Associations of AMP and adenosine induced dyspnea sensation to large and small airways dysfunction in asthma

Airway hyperresponsiveness (AHR) is a distinct asthma characteristic. Bronchial provocation tests can be used to assess AHR, can help to diagnose asthma and monitor asthma control [1]. However, the patient’s dyspnea perception associates poorly with the provocation test [2]. In clinical practice, patients often experience dyspnea before the provocative agent causes the forced expiratory volume in the first second (FEV₁) to drop 20% [3]. On the other hand, others experience no dyspnea even when the FEV₁ has dropped more than 20% [4]. A provocation test is based on the FEV₁, which is believed to be a marker for the larger airway [5]. However, dyspnea sensation is associated with both large- and small airways dysfunction [6‐8]. To evaluate the small airways, for example, the forced expiratory flow between 25 and 75% of the expiration (FEF_25–75) or the difference in resistance between 5 Hz and 20 Hz (R₅-R₂₀) measured with impulse oscillometry (IOS) can be used [5]. Provocation tests with subsequent IOS measurements have suggested that dyspnea induced with a provocative agent corresponds better to small- than to large airways dysfunction [3, 9, 10].

Provocation tests can be performed with either direct or indirect acting agents. Direct stimuli, such as histamine and methacholine, stimulate the airway smooth muscle, resulting in airway contraction [11]. Indirect stimuli, on the other hand, induce the release of mediators from inflammatory cells, such as histamine, leukotrienes, and prostaglandins causing airway contraction [12]. Examples of indirect stimuli are mannitol, nebulized adenosine 5′-monophosphate (AMP), and dry powder adenosine. The well-established AMP is dose restricted (as AMP becomes insoluble above 320–400 mg/mL) [13], whereas mannitol and the newly available dry powder adenosine are not [14]. AMP and dry powder adenosine are well tolerated by patients [15], but mannitol evokes discomforting cough [16, 17]. AMP and dry powder adenosine appear to act via the same indirect pathways, but can consist of differently sized particles. Nebulized AMP commonly has a mass median aerodynamic diameter (MMAD) between 5.1–8.5 μm [18], depending on the nebulizer settings and AMP concentration [18, 19]. Dry powder adenosine, on the other hand, can be produced with an MMAD as small as 2.6–2.9 μm [20], with a much smaller distribution in particle size which is independent of the dose [20]. Therefore, dry powder adenosine was postulated to reach the small peripheral airways to a larger extent compared to nebulized AMP, especially when inhaled at a low flow [21]. Thus, to target the small airways specifically, without a dose restriction and cough, adenosine may be valuable.

In this study we evaluated whether there is a difference between the perception of dyspnea induced with the assumed small airways trigger dry powder adenosine or the assumed larger airways trigger nebulized AMP. In addition, we evaluated for both triggers if the perception of dyspnea during a provocation test is more closely associated to changes in large- or small airways function.

We found that dry powder adenosine and AMP evoke equal increases in dyspnea sensation, with a similar decrease in FEV₁. However, during adenosine and AMP provocation, the increase in dyspnea sensation was differentially associated with large- and small airways dysfunction. The only independently association with dyspnea induced by dry powder adenosine was the decrease in FEF_25–75, whereas dyspnea induced by AMP was not associated with changes in either large- or small airways dysfunction.

Our aim was to selectively target the small airways with dry powder adenosine. Therefore, we expected that dyspnea induced by dry powder adenosine would associate primarily with small airways parameters. Our findings were partly in line with this as we found the increase in Borg dyspnea score after inhalation of adenosine to associate with the decrease in FEF_25–75, both in the univariate and multivariate analysis. However, the adenosine-induced change in other small airways parameters, such as R₅-R₂₀, S_cond and S_acin, did not associate with ΔBorg. This was in contrast to our expectations, as these parameters are considered to be measures of the more peripheral small airways. A possible explanation could be that the measurements provide different information, yet there is no gold standard to determine which parameter is most accurate. Another possible explanation could be that the adenosine did not reach the more peripheral small airways even though it was designed to reach the small airways, consist of relatively small particles (MMAD of 2.6–2.9 μm) [20], and was inhaled with a low flow of 30 L/min [21]. Unfortunately we lack information on the exact deposition as radiolabeling for adenosine was not performed and our conclusions are thus based on assumed differential deposition.

With respect to AMP-induced dyspnea, multivariate analysis showed no large or small airways parameters that independently associated with ΔBorg. This may suggest that other factors than airway caliber or resistance play a role in the sensation of induced dyspnea. AMP acts on adenosine receptors which are located on various inflammatory cells including mast cells, eosinophils, and neutrophils, and their activation induces a cascade resulting in airway contraction [12]. Adenosine receptors are also found on afferent nerve endings [26]. It could be speculated that activation of afferent nerve endings plays a role in the dyspnea sensation after inhalation of AMP, independent of the presence of airway contraction. This activation may be direct or indirect through bronchial interstitial edema. In the context of direct activation, the findings of Burki et al. [27] are of interest. They administered intravenous adenosine to six asthmatic and six healthy subjects. Both groups reported a significant increase in dyspnea, with a higher intensity of the dyspnea in asthmatics. The FEV₁, however, remained unchanged, indicating the absence of airway constriction. Based on these observations, they concluded that afferent nerve endings may be involved in the adenosine-induced sensation of dyspnea, which in asthmatics might be sensitized due to inflammation. In the context of indirect activation, interstitial edema may arise when the adenosine-induced inflammatory response induces alveolar-capillary leakage [28], which triggers the J-receptors to induce dyspnea sensation [29]. This, combined with the knowledge that afferent nerve endings are mainly seen in the upper and central airways [30], where we assume AMP primarily deposits, supports our speculation.

Although dry powder adenosine and AMP provocation may induce dyspnea through different processes, the degree of dyspnea after the final dose was not different. In addition, both tests were well tolerated and, apart from dyspnea, only led to minor cough in some subjects. This confirms previous findings in a small proof of concept study, that the relatively new adenosine provocation test is well tolerated [23].

We only included current and ex-smokers with asthma. It is therefore unclear whether these findings can be extrapolated to never-smoking asthmatics, as previous studies have shown a decreased dyspnea perception attributed to smoking, in asthmatics [31]. Never-smoking asthmatics may have had greater increases in dyspnea as a result of the provocations, but what this would have done to the association of dyspnea to large- and small airways parameters cannot be speculated.

Our study shows that provocation with dry powder adenosine and AMP evoke similar levels of dyspnea. Dyspnea sensation evoked with dry powder adenosine shows small airways involvement independent of large airways involvement, while AMP evoked dyspnea associated with neither large- nor small airways dysfunction. This may indicate that dry powder adenosine and AMP evoke dyspnea via different processes.