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The authors analyzed by spectral techniques the cardiovascular variability signals recorded in a patient carrying a left ventricular assist device. This device, while constituting a simplified model of the cardiovascular function, allows the accurate recording of beat-by-beat stroke volume. The authors recorded 8 min of the native heart RR intervals, the pump cycle duration (PCD), the stroke volume (SV), the systolic and diastolic blood pressure (SP, DP), the instantaneous lung volume (ILV) and the arterial O/sub 2/ saturation (SpO/sub 2/). The authors found a dominant respiratory peak (0.32 Hz, HF peak) in the SV and PCD while SP, DP, RR and LV showed both the HF peak and a spectral component around 0.02 Hz (VLF peak). Finally, SpO/sub 2/ was characterized only by the VLF peak. The coherence was high (>0.8) between all signals at the HF peak and from moderate (0.5-0.6) to high, depending on the signals considered at the VLF peak. The presence of a VLF oscillation in the SP and DP but not in the SV nor in the PCD supported the hypothesis that this pressure oscillation has not simply a mechanical origin due to the venous return to the heart but is caused by a modulation of the vasomotor tone.