We present a novel method which provides an observer of the autonomic cardi
ac outflow using heartbeat intervals (RR) and QT intervals. The model of th
e observer is inferred from qualitative physiological knowledge. It consist
s in a problem of blind source separation of noisy mixtures which is resolv
ed by a simple and robust algorithm. The robustness of the algorithm has be
en assessed by numerical simulations in adverse noisy environments. In clin
ical applications, we have validated the observer on subjects exposed to ex
perimental conditions known to elicit sympathetic or parasympathetic respon
se.