Influence of kappa-rhythm when assessing drowsiness

Kappa-rhythm appears at the highest amplitude on a bipolar T3-T4 derivation and its frequency range is 7-10 Hz. When a contra-lateral earlobe is used as a reference, the kappa-rhythm causes serious problems in assessing drowsiness. Assessment of drowsiness using contra- and ipsi-lateral earlobes as references in 129 subjects who showed kappa-rhythm was compared. Drowsiness could be assessed properly in only 26% of subjects using the contra-lateral earlobe and in 90% of subjects using the ipsi-lateral earlobe. These results suggest that the ear on the ipsi-lateral earlobe should be used as a reference in subjects who show kappa-rhythm.