THE CORTICAL REPRESENTATION OF SOMATOSENSORY-EVOKED POTENTIALS OF THEPHRENIC-NERVE

Respiratory electrophysiological studies are of essential value in dia gnosing and managing patients with respiratory failure, but assessment of the sensory phrenic nerve fibres has been neglected. We recorded p hrenic nerve somatosensory evoked potentials (SSEPs) by combining neur ophysiological and neuroimaging techniques in three healthy subjects. Evoked potentials of the phrenic nerve showed the highest amplitude at CP3, determined by the modified 10-20 EEG system, and occurred at a c onstant latency, PI at 12.0 +/- 0.6 ms, and NI at 17.3 +/- 0.8 ms. Sin gle photon emission computer tomography (SPECT) performed during phren ic nerve stimulation revealed focal neuronal activation in the somatos ensory pathways. Intravenously administered Tc-99m Ethyl Cysteinnate D immer (ECD) was used as a blood flow tracer to obtain baseline and act ivated images. After image registration, baseline images were compared voxel-by-voxel with the activation images. The mean inter-subject sum mation image of the activated state was compared with that of the base line state using ten normal subjects. The extent of the total voxel vo lume increase on the mean images of the 3 activated SPECT images was 0 .7%, and a mean signal increase of 22%. For further anatomic localizat ion of regional increases in signal, the magnetic resonance image (MRI ) scan of each subject was registered and superimposed on the activate d stage SPECT image. This method may be used clinically to study the p athophysiology of impaired central respiratory drive.