ANZCTR - Registration

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Trial registered on ANZCTR

Registration number

ACTRN12624000258550

Ethics application status

Approved

Date submitted

29/01/2024

Date registered

15/03/2024

Date last updated

9/08/2024

Date data sharing statement initially provided

15/03/2024

Type of registration

Prospectively registered

Titles & IDs

Public title

Exploring how the human body responds to extreme heat as a result of climate change

Scientific title

Evaluating the human physiological heat strain responses to environmental conditions equivalent to 35 C wet-bulb temperature

Secondary ID [1]

Nil known

Universal Trial Number (UTN)

U1111-1302-4047

Trial acronym

35CTW

Linked study record

Health condition

Health condition(s) or problem(s) studied:

Heat-related cardiovascular strain

Heat-related thermal strain

Heat-related changes in thermal perceptions and symptoms of distress

Condition category

Condition code

Cardiovascular

Normal development and function of the cardiovascular system

Public Health

Other public health

Injuries and Accidents

Other injuries and accidents

Neurological

Studies of the normal brain and nervous system

Respiratory

Normal development and function of the respiratory system

Intervention/exposure

Study type

Interventional

Description of intervention(s) / exposure

Participants will undergo 7 consecutive days of exercise heat acclimation in pairs in the Thermal Ergonomics Laboratory's climate chamber (University of Sydney) set to 40 °C, 50% relative humidity (RH). The protocol will involve 90 minutes of treadmill running at an individualized target of ~70-75% of maximal heart rate. These sessions will be supervised by qualified research personnel (i.e., undergraduate or post PhD-level exercise science/physiology trainees), and adherence will be assessed as the number of sessions completed out of 7.

In the same pairs, participants will then undergo 3-hour (maximum) exposures to all 4 different extreme heat conditions in the climate chamber (University of Sydney), with exposures separated by at least 48 hours:
1. Hot, very humid (HVH): 38 °C, 81.1% RH (Twet equal to 35.00 °C)
2. Hot, humid (HH): 46 °C, 45.8% RH (Twet equal to 35.00 °C)
3. Very hot, dry (VHD): 54 °C, 26.2% RH (Twet equal to 34.99 °C)
4. Very hot, very dry (VHVD): 54 °C, 13.1% RH (Twet equal to 28.54 °C)
For all exposures, participants will remain seated in the chamber and will be allowed to drink water ad libitum. Intervention fidelity will be assessed through laboratory observation and monitoring by the researcher.

Intervention code [1]

Lifestyle

Comparator / control treatment

In pairs, participants will undergo a 3-hour (maximum) exposure to hot and very humid (HVH) conditions (38 °C, 81.1% RH (Twet equal to 35.00 °C)) in the Thermal Ergonomics Laboratory's climate chamber (University of Sydney). Intervention fidelity will be assessed through laboratory observation and monitoring by the researcher.

Control group

Active

Outcomes

Primary outcome [1]

Core temperature (Trec)

Timepoint [1]

Every 5 seconds throughout the exposure for each condition

Primary outcome [2]

Heart rate

Timepoint [2]

Every 1 second throughout the exposure for each condition

Primary outcome [3]

Whole-body sweat loss

Timepoint [3]

Body mass measurement pre- and post-exposure, and fluid and urine measurements as needed (when participant drinks water or needs to urinate) throughout the exposure for each condition

Secondary outcome [1]

Mean skin temperature

Timepoint [1]

Every 5 seconds throughout the exposure for each condition

Secondary outcome [2]

Blood pressure

Timepoint [2]

Every 10 minutes throughout the exposure for each condition

Secondary outcome [3]

Blood gases (pH, PO2, and PCO2)

Timepoint [3]

Every 60 minutes throughout the exposure for each condition

Secondary outcome [4]

Signs and symptoms of distress

Timepoint [4]

Every 20 minutes during exposures and exercise heat acclimation, with increasing frequency up to every 3 minutes if/when signs or symptoms begin to manifest

Secondary outcome [5]

Mean skin temperature

Timepoint [5]

Every 60 minutes throughout the exposure for each condition

Eligibility

Key inclusion criteria

18-40 years old
Generally healthy
Able to read and understand English in order to provide informed consent to participate in the study

Minimum age

18 Years

Maximum age

40 Years

Sex

Both males and females

Can healthy volunteers participate?

Yes

Key exclusion criteria

Those with contraindications to rectal temperature measurement (e.g., haemorrhoids, heterotopic ossification, rectal bleeding or bleeding disorder, fissures or active infections, anticoagulant therapy, colitis)
Smokers
Pregnancy
Currently diagnosed or those that possess a history of hypertension, cardiovascular, respiratory and/or metabolic disorders
Medication use related to any of the above conditions

Study design

Purpose of the study

Prevention

Allocation to intervention

Randomised controlled trial

Procedure for enrolling a subject and allocating the treatment (allocation concealment procedures)

Allocation is not concealed

Methods used to generate the sequence in which subjects will be randomised (sequence generation)

Simple randomisation using a randomisation table created by computer software (i.e. computerised sequence generation) for N=12 which will then be mirrored between sexes

Masking / blinding

Open (masking not used)

Who is / are masked / blinded?

Intervention assignment

Crossover

Other design features

Phase

Not Applicable

Type of endpoint/s

Safety

Statistical methods / analysis

Sample Size Justification
Sample size was determined a priori based on expected differences across conditions and the minimally clinically important difference between sexes in the rates of core temperature change in response to the exposures. Using a simulation-based R package, we entered the following information: (1) anticipated rates of change (°C/h) of 0.49 and 0.57 for HVH, 0.67 and 0.75 for HH, and 0.86 and 0.93 for VHD and VHVD in males and females, respectively; (2) a common standard deviation of 0.05 °C/h based on typical measurement error; (3) a level set at 0.05; and (4) N = 12 per group. These parameters yielded >80% power to detect differences between all relevant comparisons.

Statistical Analysis
For each trial, the exposure times to reach a Trec of 39.0 °C will be compared across conditions with a one-way ANOVA. If the Trec target is not attained, the rate of change in Trec in the final hour of the trial will be used to calculate the predicted time to reach 39.0 °C.

Recruitment

Recruitment status

Recruiting

Date of first participant enrolment

Anticipated

1/04/2024

Actual

22/05/2024

Date of last participant enrolment

Anticipated

2/12/2024

Actual

Date of last data collection

Anticipated

20/12/2024

Actual

Sample size

Target

Accrual to date

Final

Recruitment in Australia

Recruitment state(s)

NSW

Funding & Sponsors

Funding source category [1]

Government body

Name [1]

National Health and Medical Research Council

Address [1]

16 Marcus Clarke St, Canberra ACT 2601

Country [1]

Australia

Primary sponsor type

University

Name

The University of Sydney

Address

The University of Sydney, Camperdown NSW 2006

Country

Australia

Secondary sponsor category [1]

None

Name [1]

Address [1]

Country [1]

Ethics approval

Ethics application status

Approved

Ethics committee name [1]

The University of Sydney Human Research Ethics Committee

Ethics committee address [1]

Research Integrity and Ethics Administration; Level 3, F23 Administration Building, The University of Sydney, Camperdown, NSW 2006

Ethics committee country [1]

Australia

Date submitted for ethics approval [1]

23/10/2023

Approval date [1]

20/12/2023

Ethics approval number [1]

2023/884

Summary

Brief summary

Climatologists currently estimate that the environmental threshold for human heat tolerance is 6 hours at a wet-bulb temperature (Twet) of 35 °C. When accounting for human physiological thermoregulatory variables such as metabolic heat production and sweating capacity, however, it is likely that this threshold is severely underestimated. Contrary to the assumptions made by the 35 °C Twet model, in real-world scenarios, people expend anywhere between 1.5 to 5.0 METs to perform activities of daily living, and the ability to cool down through sweating is limited in certain conditions. Specifically, in very humid environments, evaporative capacity is limited by a low vapour pressure gradient between the air and skin; whereas in very dry environments, it is limited by the ability of the body to produce sweat. This is an important distinction because the same Twet can be represented by different combinations of air temperature and relative humidity, which means that the limiting factor to achieving heat balance will be different depending on the environmental conditions being experienced. It is more than likely that the limits to human heat tolerance are not represented by a universal Twet temperature threshold, but rather differ based on the type of heat stress an individual is exposed to (i.e., humid vs. dry). This theory has only ever been modelled and has never been tested in human subjects which is imperative to moving this area of research forward.

Trial website

Trial related presentations / publications

Public notes

Contacts

Principal investigator

Name

Prof Ollie Jay

Address

Thermal Ergonomics Laboratory (Room 901), Susan Wakil Health Building, The University of Sydney, Camperdown, NSW 2006

Country

Australia

Phone

+61 293519328

Fax

ollie.jay@sydney.edu.au

Contact person for public queries

Name

Ollie Jay

Address

Thermal Ergonomics Laboratory (Room 901), Susan Wakil Health Building, The University of Sydney, Camperdown, NSW 2006

Country

Australia

Phone

+61 293519328

Fax

ollie.jay@sydney.edu.au

Contact person for scientific queries

Name

Ollie Jay

Address

Thermal Ergonomics Laboratory (Room 901), Susan Wakil Health Building, The University of Sydney, Camperdown, NSW 2006

Country

Australia

Phone

+61 293519328

Fax

ollie.jay@sydney.edu.au

Data sharing statement

Will individual participant data (IPD) for this trial be available (including data dictionaries)?

Yes

What data in particular will be shared?

De-identified individual participant data will be available in published study results.

When will data be available (start and end dates)?

Data will be available immediately following publication (no end date).

Available to whom?

Data will be available within published journals and case by case at the discretion of the primary sponsor.

Available for what types of analyses?

Meta-analysis

How or where can data be obtained?

Access to data is subject to approval by the Principal Investigator with a requirement to sign a data access agreement. Access can be obtained by emailing Prof Ollie Jay at ollie.jay@sydney.edu.au.

What supporting documents are/will be available?

No Supporting Document Provided

Results publications and other study-related documents

Documents added manually

No documents have been uploaded by study researchers.

Documents added automatically

No additional documents have been identified.

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