CLINICAL-ASSESSMENT OF DIAPHRAGM STRENGTH BY CERVICAL MAGNETIC STIMULATION OF THE PHRENIC NERVES

Background-Accurate assessment of diaphragm strength can be difficult. Transdiaphragmatic pressure (PDI) measurements during volitional mano euvres are useful but it may be difficult to ensure maximum patient ef fort. Magnetic stimulation of the phrenic nerves is easy to perform an d the results are reproducible in normal subjects. The purpose of the present study was to evaluate the usefulness of magnetic stimulation o f the phrenic nerves in the assessment of diaphragm weakness in patien ts. Methods - Sixty six patients referred for assessment of respirator y muscle strength and 23 normal subjects were studied. Twitch PDI (TwP DI) following magnetic stimulation of the phrenic nerves and sniffPDI were obtained in all individuals. TwPDI following bilateral electrical stimulation of the phrenic nerves was also obtained in eight patients . Results - Mean (SD) TwPdi for the normal subjects was 31 (6) cm H2O and 18 (11) cm H2O for the patients. TwPDI and sniffPDI were correlate d (r = 0.77). Seven of the 37 patients (19%) with a reduced sniffPDI h ad a TwPDI within the normal range whereas two of the 32 patients (6%) with a reduced TwPDI had a normal sniffPDI. TwPDI was similar with ma gnetic and electrical stimulation. Conclusions - TwPDI following magne tic stimulation of the phrenic nerves is a clinically useful measureme nt when assessing diaphragm weakness.