Investigating heat tolerance in females

The exposure variable in this study is exertional heat stroke (EHS) which is characterized by an elevated internal body temperature above 40°C accompanied by dysfunction or disturbance of the central nervous system. As part of the return to duty process, the Australian Defence Force recommends that all Australian Defence Force members who have experienced exertional heat stroke are sent for a heat tolerance test. The heat tolerance test is a standard procedure that is performed for all cases of heat stroke to determine their heat tolerance status. Study participants will be grouped into cases and an unexposed (control) group. The exposed group (cases) will be female Defence Force members who have been referred to the chief investigator for heat tolerance testing as part of the routine procedure following a heat stroke event. The unexposed group will be female participants recruited from the general community who have no history of heat illness. Using the standard heat tolerance test the heat tolerance responses of both the cases and controls will be observed to identify their heat tolerance status (heat tolerant or intolerant). The standard heat tolerance test comprises treadmill walking for 2 hours at 5 km/h and a 2% grade in a climate control chamber set to hot/dry conditions (40°C, 40% relative humidity). The key outcome variables that will be assessed during the 2 hour test will include core temperature, heart rate, skin temperature, sweat rate, and subjective ratings of thermal sensation and thermal comfort. The HTT involves a total of 3.5-4 hours observation. All participants will also undertake a VO2max test to assess aerobic fitness. Assessment of aerobic fitness will enable us to establish whether the control group is a suitable comparator for the cases as aerobic fitness is known to affect heat tolerance. VO2 will be measured using a metabolic cart and either treadmill or cycle ergometer. The participants will commence the max test at a low intensity to warm up. The exercise intensity will then be progressively increased until the participants reach a point of voluntary exhaustion. This test usually involves 8-15 minutes of exercise and a total of 30-45 mins of total time. The VO2max test can occur within a 4 week period of the heat tolerance test, either beforehand or afterwards. Alternative heat tolerance test protocol The current HTT will be considered inappropriate for females if a large proportion of the controls are categorized as heat intolerant (false positive). If the protocol is appropriate, most if not all the control participants should pass the HTT. However, previous studies have shown that a higher proportion of women were classified as heat intolerant using the HTT compared to men. If the protocol is not valid for females, then the participants will be asked to attend the laboratory on a second occasion to repeat the test with a lower workload. A modified protocol will then be developed.