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Human body can tolerate wide variations in environmental temperature, but only very small variations in internal temperature. Several of the gentlemen present, as During 1979--1999, a total of 8,015 deaths in the United States were associated

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Human body can tolerate wide variations in environmental temperature, but only very small variations in internal temperature. Several of the gentlemen present, as well as myself, went into the room without shirts … when the thermometer had risen much higher, almost to 260 ° F (127 ° C), and found that we could bear that very well, …

C. Blagden; Philos Trans Royal Soc

(Lond) 65:111-123, 1775.

During 1979--1999, a total of 8,015 deaths in the United States were associated with excessive heat exposure.

Source: National Center for Health Statistics

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What are the primary mechanisms for heat dissipation in humans?

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Vasoconstrictor Vasodilator

Nerve activity Skin Blood Flow Hypothermia Normothermia Hyperthermia 5-15% 85-95%

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Ach

Sweat gland

Other influence (i.e. vasoactive peptides, nitric oxide)

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Condition A

Internal Temperature Skin Blood Flow and/or Sweat Rate

Threshold Slope

Condition B

Threshold Slope

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Cutaneous Vasculature Central Nervous System Vasodilation Vasoconstriction Afferent Efferent Internal Temp Skin Temp Sweat Gland Other Variables

* Aging * Pathology * Drugs * Skin Injury

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Rowell Approach

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Skin Temp (Ts) Blood Temp (Tb)

Where is this blood going?

Skin Blood Flow Normothermia: 300-500 ml/min Heat Stress: 6,000+ ml/min

Rowell, Physiol Rev 1974

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 Brain blood flow  heart rate;  or  BP  Splanchnic and renal blood flow and volume  Skin blood flow & volume  Central blood volume  Sympathetic Activity

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Aging and Heat Stress Why are we concerned?

Weather related mortality by age in the US (2006-2010) Center of Disease Control

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Subjects exercised @ 50%VO2max in 36 deg C room

20-25 years 60-74 years Kenney et al. 1997

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Inoue et al. Eur J Appl Physiol 1998

What is the problem reporting data as such?

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Think about possible mechanisms responsible for the attenuated skin blood and sweating responses in elderly compared to young subjects – how would you design studies to identify these mechanisms?

Peripheral versus Central

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Kenney & Fowler J Appl Physiol 1988

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Minson et al. J Appl Physiol 1998

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Insufficient increases in cardiac output and thus attenuated increases in skin blood flow …. or Restricted skin blood flow responses resulting from less vasodilation and thus less of a need to increase cardiac output How would you address this question?

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Gagnon et al. 2017

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Gagnon et al. 2017

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Heat stress, independent of a hemorrhage insult

 Brain blood flow  heart rate;,  BP  Splanchnic and renal blood flow and volume  Skin blood flow & volume  Central blood volume  Sympathetic Activity

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Schlader, Wilson, Crandall 2016

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Normothermia Heat Stress Pre-LBNP 15 mmHg LBNP 30 mmHg LBNP

Crandall, Wilson, Marving, Bundgaard-Nielsen, Seifert, Klausen, Andersen, Secher, Hesse J Physiol (2012)

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Discus possible mechanisms by which heat stress compromises tolerance to a hemorrhagic challenge – how would you design studies to identify these mechanisms?

Autonomic Function Baroreceptors Cardiac Function Central Blood Volume Cerebral Vasculature Hydration

Blood Pressure = Cardiac Output x Systemic Vascular Resistance

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“Central” Blood Volume Normothermia

Skin blood volume

“Central” Blood Volume Heat Stress

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LBNP (mm Hg)

10 20 30 40 50 60 70 80 90 100

Survival Probability

0.0 0.2 0.4 0.6 0.8 1.0

Normothermia Heat stress

N=7

Keller, Low, Wingo, Brothers, Hasting, Davis, Crandall J Physiol (2009)

What if central blood volume is “normalized” relative to pre-heat stress levels

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“Central” Blood Volume Normothermia

Skin blood volume

“Central” Blood Volume Heat Stress

Skin blood volume

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LBNP (mm Hg)

10 20 30 40 50 60 70 80 90 100

Survival Probability

0.0 0.2 0.4 0.6 0.8 1.0

Normothermia Heat stress

N=7

Heat stress + volume

Keller, Low, Wingo, Brothers, Hasting, Davis, Crandall J Physiol (2009)

What if central blood volume is “normalized” relative to pre-heat stress levels

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“Central” Blood Volume Normothermia

Skin blood volume

“Central” Blood Volume Heat Stress

Skin

Current Protocol: Army is heating hemorrhaging victims

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Young Elderly

Minson et al. J Appl Physiol 1998

Increases in Core Temperature Increases in Core Temperature

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Rowell, Human Circulation, 1986

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Discus possible mechanisms responsible for the plateau in SKBF during exercise (in contrast to passive heat stress) – how would you design studies to identify if it is ”lack of further vasodilation” or withdrawal of vasoconstriction – and study (studies) to identify the importance

f cardiac output.

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38 37 100 50 Internal Temperature, °C A B C Skin blood flow, % of maximal REST DYNAMIC EXERCISE

Gonzalez-Alonso et al J Physiol 2008

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Discus mechanisms responsible for the reduction in stroke volume during exercise in the heat – why does it decline during exercise but not rest ? How would you design studies to support/test your hypothesis

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Gonzalez-Alonso et al J Physiol 1998 Gonzalez-Alonso et al J Appl Physiol 1999

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Gonzalez-Alonso et al J Physiol 2008

Dehydration trial