Optimising passive heating protocols: balancing health outcomes and thermal perception

Introduction: Passive heat therapy, achieved by regularly exposing the body to high temperatures, is linked to reduced cardiovascular disease (CVD) risk through mechanisms including enhanced endothelial function, nitric oxide-dependent vasodilation, and reductions in blood pressure, arterial stiffness, and inflammation. While intense heat protocols are often employed to elicit acute responses, thermal discomfort remains a barrier to adherence. Furthermore, limited research has addressed sex-specific differences in passive heating responses, particularly the influence of the menstrual cycle. This thesis investigates the inflammatory, vascular, and perceptual responses to different heat exposure protocols (dry heat (DH), hot water immersion (HWI), dry heat with facial cooling (FAN), and intermittent dry heat (IH)), with a specific focus on sex differences and menstrual cycle influences.
Methods: In Chapter 2, thermal perception and Interleukin (IL)-6 responses to HWI and DH were compared to evaluate differences in comfort and inflammatory outcomes. Chapter 3 examined sex-specific differences in responses to DH and the impact of facial cooling on thermal perception and IL-6 responses. Chapter 4 explored IH versus continuous heating (CH) protocols, assessing their effects on cardiovascular disease risk indicators (skin
perfusion, blood pressure, arterial stiffness and plasma nitrite concentration) and inflammatory markers (IL-6, IL-10 and Interleukin-1 receptor antagonist (IL-1ra)). Finally, Chapter 5 compared vascular (skin perfusion, blood pressure, arterial stiffness and plasma nitrite) and inflammatory (IL-6, IL-10 and IL-1ra) responses to passive heating between the early follicular phase (EFP) and the mid-luteal phase (MLP) of the menstrual cycle.
Results: In Chapter 2, DH resulted in less favourable thermal perception and higher Tskin compared to HWI, despite similar rectal temperatures (Trec ), but both methods induced significant increases in IL-6. Chapter 3 demonstrated that FAN improved thermal comfort with lower forehead and cheek Tskin during DH without suppressing IL-6 responses, while females experienced greater thermal discomfort and higher skin temperatures (Tskin ) than males during DH. Chapter 4 found that IH reduced Trec cardiovascular stress (systolic blood pressure), improved thermal perception and induced similar IL-1ra responses compared to CH, whereas the IL-6 response was inhibited in IH. Additionally, females exhibited higher cutaneous vascular conductance and plasma nitrite concentrations than males in both conditions. Chapter 5 revealed that menstrual cycle phases did not influence IL-6 and IL-1ra responses; however, some vascular outcomes differed, with lower arterial stiffness and mean arterial pressure during MLP compared to EFP and higher skin perfusion observed in EFP.
Conclusion: Heat conditions that lead to greater increases in Trec are associated with an enhanced vascular response, despite less favourable thermal perception. Interventions such as reducing Tskin , facial cooling or intermittent heating improved thermal perception while eliciting significant inflammatory and vascular responses. Moreover, females exhibit greater thermal discomfort, higher Tskin , and increased skin perfusion compared to males, with menstrual cycle fluctuations influencing vascular function. The optimisation of passive heating protocols in the present thesis may improve adherence, making heat therapy a more practical and effective health intervention.