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It has to be overcome by the inspiratory muscles at the start of the inspiration before inspiration can proceed, adding to the energy cost of breathing. As muscle weakness progresses, the ratio of FEV1 to FVC increases such that with advanced muscle weakness, FEV1 approximates FVC, at which point they yield similar information. Earlier in disease progression, it arguably provides more information about capacity of respiratory muscles to ventilate the lungs than does FVC, as it reflects their effectiveness in overcoming both elastic and resistive forces, rather than predominantly elastic forces as is the case with a volume measurement such as FVC. These tests allow respiratory muscle strength and its effect on ventilatory capacity to be quantified. Hence, they have the potential to track changes as respiratory muscle weakness evolves. Sniff nasal inspiratory pressure and VC appear to increase with age until between click here 10.5 and 12 years in Duchenne muscular dystrophy before declining thereafter.[17] These and the other measures described earlier each have predictive power for the occurrence of daytime hypercapnia.[15, 17, 19] Certain thresholds appear valuable in defining vulnerability Dipivefrine to sleep hypoventilation and therefore indicating when studies of ventilation during sleep should first be performed. In turn, these studies define when ventilatory assistance during sleep might first be required. Figure?2 illustrates the relationship between FEV1 as a percentage of predicted normal and the percentage of total sleep time spent below an arterial oxygen saturation of 90% (TST?Selleckchem Forskolin present below this threshold. It suggests that these patients should be followed with periodic spirometry with the first sleep study undertaken once FEV1 first decreases below 40% predicted normal. The degree of sleep hypoventilation revealed by this or subsequent sleep studies then determines the need for assisted ventilation (NIV) during sleep. Similar thresholds are likely to be applicable to neuromuscular disorders associated with slowly progressive, reasonably symmetrically distributed respiratory muscle weakness, such as other muscular dystrophies, spinal muscular atrophy and myotonic dystrophy. Disorders with ��patchy�� involvement of respiratory muscles may not behave as predictably. For example, diaphragmatic weakness can occur disproportionately to weakness in other respiratory muscles in conditions such as ALS, with NIV being required during sleep in the recumbent position to reduce associated orthopnoea.