Positive Expiratory Pressure: A Potential Therapy to Mitigate Acute Bronchoconstriction in the Asthma of Obesity

The fix is not yet in: recommendation for fixation of lungs within physiological/pathophysiological volume range in preclinical pulmonary structure-function studies

Quantitative characterization of lung structures by morphometrical or stereological analysis of histological sections is a powerful means of elucidating pulmonary structure-function relations. The overwhelming majority of studies, however, fix lungs for histology at pressures outside the physiological/pathophysiological respiratory volume range. Thus, valuable information is being lost. In this perspective article, we argue that investigators performing pulmonary histological studies should consider whether the aims of their studies would benefit from fixation at functional transpulmonary pressures, particularly those of end-inspiration and end-expiration. We survey the pressures at which lungs are typically fixed in preclinical structure-function studies, provide examples of conditions that would benefit from histological evaluation at functional lung volumes, summarize available fixation methods, discuss alternative imaging modalities, and discuss challenges to implementing the suggested approach and means of addressing those challenges. We aim to persuade investigators that modifying or complementing the traditional histological approach by fixing lungs at minimal and maximal functional volumes could enable new understanding of pulmonary structure-function relations.

Gaps and Future Directions in Clinical Research on Obesity-Related Asthma

Obesity is a major comorbidity for the development and worsening of asthma. It is associated with increased disease incidence, reduced response to inhaled and systemic steroids, increased asthma exacerbations, and poor disease control. Over the past two decades, we have learned that there are clinical asthma phenotypes associated with obesity, which have unique immune, inflammatory, and metabolic disease mechanisms. The objectives of this review are to provide a brief overview of the associations and gaps between these chronic inflammatory diseases and the role that traditional therapies have on treating patients with obesity-related asthma, and to describe new clinical research of therapeutic developments targeting mechanisms that are more specific to this patient population.

Lung de-recruitment in the allergic asthma of obesity: evidence from an anatomically based inverse model

The increase in asthma associated with the obesity epidemic cannot simply be due to airway hyperresponsiveness from chronic lung compression because chronic lung compression is a feature of obesity in general. We therefore sought to investigate what other factors might be at play in the impaired lung function seen in obese individuals with asthma. We measured respiratory system impedance in four groups-Lean Control, Lean Allergic Asthma, Obese Control, and Obese Allergic Asthma-before and after administration of albuterol. Impedance measurements were fit with an anatomically based computational model of lung mechanics that represents the airway tree as a branching structure with a uniform degree of asymmetry and a fixed radius scaling ratio, γ, between branches of sequential order. The two model parameters that define the airway tree, γ and tracheal radius, varied only modestly between the four study groups, indicating relatively minor differences in airway caliber. In contrast, respiratory system elastance was 57, 34, 143, and 271 cmH2O/L, respectively, for the four groups, suggesting that obesity induced significant lung de-recruitment that was exacerbated by allergic asthma. In addition, when the radii of the individual branches of the airway tree were varied randomly, we found that roughly half the terminal airways had to be closed to have the model fit the data well. We conclude that de-recruitment of small airways is a particular feature of Obese Allergic Asthma, and this can be inferred from respiratory system impedance fit with an anatomically based computational model.NEW & NOTEWORTHY Using a novel anatomically based computational model to interpret oscillometry measurements of impedance, we show that respiratory system elastance is increased in obesity and is increased dramatically in individuals with obese allergic asthma. A significant component of this increased elastance in obese allergic asthma appears to be due to closure of small airways rather than alveolar atelectasis, and this closure is partially mitigated by albuterol. These findings potentially point to nonpharmacological therapies in obese allergic asthma aimed at recruiting closed airways.

Understanding the fundamentals of oscillometry from a strip of lung tissue

Metrics used in spirometry caught on in respiratory medicine not only because they provide information of clinical importance but also because of a keen understanding of what is being measured. The forced expiratory volume in 1 s (FEV₁), for example, is the maximal volume of air that can be expelled during the first second of a forced expiratory maneuver starting from a lung inflated to total lung capacity (TLC). Although it represents a very gross measurement of lung function, it is now used to guide the diagnosis and management of many lung disorders. Metrics used in oscillometry are not as concrete. Resistance, for example, has several connotations and its proper meaning in the context of a lung probed by an external device is not always intuitive. I think that the popularization of oscillometry and its firm implementation in respiratory guidelines starts with a keen understanding of what exactly is being measured. This review is an attempt to clearly explain the basic metrics of oscillometry. In my opinion, the fundamentals of oscillometry can be understood using a simple example of an excised strip of lung tissue subjected to a sinusoidal strain. The key notion is to divide the sinusoidal reacting force from the tissue strip into two sinusoids, one in phase with the strain and one preceding the strain by exactly a quarter of a cycle. Similar notions can then be applied to a whole lung subjected to a sinusoidal flow imposed at the mouth by an external device to understand basic metrics of oscillometry, including resistance, elastance, impedance, inertance, reactance and resonant frequency.