Temp Regulation TCU Student .pptx
Transcript
Temperat ure Managem ent NRAN 80323 Casey Crow DNP, CRNA Objectives • Laws of Thermodynamics • Temperature vs Heat • Temperature Measuring and Monitoring • Heat Loss Mechanisms • Perioperative Hypothermia • Normothermia Management Laws of Thermodynamics Explain the relationship between HEAT an...
Temperat ure Managem ent NRAN 80323 Casey Crow DNP, CRNA Objectives • Laws of Thermodynamics • Temperature vs Heat • Temperature Measuring and Monitoring • Heat Loss Mechanisms • Perioperative Hypothermia • Normothermia Management Laws of Thermodynamics Explain the relationship between HEAT and ENERGY as well as the exchange during work processes… • Law of conservation of energy: Energy cannot be created or destroyed. The energy within a thermodynamic system is equal to the heat energy added to the system minus the work done by the system of the surroundings. • Not created or destroyed, but transferred Laws of Thermodynamics Explain the relationship between HEAT and ENERGY as well as the exchange during work processes… • Energy moves toward greater entropy or randomness: The entropy of an isolated system not in equilibrium will tend to increase over time, approaching maximum value at equilibrium • Energy/heat moves in a manner to reach entropy/equilibrium • Entropy is the universal trend toward equilibration • Unidirectional, energy from higher concentration to lower concentration • Energy moves because of the gradient Laws of Thermodynamics Explain the relationship between HEAT and ENERGY as well as the exchange during work processes… • Absolute zero is void of all energy: Theoretical state and impossible to achieve. As a system approaches absolute zero, all processes begin to cease, and the entropy of a system approaches minimum value • Theoretical state of 0 Kelvin Temperature vs Heat • Temperature: measurement of the thermal state of an object • Thermal state of a substance which determines whether it will give heat to another substance or receive heat from it • Quantitative measure of thermal energy • Physical manifestation of energy content • Heat is a form of kinetic energy • Transferred from hotter substance to colder substance Temperature vs Heat • Heat energy measured in temperature • SI: Kelvin • Metric: Celsius • English: Fahrenheit • Heat measured in thermal energy • SI: Joule • Metric: Calorie • 1 cal is the amount of energy to increase the temperature of 1g of H20 by 1 degree C • 1 cal = 4.2 J Temperature Scales •K = C + 273 •F = 1.8 (C) + 32 •C = (F – 32) / 1.8 Measuring Temperature When heat energy is added to a substance, not only does its temperature increase but changes in physical properties also occur • Mercury expands and contracts as temperature changes • Color changing crystals by radiant visual light frequencies • Flexible metals that expand and contract • Infrared frequencies of actual heat radiation Monitoring Temperature • Homeothermic: Core body temperature is maintained within a range of 37 degrees C +/- 0.5 degrees • Core: brain, thoracic and abdominal organs and deep tissues of the limbs • Intermediate zone: the area that varies with the environmental temperature • Shell: surface layer that is maintained at a lower and more variable temperature • Can range from 32-25 degrees C Monitoring Temperature • Core body temperature depends on the balance between heat production in the core and heat loss through the shell layer • Difference between core temperature and skin temperature is a useful measure Monitoring Temperature Thermoregulation • Three phase process: Afferent – Central - Efferent Afferent Central Efferent A-δ (cold) and unmyelinated C (hot) nerve fibers Receives information from afferent tracts (spinothalamic tract) to various regions of the brain, including the hypothalamus Periaqueductal gray matter send efferent signal to effector sites causing behavioral and autonomic responses to changes in temperature Hypothalamus is the primary center for thermoregulatory control Behavioral: layering clothes, seeking warmth Peripheral sensors send thermal information via tracts in the anterior spinal cord Autonomic: shivering, vasoconstriction, sweating Thermogenesis • Shivering • Regulated by the shivering motor center adjacent to posterior hypothalamus • Involuntary shivering can produce 5 times as much heat as is produced by the body at rest • Increased metabolism results in increased in oxygen consumption • Non-shivering • Complex mechanism not involving ATP • Triggers brown fat metabolism, located around organs and large vessels • Brown fat is prevalent in neonates, less in adults Thermoregulation • Continually exchanging heat with the environment • High concentration to lower concentration (Law #2) • Hot day, heat moves from environment to body- Hyperthermia • Cold day, heat moves from body to environment- Hypothermia • Measures to prevent heat loss • Vasoconstriction of peripheral vessels • Measures to promote heat loss • Vasodilation of peripheral vessels • Directing blood to or away from the periphery aids in conservation or removal of body’s heat energy • Disrupted under anesthesia • Vasodilating drugs • Volatile anesthetics • Regional anesthetics Core Temperature Redistribution • Occurs during general and regional anesthesia • GA: IV and volatile anesthetics are potent vasodilators, disrupt hypothalamic regulation • RA: Loss of sympathetic tone to vascular smooth muscle resulting in vasodilation at/below level of the block • Process of increased heat loss from the body • Results from vasodilating effects which causes greater blood flow and heat flow from the core to the periphery Greatest drop in core temperature occurs during the first hour after anesthetic administration Core Temperature Redistribution Heat Loss 40 % 10 % 30 % 20 % Heat Loss • Radiation • Infrared electromagnetic wavelength transfers heat energy from warm body to less warm OR • Infrared radiation is greatest from areas of highest blood flow • Blood carries body heat • Head loses the greatest amount of heat due to high percentage of blood flow Heat Loss • Convection • Process by which heat creates air currents • Body transfers kinetic energy to air molecules on the surface of the skin • Heated air molecules move with greater kinetic energy, rise, and are replaced by colder (less kinetic energy) molecules • Process continues of transferring more kinetic energy, warming the molecules, rising, replaced with colder molecules Heat Loss • Conduction • Transfer of heat via contact with a less warm object • Higher energy/heat concentration to lower energy/heat concentration • Significant in pediatric patients with a large body surface area compared to overall mass Heat Loss • Evaporation • The energy process to change from liquid to gas • Moisture evaporation from a patient’s skin, mucous membranes, exhaled water vapor • Open abdominal/chest procedures • Prepping with liquids- isopropyl alcohol, povidoneiodine, and chlorhexidine gluconate • Limited by the gradient of the skin/airway H20 to environment H20 • Relative and absolute humidity Humidity • Measured with a hygrometer • Absolute humidity- mass of water vapor present in a given volume of air, expressed as mass units per volume units • The maximum amount of water vapor that can be present in a given volume of air is determined by the temperature • Temperature increases, the amount of water which can be present as vapor also increases Vapor Pressure: pressure of gaseous molecules exert on a liquid gas interface Warmer air holds more water vapor, Cooler air holds less water Humidity • Relative humidity- the ratio of the mass of water vapor in a given volume of air to the mass required to saturate that given volume of air at the same temperature • Percentage • Relative humidity = actual water vapor/maximal potential water saturation • Relative humidity increases as temperature decreases because vapor pressure falls • Fully saturated air at 37 degrees C contains 44 mg H20/L Vapor Pressure: pressure of gaseous molecules exert on a liquid gas interface Warmer air holds more water vapor, Cooler air holds less water Humidity Warmer air has the capacity to hold more water vapor Cooler air has the capacity to hold less water Humidity • ETT and condensation • Condensation is formed in the ETT because water vapor saturates the inspired gas • The temperature of the expired gas decreases as it flows out on exhalation along the ETT • Saturated vapor pressure falls • Water condenses • Temperature falls, air has reduced capacity to hold water vapor, and water condenses Airway Humidification • Normal humidification- air breathed through the nose, inspired air is warmed and saturated with water vapor before entering the trachea • 100% relative humidity and fully saturated by the time reaches carina • Saturated vapor pressure of water in alvoli is 47 mmgHg • Bypassed by ETT or tracheostomy, dry air enters the trachea, lose the humidification process • Dried and tenacious airway secretions, mucus plug, damage to cilia and epithelium of trachea • 2 methods to increase inspired humidity artificially • Humidify the environment • Humidify the inspired gas • Latent Heat: Energy absorbed or released by a substance during a change in its physical state (phase) that occurs without changing its temperature • Latent Heat of Vaporization: Amount of energy required to change the physical state of a liquid to a gas without changing its temperature Latent Heat of Vaporization • Amount of heat energy per unit mass required to convert a liquid into the vapor phase • Heat energy is withdrawn from the environment as liquid vaporizes • Results in heat loss when water evaporates from the body • The energy withdrawn from the environment to convert 1g of water into vapor is 2500J or approximately 600 cal Anesthesia Practice • Why temperature monitoring is • Risk factors for hypothermia • High ASA status important • Alert anesthetist to hyperthermia • Alert anesthetist to hypothermia • Standard of care from the AANA and the ASA • Hypothermia is core temperature of less than 36 degrees C (96.8 degrees F) • Hypothermia- strong evidence for increases in morbidity and mortality • Hypothermia- increased incidence of wound infection • Hypothermia- Longer PACU stays • Lengthy surgeries • Combined neuraxial and general anesthesia • Elderly • Neonates, preterm babies • Lean body mass • Preexisting endocrine disease • Open cases- abdominal, thoracic • Cold fluid infusion • OR less than 21 degrees C Perioperative Hypothermia CNS Decreased cerebral metabolic rate Decreased cerebral blood flow Decreased MAC requirements Delayed emergence EEG/SSEP suppression Respiratory Increased PVR, increased V/Q mismatch Depressed hypoxic drive Increased O2/CO2 solubility in plasma Left shift of oxygen-hgb dissociation curve Cardiovascular Increased SVR and PVR Bradycardia Arrhythmias Increase O2 demand with shivering Hematology Decreased platelet production and activity Decreased clotting activity Renal Potential hypoperfusion Cold diuresis Maintenance of Normothermia Preoperatively • Preoperative Warming • 90% of surgical patients experience some adverse effect of hypothermia • Preop areas cold • Preop gowns thin and drafty • Begin warming efforts preoperatively • Continue warming efforts during operative and postoperative period Maintenance of Normothermia Intraoperatively BAIR Hugger- pulls ambient air into unit, warms and exits through specialized blanket Conductive warming blanketscirculates warm water Maintenance of Normothermia Intraoperatively Reflective hat and garments- lessens heat loss from radiation and convection Heat and Moisture Exchanger (HME)helpful in decreasing heat loss from respiratory tract Maintenance of Normothermia Intraoperatively Intravenous Fluid Warmereffective at slowing heat loss compared to room temperature fluids, especially important when delivering cold fluids such a blood products Low Fresh Gas Flow Rates- effective at slowing heat loss, increases “rebreathing”, speeds and increases HME effeciacy Ambient Temperature Control- simple way to reduce concentration gradient of heat energ especially important in pediatric anesthesia Anesthesia Practice Prevent heat from escaping the system Add heat energy to the system • Minimize exposed patient surface area • Blankets on nonsurgical areas, head covered • Control the OR environment • Ambient temperature management • Warm surfaces • Control the anesthetic system • HME, low fresh gas flows • Warm IV fluids • Active warming when possible • Forced air warming devices • Conductive warming devices References • Nagelhout, Chapter 15, 209-211 • Shubert, Chapter 3, 100-104 • Basic Physics and Measurement in Anesthesia, Davis & Kenny, Chapter 9, 98-105, Chapter 10, 107-113, Chapter 12, 128-136 • Physics for Anesthesiologists, Pisano, Chapter 9, 75-83
