Thermoregulation

Question 1

What is the definition of hypothermia?

Answer 1

Perioperative hypothermia is core temp less than 36*C.

Question 2

Temperature conversion

Answer 2

F* = (C* x1.8) + 32 / C* = (F-32) / 1.8 / K (kelvin) = C + 273

Question 3

Measure #193

Answer 3

numerator = noromothermia or active body warming denominator = surgery > 60 min with gen/neuraxial anesthesia

Question 4

Location of thermoreceptors (afferent input)

Answer 4

central = hypothalamus (po/ah), spinal cord, abdo viscera, great veins / peripheral = skin (a-delta & c fibers)

Question 5

Thermoreceptor response by type

Answer 5

central –> resp to heat –> causes sweating & vasodilation / peripheral –> resp to cold –> causes vasoconstriction, shivering, and non-shiver thermogenesis

Question 6

Central Processor Locations and responses

Answer 6

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

Question 7

Interthreshold Range

Answer 7

Each response has it’s own threshold withing the central processor’s theshold range. Creates and ordered progression of maintenance responses temp changes.

Question 8

Order of Interthreshold range responses

Answer 8

Cold response order = vasoconstriction -> NST -> shivering / Heat response order = vasodilation -> sweating

Question 9

List the Efferent Responses

Answer 9

Behavoir changes / vasodilation / sweating / vasoconstriction / NST(brown fat) / Shivering

Question 10

Mechanism for vasodilation

Answer 10

Mediated by release of nitric oxide from sweat glands. Increases capillary blood flow. Part of Heat Response

Question 11

Mechanism for sweating

Answer 11

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.

Question 12

Mechanism for vasoconstricion

Answer 12

Mediated by release of NE from presynaptic adrenergic fibers. Part of Cold Response -> decr heat loss via convection/radiation

Question 13

Mechanism for NST

Answer 13

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.

Question 14

Mechanism for Shivering

Answer 14

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.

Question 15

Shivering treatment

Answer 15

Demeral 12.5-25mg. Mu-opiod and central alpha2 agonism decreases shivering threshold. Doesn’t treat actual hypothermia.

Question 16

Core body temp vs peripheral body temp

Answer 16

Core = 37C, maintained by autonomic reg, w/ circadian flux of 0.5-1. Peripheral 31-35* C, varies with behavoir, environment, time, vasoconstriction.

Question 17

Effects of General Anesthesia on Thermoregulation

Answer 17

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) .

Question 18

Heat Loss during General Anesthesia Phase 1

Answer 18

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.

Question 19

Heat Loss during General Anesthesia Phase 2

Answer 19

Phase 2 - linear decrease of continuous heat loss, gradually over next 2-3 hrs. Heat loss exceeds metabolism. Lost via Radiation > Convection > Evaporation > Conduction

Question 20

4 Mechanisms of heat loss in Phase 2

Answer 20

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)

Question 21

Heat Loss during General Anesthesia Phase 3

Answer 21

Phase 3 - equilibrium reached, heat loss = metab heat production

Question 22

Regional Anesthesia and Thermoregulation

Answer 22

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)

Question 23

Positive Effects of Hypothermia

Answer 23

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.

Question 24

Side Effects by Organ/system

Answer 24

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.

Question 25

Patient’s at Risk for Hypothermia

Answer 25

Age extremes (large SA:weight ratio) / Decr subq fat and muscle mass / decr ability of vasoconstriction

Question 26

Means of Preventing Periop Hypothermia

Answer 26

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.

Question 27

Causes of Periop Hyperthermia

Answer 27

Incr basal metab rate and co2 production / hypothalamic temp reg dysfxn / altered central/peripheral perfusion / Immune/inflamm processes

Question 28

Hyperthermia Causes: Incr Metab Rate

Answer 28

malignant hyperthermia / neuroleptic malignant syndrome / thyrotoxicosis / pheochromocytoma / carcinoid syndrome / extreme heat exposure (long case)

Question 29

Hyperthermia Causes: Altered Hypothalamic Fxn

Answer 29

TBI / intracranial neoplasm / neurovasc pathology / age extremes

Question 30

Hyperthermia Causes: Immune/Inflamm

Answer 30

Infection / early sepsis / encephalitis / mengingitis / transfusion reaction

Question 31

Hyperthermia Causes: Pharmacolgic

Answer 31

Cholinergic crisis / psychotropics / acute Rx intoxication (cocaine, amphetamines)

Question 32

Temp monitoring Standards of Care

Answer 32

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

Question 33

Temperature monitoring modalities: core

Answer 33

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.

Question 34

Temperature monitoring modalities: peripheral

Answer 34

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.