The cortical representation of somatosensory evoked potentials of the phrenic nerve

Respiratory electrophysiological studies are of essential value in diagnosing and managing patients with respiratory failure, but assessment of the sensory phrenic nerve fibres has been neglected. We recorded phrenic nerve somatosensory evoked potentials (SSEPs) by combining neurophysiological 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 constant latency, PI at 12.0 +/- 0.6 ms, and NI at 17.3 +/- 0.8 ms. Single photon emission computer tomography (SPECT) performed during phrenic nerve stimulation revealed focal neuronal activation in the somatosensory pathways. Intravenously administered Tc-99m Ethyl Cysteinate Dimer (ECD) was used as a blood flow tracer to obtain baseline and activated images. After image registration, baseline images were compared voxel-by-voxel with the activation images. The mean inter-subject summation image of the activated state was compared with that of the baseline state using ten normal subjects. The extent of the total voxel volume increase on the mean images of the 3 activated SPECT images was 0.7%, and a mean signal increase of 22%. For further anatomic localization of regional increases in signal, the magnetic resonance image (MRI) scan of each subject was registered and superimposed on the activated stage SPECT image. This method may be used clinically to study the pathophysiology of impaired central respiratory drive.