Measurement of exhaled breath temperature in science and clinical practice

Evaluation of the exhaled breath temperature (EBT) has been suggested as a new method to detect and monitor pathological processes in the respiratory system. The putative mechanism of this approach is based upon changes in the blood flow in the conducting airways that are characteristic of different disease states, which influence the temperature of the exhaled gases. The first attempts to prove this concept were made in conjunction with measurement of exhaled nitric oxide fraction (FeNO) about a decade ago. They made use of an open-circuit, single-breath method in a closed indoor environment, and demonstrated associations between EBT on the one hand, and bronchial blood flow, FeNO and sputum cellular content on the other. These findings have been further extended to practical applicability with the introduction of multiple-breath, portable hand-held devices. Measurement of EBT using these is less dependent on the ambient environment and easier to perform. Studies have been conducted to explore the relationship between EBT and different physiological characteristics, and to assess possible confounding influences in the process of measurement. The value of the method has been explored in the diagnosis and monitoring of asthma, chronic obstructive pulmonary disease and other respiratory diseases. While initially, emphasis focused on airway inflammation as the primary process to be captured by this method, it was subsequently realised that destructive and other remodelling processes reducing the integral contact surface of the conducting airways and/or damaging their vasculature would also affect the end result by shifting EBT downwards. As obstructive airway disease is, in most cases, a combination of airway inflammation and remodelling/destruction, EBT should be viewed as an individual characteristic indicating fluctuating changes in the balance of these processes. Hence, an important potential application of the method is in monitoring airway diseases. EBT measurement has emerged as an attractive noninvasive new approach in respiratory and, possibly, other diseases. Systematic research will determine its place in clinical practice.