The effects of heat on the heart and lungs in healthy young and older adults

Data collection protocol: Participants will be asked to attend 2 sessions at the University of Sydney and 1 session external to the University. The time involvement will be approximately 5 hours for the university visits and 1.5-2 hours for the external visit. The visits will occur based on participant availability. Intervention adherence and fidelity will be monitored by the research team throughout each study visit. Initial Telephone Pre-Screen: When a person indicates their interest in the study, they will undergo a telephone screening to discuss the study and determine initial eligibility for the study. This phone call will take approximately 30 minutes. Preliminary session (Visit 1): Prior to the preliminary session, potential participants will do a telephone screening which will be checked and signed off by cardiologist Dr Lai Heng Foong to confirm eligibility. During the preliminary session, we will explain all procedures with the participants. In addition, they will be introduced to all of the equipment and measuring devices that we will be using for the familiarisation (visit 1) and experimental (visit 3) sessions. We will give each participant the opportunity to read the Information Sheet and Consent Form documents (which they will have been provided with prior to attending the laboratory). Besides, we will have participants perform routine pulmonary function testing (PFT) on the preliminary visit to obtain baseline characteristics from participants. During this visit, participants will be thoroughly familiarised with the procedure of assessing the psychophysical properties of respiratory muscle force production. These data are integral to the study design insofar as they help establish test-retest reproducibility for the method – information that is presently unknown. This reproducibility examination will require participants to complete 5 repeat blocks of the psychophysical assessment of respiratory muscle force, separated by at least 3-5 minutes. During this time, heart rate and blood pressure will be monitored and recorded. Screening Session (Visit 2): This screening session will take approximately 1-1.5 hours. Participants will be asked to undergo an echocardiograph which involves the participant laying supine while images of the heart are scanned using an ultrasound. These sessions will be carried out at one of the clinics within Sydney Cardiology Group (City, Bella Vista, Chatswood), under the supervision of the study cardiologist, Fiona Foo. If the cardiologist deems no abnormal findings on the echocardiogram, then the participant will be eligible to participate in visit 3. Experimental trial (Visit 3): Upon arrival to the laboratory, participants will be asked to provide a urine sample and weigh themselves nude (in private) and self-insert the rectal thermistor. ECG will be applied to the skin to measure heart rate and a cuff placed on the arm for measures of blood pressure. Temperature sensors will be placed on the skin at four sites across the body for measurement of skin temperature. Finally, participants will don a tubed lined suit covering the whole body except for the head, hands and feet, through which water will flow at different temperatures to control the participant’s core temperature. Participants will be asked to lay quietly for a period of 15 minutes while baseline measurements are recorded and 34°C water is circulated throughout the suit. Following this, 50°C water will be circulated through the suit until rectal temperature increases by 0.5°C, 1.0°C and 1.5°C. At each time point (baseline, +0.5°C, +1.0°C, +1.5°C), a cardiac assessment using an ultrasound will be performed by a trained professional, and participants will complete a short series of breathing tasks to assess their sensation of respiratory muscle effort. Following the last measurement point, cold water (20°C) will circulate throughout the suit until rectal temperature returns to baseline values. An additional cardiac assessment will be conducted when core temperature returns to baseline values. Following this, the participants will be de-instrumented, and a nude weight will be recorded. Once the experimental trial is completed, participants will be weighed once again and then de-instrumented and given the opportunity to shower in the adjacent facility. Prior to the experimental visit, participants will be asked to abstain from alcohol and caffeine, avoid strenuous exercise in the 12 h before each experimental session, and consume a light meal along with 500 ml of water ~2 h before arriving at the laboratory. Both young and older healthy participants will be assessed and these cohorts will be compared. Intervention fidelity will be ensured through a standardised study protocol applied to all participants. Water temperature, core temperature and duration of each intervention stage will be monitored by the research team. Predefined core temperature targets (+0.5°C, +1.0°C and +1.5°C) will be used to standardise the heating exposure. Any protocol deviations or premature termination of the intervention will be documented.