Downregulation of Cough by Exercise and Voluntary Hyperpnea

Methods for assessing cough sensitivity

Cough sensitivity can be described as the reaction intensity of the cough reflex to different stimuli which activate chemically and mechanically sensitive vagal afferent nerves innervating airways and lungs. Measurement of cough reflex sensitivity plays an important role in revealing the underlying mechanisms of cough and evaluating the effects of pharmacological interventions. Besides, different responses to cough suppression therapies indicate the existence of cough hypersensitivity. In consideration of these factors stated above, cough sensitivity should therefore be assessed with a variety of cough challenge tests. Based on the neuroanatomical characteristics of the cough reflex, chemical challenge tests have been developed to objectively assess cough sensitivity. In cough inhalation challenges, capsaicin and citric acid are commonly used as the tussive agents to induce cough, which are validated for describing a profile of cough sensitivity to chemical irritants. Recently, mechanical methodologies have also been tried to measure the mechanical sensitivity of the cough reflex. Methodological consideration and selection are necessary for the reasonable assessment of cough sensitivity while employing cough challenges in clinical trials. Thus, in this review, we will focus on describing various methodologies of cough sensitivity measurement and, detailing some factors influencing on the accuracy of outcomes in the experimentally induced cough.

Asthmatic cough and airway oxidative stress

The mechanisms of cough in asthma are unclear. Asthma is associated with an oxidative stress. Many reactive oxygen species sensitize or activate sensory C-fibers which are capable to induce cough. It was hypothesized that oxidative stress in the airways might contribute to the cough severity in asthma. Exhaled breath condensate samples were collected in ten healthy and 26 asthmatic subjects. The concentration of 8-isoprostane was measured. In addition, the subjects filled in Leicester Cough Questionnaire and underwent cough provocation tests with dry air hyperpnoea and hypertonic saline, among other measurements. Among the asthmatic subjects, high 8-isoprostane was associated with severe cough response to hyperpnoea (p=0.001), low Leicester Cough Questionnaire values (indicating severe subjective cough, p=0.02), and usage of combination asthma drugs (p=0.03-0.04). However, the 8-isoprostane concentrations did not differ significantly between the healthy and the asthmatic subjects. Airway oxidative stress may be associated with experienced cough severity and measured cough sensitivity in asthma.