Heat Flashcards
Locations to measure body core temp
oesophageal
rectal (most common)
gastro intenstinal
tympanic - ear
infrared camera
sublingual
When does set point for body core temp change?
exercise
climate
Set point body core temp
37
How to measure skin temp
weighted mean - at min 4 sites
thermocouples (wires)
ibuttons (wireless)
infrared thermography - radiating energy released from body
How to measure sweat rate
body mass change - respiratory loss and metabolic exchange
ventilated capsule - thermoreg control/kinetics
patch collection - dietary Na+ loss
Heat stress indexes
air temp
mean radiant temp
humidity
air velocity
clothing
metabolic rate
Heat balance equation
s = m - w +/e +/c +/r +/k
w.m-2
Environmental parameters
ambient temp
humidity
air velocity
solar radiation
Heat exchange pathways
diffused radiation
direct radiation
radiation
reflected radiation
convection
wind
conduction
evaporation
Personal factors affecting heat balance
age
biological sex
body surface area
aerobic fitness
Task dependant factors affecting heat balance
rate of metabolic heat production
clothing
What happens when body temp increases
afferent to brain
efferent to muscles
blood vessels dilate
sweat glands secrete fluid
= heat lost to environ
body temp return to normal
What happens when body temp decreases
body temp falls
afferent to brain
efferent to muscles
blood vessels constrict
sweat glands do not secrete fluid
= heat conserved
shivering generates heat
= heat retained
What happens when mean body temp increases?
increase in body temp occurs before the effector response is activated at a given onset threshold
effector output increases proportionally to increase body temp
plateau occur when effector outputs reach maximal values
Is skin temp regulated?
no
varies across the body in response to thermal environ
unevenly thermosensitive
Sweating response pattern
steep rise to own plateau
non-uniformity in sweat rates
Heat loss during exercise via
sweat evaporation
only way if environ temp exceed skin temp
Heat source during exercise
active skeletal muscle
increase in hot environ due to impaired heat conduction
start = intramuscular temp increase rapidly
more uniformed as continue
What is the risk of heat-related illness driven by?
heat exposure
from ambient heat stress and internal metabolic energy production
2 types of heat stroke
classic
exertional
Individual susceptability risk factors
age
coexisting conditions
pregnancy
medications or drugs
cognitive impairments
disabilities
social isolation
immibility
Heat exposure risk factors
ambient temperature and humidity
heat amplification
occupation (outdoor or indoor without cooling)
lack of access to cooling at home
indoor heat sources
Sociocultural risk factors
poverty
structural and environmental racism
social cohesion
housing status
literacy
limited worke protections
Exertional heat stroke
CNS disorder
high core temp
tissue and organ damage
endotoxaemia
Signs/symptoms early stages
disorientation
tachycardia
vomiting
seizures
loss of balance
coma
Signs/symptoms late stages
rhabdomyolysis
circulatory failure
multiorgan failure
disseminated intravascular coagulation
What is the second most common non-traumatic death in competitive athletes?
exertional heat stroke
Exercise-induced dehydration
induces intracellular and extracellular fluid loss in proportion to the loss of water and solutes
Hypernatremia
plasma sodium conc 145mmol.L-1 (high)
hyperpnea
restlessness
lethargy
coma
Hyponatremia
plasma sodium conc 135mmol.L-1 or less (low)
due to increase in total body water relative to amount of total body exchangeable sodium
Symptomatic hyponatremia
headache
vomiting
altered metal status - confusion/seizure
cerebral edema = death
What does the capacity of the environ to absorb heat depend on?
water vapour pressure (wet bulb)
radient temp (direct solar)
air movement (relative to skin surface)
resistance to heat transfer (clothing,postur)
ambient temp (dry bulb)
Hot/humid conditions
harder to offload heat
if low evaporative capacity
requires more sweat/skin blood flow
Why quantify heat stress?
assess clothing systems
hydration regimes
safety of exercise in warm or cold environ
Things to consider in selecting thermoregulatory measures:
sensitivity (detect change?)
practicality (cost)
usability (safe)
reliability
validity
Treatment exertional heat stroke
remove from heat
cool (whole body immersion ideal)
hydrate
Heat stress and associated hyperthermia shown to:
reduce time to exhaustion during constant work rate exercise
progressive decrease in work rate during self-paced exercise
reduced max aerobic power (VO2max)
What impaires performance?
high skin temp
high blood flow
What impairs endurance performance in the heat?
cardiovascular strain
HRmax reached at lower work rate + lower SV = decrease max CO
impairs O2 delivery
functional limit of cardiovascular system
Critical core temp
trigger premature fatigue by reducing mental drive (motivation) for motor performance
acting as safety break for catastrophic hyperthermia (cellular and CNS damage)
Perturbations in skeletal muscle function
prolonged exercise in heat
increases muscle glycogen utilization andanaerobic metabolism = greater accumulation of ammonia and muscle lactate
Physiological demands of exercise in heat
need increase muscle blood flow
= need increase skin blood flow (offload heat to prevent hyperthermia)
maintain arterial pressure
adequate blood supply to other tissues CNS
Order of priority for regulation during exercise
- blood pressure
- metabolism
- body temp (risk for heat injury)
Dehydration
decrease plasma volume
increase plasma osmolality
What can a rise in sweat rate lead to?
progressive dehydration
= more rapid increase in core temp
= affect performance
Plasma hyperosmolality
reduces sweat rate
decreases evaporative heat loss
How can single-sprint performance be improved?
passive local muscle heating (warm baths/heated blankets)
active warm-up
passive heating (elevation of core temp by 1)
hot ambient conditions
Heat-related mechanisms contributing to sprint improvements:
faster rate of PCr utilisation
greater ATP turnover and adenine nucleotide degradation
accelerated muscle fibre conduction velocity
Optimise performance weeks/days
training
acclimation
acclimatisation
Optimise performance hours/mins
hydration
pre-cooling
Optimise performance during
clothing
hydration
pacing
cooling
Acclimation
several consecutive days of heat exposure
artificial, periodic exposures
Acclimatisation
requires several consecutive days of heat exposure
natural, periodic exposures
Heat adaptations of core temp
decrease resting body temp = more heat can be stored
decrease exercising cor temp
What does heat acclimation do?
increases onset threshold, sensitivity and max capacity of sweating and skin blood flow responses
Heat acclimation increases total body water
rapid increase in plasma volume expansion
increase secretion or renal sensitivity to fluid regulatory hormones and proteins (aldosterone, vasopressin, albumin)
increasing renal retention of water and electrolytes
changing the oncotic pressure and drwing fluid into the intravascular space
Heat acclimation changes sweat composition
reduces sweat Na+, Cl- and K+ loss
Heat adaptation of cardiovascular responses
increase blood volume
decrease HR at rest? and during exercise
increase SV during exercise
= reduce cardiovascular strain during exercise in heat
Heat shock proteins
heat acclimation increase expression of HSPs
provide cytoprotection protecting and accelerating repair from heat stress
Heat acclimation may alter substrate metabolism via:
glycogen sparing
increasing lactate threshold
reduce lactate conc during exercise
improved economy
Heat acclimation can:
reduce thermal discomfort
reduce rating of perceived exertion
Classic markers of heat acclimation
plasma volume expansion
decrease core temp
decrease heart rate
increase sweat rate
improved thermal comfort
