Exhaled heat flux and expiratory thermal dynamics in patients with asthma: An exploratory cross-sectional study

Background: Asthma is characterized by chronic airway inflammation, which has traditionally been evaluated primarily for functional impairment. In contrast, relatively little attention has been paid to local heat generation within the airways. Objective: This study aimed to explore expiratory thermal dynamics in patients with asthma by assessing exhaled air temperature and expiratory heat flux (the rate of heat transfer per unit area). Methods: We developed a novel device incorporating an exhaled air temperature sensor and a heat flux meter, enabling real-time graphical visualization of expiratory thermal dynamics. Expiratory heat flux and the rate of increase in exhaled air temperature per unit volume of vital capacity were measured in a single-session assessment in healthy control subjects (n = 30) and patients with asthma (n = 27) prospectively enrolled in this exploratory cross-sectional study. Results: Preliminary experiments demonstrated that expiratory heat flux was less influenced by oral temperature and smoking status than exhaled air temperature alone. Although the maximum exhaled air temperature did not differ significantly between patients with asthma and controls, both expiratory heat flux and the rate of temperature increase per unit volume of vital capacity were significantly higher in patients with asthma than in healthy subjects. Conclusion: These findings indicate that expiratory heat flux, together with dynamic changes in exhaled air temperature, reflects altered expiratory thermal dynamics in asthma. Noninvasive assessment of these thermal parameters may complement existing physiological measures and provide novel pathophysiological insights into allergic airway diseases.