The disadvantages of using MSV as an indicator of endurance are related to the difficulty in estimating the relative contribution of lung and chest wall mechanics to the measurement. MVV measurements are highly susceptible to relatively small changes in flow resistance, the effects of which are amplified exponentially as ventilation increases (24). Therefore, the load on the respiratory muscles is not uniform across patients or even in the same patients over time.

This is of considerable importance in COPD or other obstructive lung diseases in which day-to-day and diurnal variations in airway mechanics are common. Second, the wide variety of strategies utilized in a given patient to perform MVV-like maneuvers leaves many potential sources of variance between subjects. For example, in patients with COPD, effective use of the expiratory muscles is often limited during elevated ventilations (compared with normal subjects) because of early maximum flow limitation. This is accompanied by hyperinflation and shorter inspiratory muscle lengths with a greater proportional burden on the inspiratory muscles than would be seen in normal subjects. In patients with COPD, measurements of MSV/MVV% as an indicator of respiratory muscle endurance have suggested excellent ventilatory muscle endurance relative to strength, as compared with control subjects (34). However, because their mechanical abnormalities have greater relative influence at higher ventilations, the denominator of the MSV/MVV fraction may be artificially low in these patients, giving the impression that endurance properties are normal. When external resistive loading techniques are utilized, which reduce the contribution of lung and chest wall mechanics as a factor in the measurement, it is found that the endurance capacity of the respiratory system is relatively low in the COPD population compared with normal subjects (8).

The problem of the contribution of the inherent impedance of the respiratory system could be overcome by careful measurement of Wrs during the test. As shown in Figure 6, redrawn from the experiments of Tenney and Reese (33), a strikingly different view of endurance can be seen when work rate or power output is quantified. Although this subject could sustain approximately 68% of his MVV, he could sustain only approximately 30% of maximum Wrs. The investigators also showed that despite experimental alterations in pulmonary

Figure 5. A typical apparatus for measuring ventilatory endurance. Reprinted by permission from Reference 34.

impedance, the Wrs-versus-Tlim relationship did not change appreciably (33).

In summary, ventilatory endurance testing can be useful as a functional measurement, particularly in the setting of rehabilitation or other forms of treatment. Results should be viewed with some understanding of the test's limitations with respect to separation of muscle properties versus intrinsic mechanical properties of the lungs and chest wall. This limitation could potentially be overcome by measuring Wrs, although this may not be practical in many clinical settings. The most promising approach appears to be the incremental method. However, full support of the technique awaits verification in other laboratories.

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