Comparison of the respiratory responses to external resistive loading and bronchoconstriction.

Abstract

A B S T R A C T The effects of resistive loads applied at the mouth were compared to the effects of bronchospasm on ventilation, respiratory muscle force (occlusion pressure), and respiratory sensations in 6 normal and 11 asthmatic subjects breathing 100% 02. External resistive loads ranging from 0.65 to 13.33 cm H20/liter per s were applied during both inspiration and expiration. Bronchospasm was induced by inhalation of aerosolized methacholine. Bronchospasm increased ventilation, inspiratory airflow, respiratory rate, and lowered PACO.. External resistive loading, on the other hand, reduced respiratory rate and inspiratory flow, but left ventilation and PACO, unaltered. FRC increased to a greater extent with bronchospasm than external flow resistive loads. With both bronchospasm and external loading, occlusion pressure increased in proportion to the rise in resistance to airflow. However, the change in occlusion pressure produced by a given change in resistance and the absolute level of occlusion pressure at comparable levels of airway resistance were greater during bronchospasm than during external loading. These differences in occlusion pressure responses to the two forms of obstruction were not explained by differences in chemical drive or respiratory muscle mechanical advantage. Although the subjects' perception of the effort involved in breathing was heightened during both forms of obstruction to airflow, at any given level of resistance the sense of effort was greater with bronchospasm than external loading. Inputs from mechanoreceptors in the lungs (e.g., irritant receptors) and/or greater stimulation of chest wall mechanoreceptors as a result of increases in lung elastance may explain the differing responses elicited by the two forms of resistive loading.

Changes in respiratory drive account for the magnitude of dyspnoea during bronchoconstriction in asthmatics

“…The results of this study suggest that changes in respiratory drive are major determinants of the intensity of the sensation of dyspnoea experienced during progressive bronchoconstriction in asthmatic subjects. Our findings are in agreement with previous observations by KELSEN and co-workers [17]. These authors observed that both P0.1 and the sensation of breathing effort increased to a greater extent during MCh-induced bronchoconstriction than during breathing against an external resistance, thus indicating that the sense of breathing effort is related to the intensity of the respiratory drive rather than to the resistive load applied to respiratory system.…”

“…The increase in P0.1 observed in the present study during MCh-induced bronchoconstriction was similar to that previously reported by others [6,16], and by ourselves [7]. The increase in respiratory drive during bronchoconstriction has been suggested to be due to the activation of reflexes arising from muscular and joint receptors stimulated by hyperinflation [16,17], and from airway receptors stimulated by inhaled substances and by bronchoconstriction [18][19][20]. The results of this study suggest that changes in respiratory drive are major determinants of the intensity of the sensation of dyspnoea experienced during progressive bronchoconstriction in asthmatic subjects.…”

“…These authors observed that both P0.1 and the sensation of breathing effort increased to a greater extent during MCh-induced bronchoconstriction than during breathing against an external resistance, thus indicating that the sense of breathing effort is related to the intensity of the respiratory drive rather than to the resistive load applied to respiratory system. This is not surprising, in that it has been observed that asthmatic subjects adopt different ventilatory strategies in response both to airway narrowing [15,21] and the addition of external resistive loads [17,21]. On the other hand, it has been shown that the sensation of dyspnoea experienced during exercise [9], loaded breathing [8], or progressive bronchoconstriction [14] is a function of the intensity of respiratory drive.…”

The Relationship among the Inspiratory Muscle Strength, the Perception of Dyspnea and Inhaled Beta2‐Agonists in Patients with Asthma

“…However, despite a close linear relationship between the decrease in FEV 1 and the increase in the POD, there is a considerable variation in the severity of breathlessness for any particular degree of airflow obstruction (1). Measuring the POD during external mechanical loading (resistive, elastic or threshold) less accurately simulates the mechanical characteristics of an asthma attack (20). However, it was previously shown that the imposed inspiratory threshold load is a good predictor for explaining the variability of the perceived dyspnea (21).…”

“…Inspiratory muscle training was associated with decreased dyspnea in patients with COPD and pretraining respiratory muscle weakness (19), and in patients with mild asthma and high beta 2 -agonist consumption but normal respiratory muscle strength (12). Inspiratory muscle training was evaluated in patients with various other respiratory disorders such as neuromuscular disorders (20) and cystic fibrosis (21), and in elite athletes (22), with favourable results.…”