Thermoregulation Flashcards
What is the definition of hypothermia?
Perioperative hypothermia is core temp less than 36*C.
Temperature conversion
F* = (C* x1.8) + 32 / C* = (F-32) / 1.8 / K (kelvin) = C + 273
Measure #193
numerator = noromothermia or active body warming denominator = surgery > 60 min with gen/neuraxial anesthesia
Location of thermoreceptors (afferent input)
central = hypothalamus (po/ah), spinal cord, abdo viscera, great veins / peripheral = skin (a-delta & c fibers)
Thermoreceptor response by type
central –> resp to heat –> causes sweating & vasodilation / peripheral –> resp to cold –> causes vasoconstriction, shivering, and non-shiver thermogenesis
Central Processor Locations and responses
Spinal cord and hypothalamus (majority hypothal). Hypothalamus sets body’s thermostat, integrating signals from central/peripheral thermoreceptors. desires euthermia of 37C, w/ threshhold range of +/- 0.2 C
Interthreshold Range
Each response has it’s own threshold withing the central processor’s theshold range. Creates and ordered progression of maintenance responses temp changes.
Order of Interthreshold range responses
Cold response order = vasoconstriction -> NST -> shivering / Heat response order = vasodilation -> sweating
List the Efferent Responses
Behavoir changes / vasodilation / sweating / vasoconstriction / NST(brown fat) / Shivering
Mechanism for vasodilation
Mediated by release of nitric oxide from sweat glands. Increases capillary blood flow. Part of Heat Response
Mechanism for sweating
Mediated by cholinergic/muscarinic post-ganglionic SYMPATHETIC fibers. Only mechanism where we can lose heat to environemnt exceeding core body temp. Part of Heat Response.
Mechanism for vasoconstricion
Mediated by release of NE from presynaptic adrenergic fibers. Part of Cold Response -> decr heat loss via convection/radiation
Mechanism for NST
Mediated by thyroid gland -> incr SNS activity to brown fat. Increases metabolic heat production via brown fat oxidation. Increase oxygen consumption. Part of Cold Response.
Mechanism for Shivering
Stimulated by peripheral thermoreceptors, inh by central thermoreceptors. Motor center is post hypothalamus. Increased metab -> increase heat. Increase o2 use by 200% & exac post op pain.
Shivering treatment
Demeral 12.5-25mg. Mu-opiod and central alpha2 agonism decreases shivering threshold. Doesn’t treat actual hypothermia.
Core body temp vs peripheral body temp
Core = 37C, maintained by autonomic reg, w/ circadian flux of 0.5-1. Peripheral 31-35* C, varies with behavoir, environment, time, vasoconstriction.
Effects of General Anesthesia on Thermoregulation
Depresses hypothalmic fxn, tenfold increase in interthreshold range +/- 2C, now 35-39. Depr/Inh behavioral and autonomic response. Body temp changes passivley r/t heat distrubtion (poikilothermic) .
Heat Loss during General Anesthesia Phase 1
Phase 1 - redistribution phase - d/t vasodilation causing redistribution of core warm blood to cool peripheral blood. Core @ 37C, peripheral 31-35C. Contributed to 80% of hypothermia in first hour of case, decr 1-1.6*C.
Heat Loss during General Anesthesia Phase 2
Phase 2 - linear decrease of continuous heat loss, gradually over next 2-3 hrs. Heat loss exceeds metabolism. Lost via Radiation > Convection > Evaporation > Conduction
4 Mechanisms of heat loss in Phase 2
Radiation 40%-loss of heat from warm surface (body) to cooler one environment (naked pt) / Convection 30%-temp gradient between body and ambient air (cold/dry gas inh) / Evaporation 20%- transfer of heat during liq change to gas (skin prep) / Conduction 10%- loss of heat from warm surface to cooler thru contact (IV or irrig fluids)
Heat Loss during General Anesthesia Phase 3
Phase 3 - equilibrium reached, heat loss = metab heat production
Regional Anesthesia and Thermoregulation
has 3-4x increase in interthreshold range +/- 0.6-0.8*C. Depr/Inh behavioral and autonomic responses. PN blocks prevent regional autonomic responses. Neuroaxial blocks prevent autonomic resp to more than 1/2 body, dependent on level of block. (block A delta and C fibers, peripheral reg in addition to SNS fibers)
Positive Effects of Hypothermia
Decr 1C = decr cerebral metab/oxygen demand by 6-7%. At 25C bodily oxygen requirements decr 60% (CPB). Only documented benefits of therapeutic mild (33-34*C) hypothermia are outside hospital cardiac arrest and aspyxy newborns. Has protective decr oxygen requirements, protective during cardiac and cerebral ischemia.
Side Effects by Organ/system
Cardiac - SNS activation –> tachy, HTN, MI
Resp - Incr PVR, V/Q mismatching, left o2 curve shift. Hepato/Renal - decr metabolism (DME) –> incr Rx effects as enzymes prefer normothermia. Hematologic - decr platelet fxn, decr coag cascade fxn (d/t cold enzymes) –> incr blood loss.
Immune - altered neutrophil fxn, poor peripheral perfusion/o2 delivery, surgical site infection.
Patient’s at Risk for Hypothermia
Age extremes (large SA:weight ratio) / Decr subq fat and muscle mass / decr ability of vasoconstriction
Means of Preventing Periop Hypothermia
Passive - warm blankets, drapes, reflective blanket
Active - forced air warmers (most effective), warm water circulators, fluid warmes, HME
Pre-warm with forced air warmer 30min prior to induction reduces/prevents phase 1 redistribution hypothermia.
Causes of Periop Hyperthermia
Incr basal metab rate and co2 production / hypothalamic temp reg dysfxn / altered central/peripheral perfusion / Immune/inflamm processes
Hyperthermia Causes: Incr Metab Rate
malignant hyperthermia / neuroleptic malignant syndrome / thyrotoxicosis / pheochromocytoma / carcinoid syndrome / extreme heat exposure (long case)
Hyperthermia Causes: Altered Hypothalamic Fxn
TBI / intracranial neoplasm / neurovasc pathology / age extremes
Hyperthermia Causes: Immune/Inflamm
Infection / early sepsis / encephalitis / mengingitis / transfusion reaction
Hyperthermia Causes: Pharmacolgic
Cholinergic crisis / psychotropics / acute Rx intoxication (cocaine, amphetamines)
Temp monitoring Standards of Care
General anesthesia 30+ minutes / surgical procedure 60+ minutes / regional anesthesia at high risk of hypothermia / all pediatric cases / at intervals of 15 min or less
Temperature monitoring modalities: core
Core - PA, esophageal, nasopharynx, tympanic membrane & bladder. PA gold standard but invasive. Esophageal easy/reliable for ETT patients, lower 1/3, away from trachea or stomach/liver for accuracy. Nasophy must be in contact with posterior wall (by hypothal). Don’t use ear. Bladder temp influenced by high/low urine flow. High is more accurate to core temp.
Temperature monitoring modalities: peripheral
Peripheral: axillary, rectum, skin. Axilla takes 15 min for calibration to core,variable at best. Avoid rectum. (8cm adults/3cm peds). Forehead for skin, 1-2*C off core temp.
