Cough and ventilatory adjustments evoked by aerosolised capsaicin and distilled water (fog) in man

Abstract

Airway receptors mediate cough and ventilatory adjustments. Simultaneous assessment of cough sensory-motor components and changes in breathing pattern may provide insights into the receptor(s) prevailingly stimulated by inhaled irritants. Nineteen subjects inhaled capsaicin and fog up to threshold concentrations for cough. Cough intensity, respiratory sensations and changes in breathing pattern induced by the two irritants were compared. Capsaicin and fog cough threshold values did not correlate. Coughing induced by both agents was preceded by qualitatively similar sensations and by significant increases in minute ventilation and respiratory drive due to selective increases in tidal volume (P < 0.01). Cough intensity was similar with both agents. Cough frequency and the intensity of the urge to cough were higher with capsaicin (P < 0.01). The lack of correlation between fog and capsaicin cough threshold values suggests differences in the neural mechanisms activated. The selective increase in tidal volume suggests prevailing involvement of rapidly adapting receptors. The stronger sensations evoked by capsaicin may contribute to the higher cough frequency observed with this agent.

Voluntary and reflex cough: Similarities and differences

“…The importance of abdominal muscles in the production of the cough expulsive force is well known, and it has been confirmed by many electromyographic (EMG) studies [6][7][8][9][10] . For instance, several human investigations have shown activation of the rectus abdominis, internal oblique, external oblique, and transversus abdominis muscles during voluntary cough [16][17] Furthermore, abdominal muscle activation has been found to be proportional to cough flow rate, and a linear relationship has been demonstrated between increases in EMG activity and voluntary cough flow rates [18,19] In addition to abdominal muscles, EMG bursts over the latissimus dorsi and pectoralis major muscles have been demonstrated during voluntary cough efforts in healthy volunteers and tetraplegic subjects in whom corticospinal abdominal inputs were interrupted [20,21] At variance with the inspiratory and compressive phases, several comparisons of the motor features of the expulsive efforts of cough produced voluntarily and reflexly in the same subjects can be found in the literature.…”

“…For instance, some investigations carried out in normal subjects [6] and in patients with neurological [7,8] or upper airway disorders [9] have compared the intensity of expulsions produced voluntarily or in response to threshold and supra-threshold cough stimuli. A recent electromyographic study [10] is notable for the evaluation of the pattern of activation of the respiratory musculature of reflex and voluntary cough efforts in a group of normal subjects.…”

A framework for understanding shared substrates of airway protection

“…Similarly, cough motor output is modified by type of irritant (i.e., capsaicin, citric acid, fog, brandykinin, etc.) 60,81 , irritant concentration 13,20,25,110 , volume and duration of irritant presentation 105 , nasal afferent stimulation 87,116 , and lung volume at cough initiation 99 . These characteristics then result in modifications to cough inspiratory flow rate 3,4 , number of coughs produced 22,25,60,74,81,105,110 , self-reported urge to cough 21,22 , amplitude and duration of expiratory muscle activation during cough 34,110 , and cough expiratory airflow parameters 43,60,110 .…”

Sensations and regional brain responses evoked by tussive stimulation of the airways

“…Studies of endogenously derived urge-tocough (for example, in airways disease) are limited and have only reported measures of severity rather than investigating mechanistic processes. For this reason we will limit our discussion to experimentally induced urge-to-cough, which typically involves the inhalation of nebulised solutions containing varied doses of tussive substances such as capsaicin (Davenport et al, 2002) and citric acid (Yamanda et al, 2008), or the use of other stimuli such as inhalation of distilled water (fog) (Lavorini et al, 2007) or application of puffs of air (Hegland et al, 2011). The quality of the sensation evoked by tussive stimuli is not well characterised but has been described as itching, scratching or burning, and is usually localised to the pharynx and/or larynx.…”

“…Interestingly, both cough frequency and urge-to-cough ratings failed to show intensity-dependence at two levels of air pressure, but instances of stimuli evoking coughing were associated with a stronger urgeto-cough than stimuli that did not elicit a cough. Also of note is the absence of a correlation between cough thresholds derived with capsaicin inhalation versus fog (distilled water, and a likely stimulus for mechanosensitive cough receptors) (Lavorini et al, 2007), which suggests dissociable processing of C and Adelta fibre inputs, although the implications of this dissociation of motor responses for urge-to-cough is a matter for speculation. Further studies will be required to establish the consistency of these observations and to test for any additional distinctions between mechanically and chemically evoked urge-to-cough.…”