Determination of maximal diaphragm strength in chronic obstructive pulmonary disease:: Cervical magnetic stimulation versus traditional sniff maneuver

OBJECTIVE: Magnetic stimulation of the diaphragm allows its strength to be assessed. The clinical applications of this technique are becoming more widespread given that the patient's cooperation is not required. The aim of the present study was to compare this inhalation technique with traditional voluntary forced inspiration (sniff test) in a group of patients with chronic obstructive pulmonary disease (COPD). PATIENTS AND METHODS: Sixteen men with moderate-to-severe COPD were studied (mean [SD] forced expiratory volume in 1 second, 35% [15%] of the reference value). For all patients, the maximal transdiaphragmatic pressure (a measure of the contractility of the muscle) was determined at peak inspiration and during cervical magnetic stimulation. RESULTS: A moderate correlation between measurements with the 2 techniques was observed. The value obtained with stimulation was approximately 20% of that obtained with the sniff maneuver (22 [7] cm H2O vs 97 [27] cm H2O, respectively). The stimulation technique yielded an intraindividual coefficient of variability of 12% (7%) and an interindividual one of 33% (6%). Very similar values for these coefficients were obtained with the sniff maneuver. Qualitative analysis of the stimulation technique showed it to have a high sensitivity (89%) for diagnosing muscle weakness, with few false negatives. In contrast, specificity was very low (43%), and false positives for muscle weakness were relatively common. The overall effectiveness of the prediction was acceptable (69%). CONCLUSIONS: Cervical magnetic stimulation appears to be a good clinical option for ruling out diaphragm weakness. It is particularly indicated in patients with limited capacity for understanding instructions or those unable to cooperate.