Respiratory muscle overloading and dyspnoea during bronchoconstriction in asthma: protective effects of fenoterol

Cardio-respiratory interactions in acute asthma

Asthma encompasses of respiratory symptoms that occur intermittently and with varying intensity accompanied by reversible expiratory airflow limitation. In acute exacerbations, it can be life-threatening due to its impact on ventilatory mechanics. Moreover, asthma has significant effects on the cardiovascular system, primarily through heart-lung interaction-based mechanisms. Dynamic hyperinflation and increased work of breathing caused by a sharp drop in pleural pressure, can affect cardiac function and cardiac output through different mechanisms. These mechanisms include an abrupt increase in venous return, elevated right ventricular afterload and interdependence between the left and right ventricle. Additionally, Pulsus paradoxus, which reflects the maximum consequences of this heart lung interaction when intrathoracic pressure swings are exaggerated, may serve as a convenient bedside tool to assess the severity of acute asthma acute exacerbation and its response to therapy.

The differential effects of inspiratory, expiratory, and combined resistive breathing on healthy lung

Combined resistive breathing (CRB) is the hallmark of obstructive airway disease pathophysiology. We have previously shown that severe inspiratory resistive breathing (IRB) induces acute lung injury in healthy rats. The role of expiratory resistance is unknown. The possibility of a load-dependent type of resistive breathing-induced lung injury also remains elusive. Our aim was to investigate the differential effects of IRB, expiratory resistive breathing (ERB), and CRB on healthy rat lung and establish the lowest loads required to induce injury. Anesthetized tracheostomized rats breathed through a two-way valve. Varying resistances were connected to the inspiratory, expiratory, or both ports, so that the peak inspiratory pressure (IRB) was 20%-40% or peak expiratory (ERB) was 40%-70% of maximum. CRB was assessed in inspiratory/expiratory pressures of 30%/50%, 40%/50%, and 40%/60% of maximum. Quietly breathing animals served as controls. At 6 hours, respiratory system mechanics were measured, and bronchoalveolar lavage was performed for measurement of cell and protein concentration. Lung tissue interleukin-6 and interleukin-1β levels were estimated, and a lung injury histological score was determined. ERB produced significant, load-independent neutrophilia, without mechanical or permeability derangements. IRB 30% was the lowest inspiratory load that provoked lung injury. CRB increased tissue elasticity, bronchoalveolar lavage total cell, macrophage and neutrophil counts, protein and cytokine levels, and lung injury score in a dose-dependent manner. In conclusion, CRB load dependently deranges mechanics, increases permeability, and induces inflammation in healthy rats. ERB is a putative inflammatory stimulus for the lung.

Do obstructive and restrictive lung diseases share common underlying mechanisms of breathlessness?

This review tries to answer two main questions: (i) What are the neurophysiological underpinnings of the most commonly selected cluster descriptors which define the qualitative dimension of dyspnea in patients? (ii) How do mechanical constraints affect dyspnea? (iii) Do obstructive and restrictive lung diseases share some common underlying mechanisms? Qualitative dimensions of dyspnea, which allude to increased respiratory work/effort breathing, reflect a harmonious coupling between increased respiratory motor output and lung volume displacement in healthy subjects. Descriptors that allude to unsatisfied inspiration are the dominant qualitative descriptors in patients with a variety of respiratory diseases. It is possible that sensory feedback from a multitude of mechanoreceptors throughout the respiratory system (in the muscle, chest wall, airways and lung parenchyma) collectively convey information to the consciousness that volume/flow or chest wall displacement is inadequate for the prevailing respiratory drive. The data would lend support to the idea that: (i) an altered afferent proprioceptive peripheral feedback signals that ventilatory response is inadequate to the prevailing motor drive, reflecting neuromechanical uncoupling (NMU), (ii) mechanical constraints on volume expansion (dynamic restriction) play a pivotal role in dyspnea causation in patients with a variety of either obstructive or restrictive respiratory disorders, and (iii) all of the physiological adaptations that optimize neuromechanical coupling in obstructive and restrictive disorders are seriously disrupted so that an NMU underpins cluster descriptors of dyspnea which are similar in obstructed and in restricted patients.

Inspiratory muscle training for asthma

BACKGROUND

In moderate to severe chronic obstructive pulmonary disease there is good evidence of a generalised loss of muscle bulk (including the respiratory muscles). It is possible that similar loss of respiratory muscle strength occur particularly in more severe asthma related in part to the effects of steroid therapy. Thus the respiratory muscle function may well be of relevance in asthma and if dysfunctional, may be a suitable target for training.

OBJECTIVES

To evaluate the efficacy of inspiratory muscle training with an external resistive device in patients with asthma.

SEARCH STRATEGY

We searched the Cochrane Central Register of Controlled Trials (The Cochrane Library Issue 1, 2002), MEDLINE (January 1966 to March 2002), EMBASE (January 1985 to March 2002), CINAHL (to March 2002) and the UK National Research Register of trials (January 1982 to March 2002) and reference lists of articles. We also searched on line respiratory journals and contacted manufacturers of training devices to obtain trials.

SELECTION CRITERIA

All randomised-controlled trials that involved the use of an external inspiratory muscle training device versus a control (sham or no inspiratory training device) were considered for inclusion.

DATA COLLECTION AND ANALYSIS

Two reviewers independently selected articles for inclusion, evaluated methodological quality of the studies and abstracted data.

MAIN RESULTS

Five studies were included in the review with four of the studies being produced by the same group. PI(max) (maximum inspiratory pressure) reported in three studies with 76 patients showed significant improvement with inspiratory muscle training when compared to the control group (WMD 23.07 cmH(2)O, 95%CI 15.65 to 30.50). Unfortunately, due to the paucity of included studies and data no other outcome was reported by more than one study. Therefore it is not possible to confirm whether this increase seen with PI(max) translates into any measurable clinical benefit.

REVIEWER'S CONCLUSIONS

Currently there is insufficient evidence to suggest that inspiratory muscle training provides any clinical benefit to patients with asthma. Due to the limited availability of studies in this area there is a need for further trials evaluating the efficacy of inspiratory muscle training devices in patients with asthma. These studies should investigate asthmatics with a range of severity. They should investigate clinically relevant outcomes such as lung function, symptoms, exacerbation rate and concomitant medications.