Lung function testing: selection of reference values and interpretative strategies.

Summary Background Few patients with amyotrophic lateral sclerosis currently receive non-invasive ventilation (NIV), reflecting clinical uncertainty about the role of this intervention. We aimed to assess the effect of NIV on quality of life and survival in amyotrophic lateral sclerosis in a randomised controlled trial. Methods 92 of 102 eligible patients participated. They were assessed every 2 months and randomly assigned to NIV (n=22) or standard care (n=19) when they developed either orthopnoea with maximum inspiratory pressure less than 60% of that predicted or symptomatic hypercapnia. Primary validated quality-of-life outcome measures were the short form 36 mental component summary (MCS) and the sleep apnoea quality-of-life index symptoms domain (sym). Both time maintained above 75% of baseline (T i MCS and T i sym) and mean improvement (μMCS and μsym) were measured. Findings NIV improved T i MCS, T i sym, μMCS, μsym, and survival in all patients and in the subgroup with better bulbar function (n=20). This subgroup showed improvement in several measures of quality of life and a median survival benefit of 205 days (p=0·006) with maintained quality of life for most of this period. NIV improved some quality-of-life indices in those with poor bulbar function, including μsym (p=0·018), but conferred no survival benefit. Interpretation In patients with amyotrophic lateral sclerosis without severe bulbar dysfunction, NIV improves survival with maintenance of, and improvement in, quality of life. The survival benefit from NIV in this group is much greater than that from currently available neuroprotective therapy. In patients with severe bulbar impairment, NIV improves sleep-related symptoms, but is unlikely to confer a large survival advantage.

https://www.thelancet.com/pdfs/journals/laneur/PIIS1474-4422(05)70326-4.pdf

Effects of non-invasive ventilation on survival and quality of life in patients with amyotrophic lateral sclerosis: a randomised controlled trial.

The act of breathing depends on coordinated activity of the respiratory muscles to generate subatmospheric pressure. This action is compromised by disease states affecting anatomical sites ranging from the cerebral cortex to the alveolar sac. Weakness of the respiratory muscles can dominate the clinical manifestations in the later stages of several primary neurologic and neuromuscular disorders in a manner unique to each disease state. Structural abnormalities of the thoracic cage, such as scoliosis or flail chest, interfere with the action of the respiratory muscles-again in a manner unique to each disease state. The hyperinflation that accompanies diseases of the airways interferes with the ability of the respiratory muscles to generate subatmospheric pressure and it increases the load on the respiratory muscles. Impaired respiratory muscle function is the most severe consequence of several newly described syndromes affecting critically ill patients. Research on the respiratory muscles embraces techniques of molecular biology, integrative physiology, and controlled clinical trials. A detailed understanding of disease states affecting the respiratory muscles is necessary for every physician who practices pulmonary medicine or critical care medicine.

Disorders of the Respiratory Muscles

Why must the chest physician become familiar with assessment of the respiratory muscles? Firstly, because dyspnoea in patients in whom no pulmonary cause can be detected may be due to respiratory muscle weakness.t2 Even moderately severe muscle weakness may be difficult to detect clinically3 and, indeed, it is possible to have total paralysis of the diaphragm without life threatening consequences.4 Secondly, because patients with clearly documented generalised neuromuscular disease usually also have respiratory muscle weakness2 and, for selected cases, treatment in the form of non-invasive ventilation is indicated.5 Finally, there has recently been increased awareness that respiratory muscle weakness can be a compounding factor in other disease processes such as malnutrition6 and steroid therapy.7 For all of these reasons it is important for respiratory physicians to initiate and to be able to interpret simple tests ofrespiratory muscle function. For most patients the suspicion of clinically important respiratory muscle weakness may be confirmed or excluded by simple tests that can be performed in the general hospital setting without the purchase of expensive equipment, but in some patients complex tests in a specialised laboratory are necessary (fig 1). In this editorial the current techniques to assess respiratory muscle strength are reviewed with

https://thorax.bmj.com/content/thoraxjnl/50/11/1131.full.pdf

Measurement of respiratory muscle strength.

Monitoring recovery from diaphragm paralysis with ultrasound.

In a study of 147 subjects (50 normals, 50 with obstructive, and 47 with restrictive lung function), the mean reduction in forced vital capacity from standing to supine (delta FVC) was 7.5% (SD +/- 5.7), 11.2% (+/- 13.4), and 8.2% (+/- 7.7) respectively, with no significant difference between groups. The respective 95% upper confidence limits were 19%, 38% and 24%. We conclude that delta FVC greater than 25% associated with normal or restrictive lung function or greater than 35% associated with airways obstruction should be an indication for further study of diaphragm function.

B. Hunt

Papers

Fall in vital capacity with posture