Dillard et al have shown that maximal exercise ventilation in patients with CAO correlates with peak inspiratory flow rate as well as the FEV. It is not clear why the training induced decrease of H/rtot in our study was not translated into improved exercise performance. Possibly threshold pressure breathing is just not specific enough with regard to exercise ventilation to induce a useful training effect.
It may be that inspiratory muscle fatigue was not a significant limitation in our patients. Pardy et al concluded that specific training of the inspiratory muscles is usually associated with improved exercise performance only in those who demonstrate electromyographic changes heralding inspiratory muscle fatigue during exercise.
The interrelationships between the presence or absence of fatigue, and the degree to which Ti/Ttot might be reduced after training, is unknown. Furthermore we did not conduct nutritional studies in our patients and it is conceivable that this may have influenced our results.
In our study, SIP was a slightly smaller proportion of MIF-FRC than that found in normal subjects by Nickerson and Keens, though our results are closer to the values predicted by Clanton et al who reported an increase in MIP-FRC and endurance time at 65 percent of MIP-FRC after ten weeks threshold IMT in normal subjects. In our subjects, there was, in fact, a training effect in the appropriate direction, as shown by the reduction of Ti/rtot. Therefore, it is unlikely that the SIPs were just too small.
We did not measure blood gas values during our threshold pressure breathing, and therefore, we cannot say whether patients allowed arterial Pco2 to rise in the face of increased C02 production, in an effort to attenuate the increased external work. This has been a matter of concern to previous authors. Although there was no overall change in mean ventilation for the group, there were individual increases or decreases in ventilation, possibly reflecting variable tolerance of hypercapnoea.
In conclusion, this study clearly demonstrates that a threshold pressure device can be used to improve inspiratory muscle performance. However, no improvement in any index of exercise performance has been observed in this group of patients with severe CAO.
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