Inspiratory muscle testing in stable COPD patients

Exploration of inspiratory muscles in stable chronic pulmonary disease patients can be important in the investigation of a respiratory handicap unsatisfactorily explained by alterations of the passive respiratory system, or in the follow-up of patients undergoing treatments that can interfere with muscle function. Compensatory mechanisms tend to counterbalance the deleterious effects of hyperinflation in these patients, and precise clinical data are needed in order to avoid mistakes due to underverified hypotheses. Investigation of inspiratory muscle function requires the study of output data under various states of activity of the system. As outputs, volume displacement lacks specificity, pressure measurements can be more specific but are at times invasive and should be associated with lung volume measurements, electromyography is methodologically complex, nonquantitative and of poor reproducibility. Voluntary manoeuvres depend upon subject co-operation, and do not allow partitioning of output between the action of different muscle groups. Transcutaneous electrical phrenic nerve stimulation is devoid of these inconveniences, but it explores only one muscle (the diaphragm) under conditions that are not "natural" (relaxed rib cage). Recently, perspectives for easier clinical assessment of inspiratory muscle function in chronic obstructive pulmonary disease patients have been opened by cervical magnetic stimulation, better understanding of the meaning of mouth pressure in relationship to phrenic stimulation, and development of noninvasive tests, such as nostril pressure during sniff or phonomyography. If validated, such tests should provide a reasonably limited panel of clinical tools to better appreciate muscle function in this setting.