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.
The drop in MIP reaches a plateau after ten minutes and is maintained throughout the 30-minute recovery phase, so there is no recovery of maximum inspiratory muscle force.
The decrease in MIP is probably related to the fatigue of rapid muscle fiber contraction (type II). For the most part, the fibers are responsible for respiratory muscle force and these muscle fibers are activated with a high frequency, but the inspiratory load test measures endurance of respiratory muscle. We cannot discard the possibility that the decrease in MIP is caused by a motivational effort; however, if this is indeed the case, we think that the MIP would decrease significantly at exhaustion and not ten minutes after. Also, it is curious that МЕР did not decrease as well.
We may conclude that in patients with COPD, the decrease in MIP lasts 30 minutes after the application of inspiratory loads and consequently, the high-frequency fatigue extends even beyond 30 minutes after exhaustion of the patient. cialis canadian pharmacy
The МЕР did not vary after application of loads, although insignificant increases were obtained in it, probably the result of the learning effort. Musculature showed no fatigue, or at least no high-frequency fatigue, which seems to be related to the normality of the MEPs of the sample (МЕР = 139.4 ±43.9). Therefore, the application of resistive inspiratory loads does not produce fatigue of the expiratory muscles as is the case in normal subjects.
On the basis of foregoing results, it may be said that the MIP decreases in the patients with COPD after the application of inspiratory loads equivalent to 65 percent of MIP The drop in MIP begins to take place at the end of the test and becomes significant ten minutes after exhaustion, without returning to its basal value even beyond 30 minutes after the end of the test. The МЕР does not vary with respect to the value obtained at rest.