The studies that have been performed on respira­tory muscle endurance are based on the capacity of the respiratory muscles to sustain and generate high levels of pressure. The most common test consists of generating a pressure against a given resistive inspi­ratory load. In our study, we used the Nickerson, Keens and Kelsen methods, that consist of breath­ing through a constant threshold load equivalent to 65 percent of the MIP. This pressure lies within the range of critical pressure required to induce respiratory muscle fatigue in normal subjects. The Ti/Tтот ratio was kept constant at 0.5, as variations in this ratio during the test change the time of endurance under the inspiratory load owing to the fact that the endurance capacity of the inspiratory muscles depends on the ventilatory pattern and on muscular force generated. It is a proven fact that the application of submaximum inspiratory loads produces low-fre­quency fatigue in normal subjects. The MIP is a measure of the maximum force generated by all the respiratory muscles and, in turn, it measures the muscular response to the high-frequency voluntary activation of the respiratory muscles. In healthy individuals, the MIP decreases significantly five min­utes after breathing has begun through the test with inspiratory load and this reduction reflects the re­duction in the mechanical capacity of the respiratory muscles to generate pressure.

We have shown that in patients with COPD the MIP is decreased at the end of the test with respect to its basal value, but not significantly (85.7 to 82.6 cm H₂0). The decrease in MIP becomes significant (79.1 cm H₂0) ten minutes after exhaustion (Table 2) with respect to its basal value.

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It can be seen in Table 1 that there is evidence that the patients suffered a moderate obstruction of air flow and the volumes are increased. Three of the patients had a Deo of less than 80 percent due to clinical and radiologic signs of pulmonary emphy­sema. The average TLIM of the sample was 3.7 ±1.96 minutes. The anthropometric measure­ments were normal.

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Sample

The study was conducted on eight male patients with COPD in stable phase, that is, the patients had not shown any symptoms of acute relapse in a period of at least two months prior to the study. The average age was 60.57 ±7.59 years, the height was 162.14 ±10.43 cm, and the weight was 65±9.7 kg. The patients did not have any other type of cardiorespiratory, endocrine, neuro­muscular, or hepatic illness. We explained the protocol to be followed and all patients gave their consent. Various tests were conducted simultaneously within the first week of inclusion in the study.

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To or some years now, resistive inhalational loads have been used to evaluate the endurance capacity of the inspiratory musculature under exertion. The capacity to sustain the muscular exertion required to inhale depends on the force and duration of the inhalational muscle contraction, which is determined by the pressure-time ratio. It is known that healthy subjects and patients with COPD become fatigued with diaphragmatic pressure-time ratios over 0.15. This technique is also used as a rehabilitative measure for the respiratory muscle function, as well as in the evaluation of the effectiveness of certain drugs such as caffeine. In patients with COPD, it is observed that the maximum inspiratory pressures (MIPs) and the exertion capacity are diminished owing probably to pulmonary hyperinflation, to changes in the ex­change of gases, and to concomitant nutritional defi­ciencies.

Nevertheless, the effects of application of resistive inhalational loads on the maximum inspiratory and expiratory pressures in such patients are not known.

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