth Most cooling of the newborn occurs immediately after birth. During the first 10 to 20 minutes, the newborn may lose enough heat for the body temperature to fall by 2-4°C if appropriate measures are not taken. Continued heat loss will occur in the following hours if proper care is not provided. The temperature of the environment during delivery and the postnatal period has a significant effect on the risk to the newborn of developing hypothermia.
• #10: Conduction Heat transfer from warm to cool objects with direct contact Accounts 5% Depends on Area of body exposed Relative difference in temperature Thermal conductivity…low in air, is approximately 25 times more rapid in water Conduction: when the newborn is placed naked on a cooler surface, such as table, scale, cold bed. The transfer of heat between two solid objects that are touching, is influenced by the size of the surface area in contact and the temperature gradient between surfaces
• #11: Convection Occurs when air flow carries heat to or away from the body Special type of conduction heat loss through moving gases Accounts 15% Second most common heat loss in anesthetized pt Depends on The temperature difference The speed of air Proportional to the square root of air speed · Convection: when the newborn is exposed to cool surrounding air or to a draft from open doors, windows or fans, the transfer of heat from the newborn to air or liquidis affected by the newborn’s large surface area, air flow (drafts, ventilation systems, etc), and temperature gradient.
• #13: Heat Production 4.1 During pregnancy maternal mechanisms maintain the intrauterine temperature. After birth the newborn must adapt to their environment by the metabolic production of heat. 4.2 Primary source of heat in the newborn is non-shivering thermogenesis which involves the utilisation of brown adipose tissue. Metabolic processes The brain, heart, and liver produce the most metabolic energy by oxidative metabolism of glucose, fat and protein. The amount of heat produced varies with activity, state, health status, environmental temperature. Voluntary muscle activity · Increased muscle activity during restlessness and cryinggenerate heat. · Conservation of heat by assuming a flexed position to decrease exposed surface area Voluntary Muscle Activity The mechanical efficiency of muscle contractions—the ratio between power output and the sum of the caloric equivalent of oxygen consumption and power output—can initially reach up to 50% for short exercises (seconds), but quickly decreases to levels in the range of 25 to 35% for sustained, steady-state exercise (e.g., cycling).175,176 This forms the basis of heat generation by voluntary muscle activity, because most of the difference between energy input and mechanical energy output is converted into heat. However, during anesthesia, this form of heat generation is, similar to thermoregulatory behavioral changes, either absent or only minimal and does not significantly con - tribute to thermoregulation Peripheral vasoconstriction · In response to cooling, peripheral vasoconstriction reduces blood flow to the skin and therefore decreases loss of heat from skin surfaces. Nonshivering thermogenesis · Heat is produced by metabolism of brown fat. · Thermal receptors transmit impulses to the hypothalamus, which stimulate the sympathetic nervous system and causes norepinephrine release in brown fat (found around the scapulae, kidneys, adrenal glands, head, neck, heart, great vessels, and axillary regions). · Norepinephrine in brown fat activates lipase, which results in lypolysis and fatty acid oxidation. · This chemical process generates heat by releasing the energy produced instead of storing it as Adenosine-5-Triphosphate (ATP). Neonates and small infants are not able to shiver because of combination of =====immaturity of musculoskeletal system, and limited muscle mass or bulk =====there are reports of shivering….but the effectiveness on temperature regulation is negligible
• #14: 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 is 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 Important factors : Integrated CNS pathways Adequate glucose Oxygen Adequate Brown
• #16: 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, Clinically significant nonshivering thermogenesis is possible within minutes after birth and may persist up to the age of 2 years. Hypoxia causes impairment of brown fat metabolism
• #18: The following characteristics put newborns at a greater risk of heat loss: · A large surface area-to-body mass ratio :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 · Decreased subcutaneous fat · Greater body water content · Immature skin leading to increased evaporative water and heat losses · Poorly developed metabolic mechanism for responding to thermal stress (e.g. no shivering): 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 · Altered skin blood-flow (e.g. peripheral cyanosis) Newborn infants are at risk for heat loss and hypothermia for several reasons. Relative to body weight, the body surface area of a newborn infant is approximately 3 times that of an adult. Generation of body heat depends in large part on body weight, but heat loss depends on surface area. In low birthweight and preterm infants, the insulating layer of subcutaneous fat is thin. The estimated rate of heat loss in a newborn is approximately 4 times that of an adult. Under the usual delivery room conditions (20-25°C [68-77°F]), an infant’s skin temperature falls approximately 0.3°C (0.54°F)/min and deep body temperature decreases approximately 0.1°C (0.18°F)/min during the period immediately after delivery; these rates generally result in a cumulative loss of 2-3°C (3.6-5.4°F) in deep body temperature (corresponding to a heat loss of approximately 200 kcal/kg). The heat loss occurs by 4 mechanisms: (1) convection of heat energy to the cooler surrounding air, (2) conduction of heat to the colder materials touching the infant, (3) heat radiation from the infant to other nearby cooler objects, and (4) evaporation from skin and lungs. Factors affecting heat loss Infant Large surface area relative to body mass Relatively large head with highly vascular fontanelle Skin maturation/thickness, epidermal barrier functionally mature at 32 to 34 weeks. Transepidermal water loss may be 10 to 15 times greater in preterm infants of 25 weeks' gestation (4). Decreased stores of subcutaneous fat and brown adipose tissue in more premature infants (7) Inability to signal discomfort or trigger heat production (shivering) (7) Environment (3,4) Physical contact with cold or warm objects (conduction) Radiant heat loss or gain from proximity to hot or cold objects (radiation) Wet or exposed body surfaces (evaporation) Air currents in nursery or in incubator fan (convection) Excessive or insufficient coverings or clothing Other factors Metabolic demands of disease: asphyxia, respiratory distress, sepsis (11) Pharmacologic agents, e.g., vasodilating drugs, maternal analgesics, and unwarmed IV infusions, including blood products Medical stability of infant prior to procedure Thermogenic response matures with increase in postconception age (4)
• #19: Premature =====less brown fat and glycogen stores decreased ability to maintain flexion, increased body surface area compared to weight
• #20: Allmost all anesthesia drugs affect thermoregulation mechanism Eg. Narcotics…reduces vasoconstriction effect for heat conservation musle relaxants …reduces musle tone and shivering regional anesthesia…causes sympathetic blockage, musle relaxant, sensory blockage Anesthesia-induced inhibition of central thermoregulation.338 2. Internal redistribution of heat from the central to the periph - eral compartment.329 3. Linear reduction in metabolic heat production as a function of mean and core body temperatures (4–8%/°C).339 4. Increased exposure to the environment (up to 90% of heat loss occurs via skin mainly by radiation and convection). Altered responses to heat loss due to anaesthesia (e.g. lack of shivering) • Increased heat loss-environment exposure • Cooling effect of cold anaesthetic gases and intravenous fluids • Reduced heat production due to reduced metabolic activity
• #21: PHASE 1 / REDISTRIBUTION This is due to vasodilatation causing redistribution of heat from core to periphery following induction of anaesthesia. Body heat content initially remains unchanged. PHASE 2 / LINEAR PHASE There is a more gradual reduction in core temperature of a further 1-2oC over the next 2-3hrs. This usually begins at the start of surgery as the patient is exposed to the cold cleaning fluids, and exposure to the cold theatre environment. Heat loss exceeds heat production. The various modes of heat loss are: Phase 1/Redistribution Phase 2/Linear Phase 3/ Plateau • Radiation: contributes to most of heat loss - approximately 40% and is proportional to the environment/core temperature difference (to the power of four). • Convection up to 30% and is due to loss of heat to air immediately surrounding the body. It is proportional to the velocity of the air. • Conduction: up to 5% and is due to heating surfaces in contact with the body such as theatre table or cold fluids. • Evaporation contributes to 8-15% and occurs from cleaning fluids, skin, respiratory, bowel and wound surfaces. • Respiratory 8-10% enhanced by cooling effect of cold anesthetic gases. PHASE 3 / PLATEAU Once core temperature falls below the thermoregulatory threshold, peripheral vasoconstriction increases and acts to limit the heat loss from the core. When core heat production equals heat loss to the periphery, core temperature reaches a plateau. This may not be achieved in diabetics with autonomic neuropathy and impaired vasoconstriction and also during combined general and regional anesthesia.
• #24: Hypoxemia from increased Oxygen consumption Hypoglycemia from increased glucose metabolism Respiratory & metabolic acidosis secondary to anaerobic metabolism Inhibition of surfactant production related to increased acidosis  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
• #25: If no equipment is available or if the newborn is clinically stable, skin-to-skin contact with the mother can be used in a warm room (at least 25°C) Severe hypothermia (body temperature below 32°C) · Using a warm incubator (should be set at 1 to 1.5°C higher than the body temperature) and should be adjusted as the newborn’s temperature increases · If no equipment is available, skin-to-skin contact or a warm room or cot can be used
• #31: PRE-OPERATIVE Keep the patient comfortably warm (36.5-37.5 0C) by providing sheets/warm clothes and by maintaining higher ambient temperature. If temperature is below 36oC commence forced air warming unless immediate surgery is imperative. A forced air warming system is a medical electrical devices used to help keep pts warm during anesthesia and surgery .it comprises a reusable controller and disposable single use blankets Controller-----components are electronic motor and fan, electronic heating element, thermostats, air filter, hose ===the fans draws in air through the filter , and heating elements heat it to a selected temperature controlled by thermostat ===heat air travels through the hose to the blanket which connects to the hose nozzle Blanket===double layer and inflates in operaton ====the patient contact surface is permeable to air and the warm air exits the blanket and moves over the patients skin and transfer heat to the patients by convection INTRA-OPERATIVE MEASURES Maintain ambient temperature above 210C. Cover the patient adequately with either sheets or cotton roll or any other passive insulating material. This traps air under the insulation material and may prevent heat loss by up to 30%. Active warming using forced air warmers such as the “Bair hugger” illustrated below are devices that blow hot air into a blanket on top of the patient. It is more efficient than passive warming and prevents heat loss both by convection and radiation. These warmers must be used with the correct blankets to prevent thermal injury. Figure 2. Forced air warming blanket Figure 3. Intravenous fluid warmer Use warmed irrigation fluids. Connect a blood and fluid warmer if large amounts of fluid and blood product use are anticipated. Warm and humidify Inspired gases may be warmed by using a heat and moisture exchange device. POST OPERATIVE: Continue to monitor temperature and use appropriate measures to prevent further heat loss and keep the patient comfortably warm. If the patient’s temperature is less than 360C then commence forced air warming until thermally comfortable.
• #32: The main mechanism of heat loss during sugery are radiation and convection upto 90% So they depend on the temperature differences. Warming of the OR reduces the difference and heat loss The ideal ambient temperature should be Term…27 preterm…29 adult…21 But these temperature range makes uncomfortable for the staff members to have discomfort….practically better to be 20-24 There is a rule of thumb…increasing of OR temperature by 1ºc reduces heat loss by 10% convection heat loss…. Cover the patient with passive insulation evaporation heat loss…..from respiratory tract, surgical field preparation, skin incision, open wounds, bowels….so,, relative OR humidity should be kept in the range of 40-60% HME….heat and moisture exchange device
• #33: PATIENT TRANSPORT: Although we have focused on the intra - operative care of patients, measures to prevent hypothermia are equally important during patient transport. Patients should be appropriately covered during any transport. For older children and adults, a warm blanket may be sufficient; however, neonates and particularly premature babies should, whenever possible, be transported in a prewarmed incubator. If an incubator is not available, chemical heating pads can be used to maintain normothermia or even rewarm a small patient. Even a short inhospital transport may be all it takes to ruin all of the successful intraoperative efforts to maintain normothermia.
• #34: Due to continued peripheral vasoconstriction, they have low efficiency and take long time to warm patient. Intraoperative warming is therefore ideal. Active warming is better compared to passive warming alone in the postoperative phase also. Active warming helps regain temperature an hour faster. Evidence suggests that active warming with convection is slightly superior to conductive and radiant warming in the postoperative period too.
• #35: Pulmonary artery catheter monitoring ====is the gold standard for assessing centeral body temperature =====providing the reference tempraue against which all other other sites are compared =====limited to use,,because of invasive ness,,,except criticlal ill child Distal esophagus =====in patient with ETT , MORE RELIABle than rectal, more practical than tympanic Tympanic ……..most ideal temp monitoring site to determine cor temp ======disadv….difficult obtaining apprpraite size thermistors ===========risk of tympanic perforation Nasopharynx =====accurately reflect the core the temperature if the probe is positioned in correct position. ----------tip should be the posterior nasopharynx close to the soft pplatee ========d/a…air leak from the ETT falsly low t, risk of bleeding