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Trial registered on ANZCTR

Registration number
Ethics application status
Date submitted
Date registered
Date last updated
Date data sharing statement initially provided
Type of registration
Retrospectively registered

Titles & IDs
Public title
Do natural changes in baseline core body temperature affect thermal tolerance, breathing or blood flow to the brain?
Scientific title
Determining whether changes in baseline core body temperature affect adult humans' thermal tolerance, cerebrovascular or respiratory control during exercise in uncompensable heat stress
Secondary ID [1] 304507 0
Nil known
Universal Trial Number (UTN)
Trial acronym
Linked study record

Health condition
Health condition(s) or problem(s) studied:
Heat stress 322432 0
Condition category
Condition code
Physical Medicine / Rehabilitation 320084 320084 0 0
Other physical medicine / rehabilitation

Study type
Description of intervention(s) / exposure
Using a repeated measures cross over design, each participant will cycle (at 25-35% of maximal oxygen consumption) on a semi-recumbent exercise bike while wearing a warm water-perfused suit (48-50 °C) until volitional exhaustion. Sessions will be performed with one participant at a time, and a minimum of two researchers, as elaborated on below. Work rate will be monitored continuously and automatically adjusted by the cycler ergometer's computer software to maintain appropriate work rate (at 25-35% of maximal oxygen consumption). This fixing and standardisation of workload will be used to ensure adherence to the trial demands, i.e., this dictates the rate of application/imposition of heat stress. Adherence then ceases when the fixed workrate cannot be sustained. Participants will be asked to participate on three separate occasions, i.e. once in the morning and twice in the afternoon. Time of sessions will be standardised to onset of instrumentation and data recording within 7-9 am and 2.30-5.00 pm. The washout period between sessions will be a minimum of 5 days. The protocol and measures taken will be identical across all three trials, however, one afternoon condition will involve the addition of a fan, which the participant can use to make themselves more comfortable (i.e., allowing behavioural thermoregulation).

The sessions will be run by at least two researchers (i.e. the lead student researcher and an experienced ultrasound technician for blood flow measures. The technician is also an academic supervisor).
Intervention code [1] 320896 0
Treatment: Other
Comparator / control treatment
Please see above i.e. participants act as own controls and will undergo all trials stated above. There are two questions, and therefore two comparator trials. First, a comparison is being made between morning and afternoon, so the comparator for this question is the afternoon condition. Second, a comparison is being made between available levels of behavioural thermoregulation, i.e., no self regulation vs localised skin cooling to improve comfort. The comparator in this situation is the self regulated, because this has not been permitted in previous research examining heat stress effects on cerebrovascular function.
Control group

Primary outcome [1] 327949 0
Ventilation, as measured by turbine spirometry.
Timepoint [1] 327949 0
Ventilation will be measured at each 0.20-0.35 °C rise in oesophageal temperature. This has no minimum or maximum number of measurements as it is based on core temperature tolerance. Exact timing/spacing of measurements is kept deliberately non-fixed so that participants cannot count the measurement periods and use this information to track of their actual level of heat strain. Final point taken at limit of volitional thermal tolerance.
Primary outcome [2] 327950 0
Thermal tolerance (i.e. the maximum core (oesophageal) temperature tolerated in each condition), assessed using an oesophageal temperature probe inserted through the nasal passage. The temperature transducer probe is a thermistor, logged at 1 Hz to a portable data logger.
Timepoint [2] 327950 0
Thermal tolerance will be determined by recording the core temperature at the start and end points of exercise.
Primary outcome [3] 327951 0
Change (%) in brain blood flow from baseline (initial reading after beginning exercise). Brain blood flow will be determined via duplex ultrasound of unilateral internal carotid (ICA) and vertebral (VA) arteries; these will be summated and then doubled to represent total brain blood flow.
Timepoint [3] 327951 0
Blood flow will only be measured at each ~0.5 °C rise in oesophageal temperature (from baseline temperature: e.g 37.0°C, 37.5°C, 38°C, 38.5°C, 39°C, 39.5°C). This will be assessed from the commencement of exercise until exercise ceases, with the final measurement taken just before exercise ceases.
Secondary outcome [1] 397072 0
Perceptions of whole body thermal discomfort as a function of body core temperature (threshold and slope, equally weighted as a determinant/outcome). Thermal discomfort will be assessed using a 10-point scale as used by Cotter & Taylor, J Physiol (2005). This scale is an extension of that first developed by Gagge et al., 1969 because that saturates at modest displacements in body temperature and would not be valid in this setting. The scale being used has been used extensively in research involving severe heat strain.
Timepoint [1] 397072 0
Thermal discomfort will be measured at each +0.25 °C increase in core temperature and at volitional exhaustion. This will be assessed from the commencement of exercise until exercise ceases.
Secondary outcome [2] 397073 0
Perceptions of whole body temperature will be assessed using a 13-point scale (extended by Cotter & Taylor, 2005) during each trial, with the extension to the original scale being for the same reasons mentioned above for secondary outcome 1. i.e., All participants attain the end-point of the original scale well before reaching thermal tolerance.
Timepoint [2] 397073 0
Thermal perceptions will be measured at each +0.25 °C increase in core temperature and at volitional exhaustion. this will be assessed from the commencement of exercise until exercise ceases.
Secondary outcome [3] 397074 0
Optic nerve sheath diameter (ONSD) will be measured using duplex ultrasound. ultrasound settings such as gain and power will be reduced as low as possible and assessment duration kept under 10 minutes to minimise the risk of heat-related damage to the eye.
Timepoint [3] 397074 0
ONSD will be measured at each 0.5 °C increase in core temperature. This will be assessed between the commencement and cessation of exercise, but not after exercise.
Secondary outcome [4] 398186 0
Partial pressure of end tidal CO2, as an index of arterial CO2 pressure. Measured from end-tidal CO2 pressure, via a sampling line, to online calibrated gas analyser.
Timepoint [4] 398186 0
Measured at intervals of 0.2-0.35°C rise in core temperature, with a non fixed timing for reasons described above (i.e., keep participant blinded to actual thermal strain). This will be assessed between the commencement and cessation of exercise.

Key inclusion criteria
Volunteers may participate if they: Meet ACSM physical activity guidelines (i.e. exercise >3 times per week for 30 minutes or more) and have experience exercising at maximal intensity; have no current or recurring injuries that may affect ability to exercise maximally; are not consuming medications (except contraceptives) where heat exposure or exercise are contraindicated; have a history of heat intolerance or fainting; have any medical conditions that will impair the ability to exercise maximally and/or endure moderate- to severe- heat stress (e.g. core temperature above 40ºC).
Minimum age
18 Years
Maximum age
55 Years
Both males and females
Can healthy volunteers participate?
Key exclusion criteria
Contraindication to exercise (as indicated by PARQ(+)), or exposure to extreme heat stress.

Study design
Purpose of the study
Allocation to intervention
Randomised controlled trial
Procedure for enrolling a subject and allocating the treatment (allocation concealment procedures)
Allocation 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
Masking / blinding
Open (masking not used)
Who is / are masked / blinded?

Intervention assignment
Other design features
Not Applicable
Type of endpoint(s)
Statistical methods / analysis
Sample size is determined by a recently completed, unpublished PhD study, in which 6 each of males and females was sufficient to reveal differences in these measures of thermal tolerance, and with a baseline core temperature difference of the same average magnitude as the diurnal rhythm. The statistical analysis will involve modelling all dependent variables as a function of oesophageal temperature, and inferentially testing the parameters of those modelled data (thresholds, slopes, peaks) between the two times of day, using paired t test. Mixed ANOVA will be used for secondary questions of assessing effect of sex on thermal tolerance. Significance will be set at 0.05 for each analysis, and outcomes will be reported using 95% CI's.

Recruitment status
Date of first participant enrolment
Date of last participant enrolment
Date of last data collection
Sample size
Accrual to date
Recruitment outside Australia
Country [1] 23804 0
New Zealand
State/province [1] 23804 0

Funding & Sponsors
Funding source category [1] 308873 0
Name [1] 308873 0
University of Otago
Address [1] 308873 0
55 Union Street West, North Dunedin, Dunedin 9016
Country [1] 308873 0
New Zealand
Primary sponsor type
Jim Cotter
School of Physical Education, Sport and Exercise Sciences,
University of Otago
PO Box 56
Dunedin 9054
New Zealand
Secondary sponsor category [1] 309792 0
Name [1] 309792 0
Kate Thomas
Address [1] 309792 0
Department of Surgical Sciences Dunedin School of Medicine University of Otago PO Box 56 Dunedin 9054
Country [1] 309792 0
New Zealand
Other collaborator category [1] 281868 0
Name [1] 281868 0
Tiarna Stothers
Address [1] 281868 0
School of Physical Education, Sport and Exercise Sciences,
University of Otago
PO Box 56
Dunedin 9054
Country [1] 281868 0
New Zealand
Other collaborator category [2] 281912 0
Name [2] 281912 0
Mr Travis Gibbons
Address [2] 281912 0
School of Physical Education, Sport and Exercise Sciences,
University of Otago
PO Box 56
Dunedin, 9056
New Zealand
Country [2] 281912 0
New Zealand

Ethics approval
Ethics application status
Ethics committee name [1] 308778 0
University of Otago Human Research Ethics Committee (Health)
Ethics committee address [1] 308778 0
University of Otago PO Box 56 Dunedin 9056
Ethics committee country [1] 308778 0
New Zealand
Date submitted for ethics approval [1] 308778 0
Approval date [1] 308778 0
Ethics approval number [1] 308778 0

Brief summary
Background: People over breathe and reduce their brain blood flow when severely heat stressed. High body core (especially brain) temperature is considered the main determinant of a person's heat tolerance. The purpose of this study is to assess how normal daily changes in core temperature affect heat tolerance, brain blood flow and respiratory control, and whether these responses differ between males and females, or with opportunity to improve their comfort. We hypothesise that heat tolerance will be impaired more in (a) the afternoon, (b) in females, and (c) in the absence of any ability to improve one's comfort.
Methods: Twelve healthy, aerobically fit individuals (six females) will exercise on a semi-recumbent cycle ergometer (30% VO2 max) in a water-perfused suit (~48 °C) until volitional exhaustion on three separate occasions (one in the morning, two in the afternoon).
Trial website
Trial related presentations / publications
Public notes

Principal investigator
Name 111850 0
Prof Jim Cotter
Address 111850 0
School of Physical Education, Sport and Exercise Science University of Otago PO Box 56 Dunedin 9056
Country 111850 0
New Zealand
Phone 111850 0
+64 3 479 9109
Fax 111850 0
+64 3 479 8309
Email 111850 0
Contact person for public queries
Name 111851 0
Prof Jim Cotter
Address 111851 0
School of Physical Education, Sport and Exercise Science University of Otago PO Box 56 Dunedin 9056
Country 111851 0
New Zealand
Phone 111851 0
+64 3 479 9109
Fax 111851 0
+64 3 479 8309
Email 111851 0
Contact person for scientific queries
Name 111852 0
Prof Jim Cotter
Address 111852 0
School of Physical Education, Sport and Exercise Science University of Otago PO Box 56 Dunedin 9056
Country 111852 0
New Zealand
Phone 111852 0
+64 3 479 9109
Fax 111852 0
+64 3 479 8309
Email 111852 0

Data sharing statement
Will individual participant data (IPD) for this trial be available (including data dictionaries)?
What data in particular will be shared?
After de-identification, individual participant data underlying published results only
When will data be available (start and end dates)?
Immediately following publication, ending 10 years following main results publication.
Available to whom?
Only researchers who provide a methodologically sound proposal
Available for what types of analyses?
IPD meta-analyses
How or where can data be obtained?
access subject to approvals by Principal Investigator (
What supporting documents are/will be available?
No other documents available
Summary results
No Results