BACKGROUND AND PURPOSE: In order to identify sleep disorders by thoracic impedance plethysmography (TIP), we propose several new techniques: the application of an adaptive filter, a scaled Fourier linear combiner (SFLC) to eliminate cardiac-derived fluctuation in the impedance waveform, and the use of heart rate variability (HRV) to ascertain whether the airflow is obstructed. PATIENTS AND METHODS: Laboratory simulation experiments on four healthy individuals and actual overnight measurements on five patients with sleep disorders were carried out. Amplified thoracic impedance change (DeltaZ), ECG, a phonocardiograph, a pneumotachograph and a standard polysomnograph were recorded. The SFLC was applied to DeltaZ to selectively extract the cardiac-synchronous component (DeltaZ(CSC)), and the remainder of the waveform (DeltaZ(REM)) was low-pass filtered to estimate the waveform driven by respiration. The HRV was divided into respiratory synchronous (HRV(R)) and low frequency (HRV(L)) components. RESULTS: The SFLC could drastically extract DeltaZ(CSC) from DeltaZ and thereby demonstrate a DeltaZ(REM) pattern quite similar to the flow-volume curve of the pneumotachograph. Central sleep apnea could be identified as the cessation of DeltaZ(REM) and concomitant attenuation of HRV(R). Obstructive sleep apnea could be identified as the maintenance of rhythmic but attenuated variations of DeltaZ(REM) accompanied by asynchronous fluctuation of HRV(R) against DeltaZ(REM). Central hypopnea could be identified as a normal but attenuated waveform in both DeltaZ(REM) and HRV(R). A large fluctuation in HRV(L) was observed during repetitive appearances of apnea/hypopnea in the nocturnal experiments. CONCLUSION: The modified TIP together with HRV provides a superior tool for accurate and convenient definition of sleep apnea syndromes.