DOES LOW-FREQUENCY VARIABILITY OF HEART PERIOD REFLECT A SPECIFIC PARASYMPATHETIC MECHANISM

Low frequency (LF, approximate to 0.1 Hz) spontaneous oscillations of heart period in humans have been attributed to and correlated with the sympathetic efferent control of the heart. However, this interpretati on is controversial, because sympathetic blockade does not suppress th ese oscillations, while parasympathetic blockade strongly affects them . The sympathetic origin of LF of arterial pressure, on the contrary. has been convincingly demonstrated. Four 10 min cycle-by-cycle time se ries of R-R interval (RR), and systolic (SAP) and diastolic (DAP) arte rial pressure were produced by automatic analysis of data obtained wit h non-invasive methods in 10 healthy humans during supine rest and whi le standing, both before and after beta(1)-selective blockade (atenolo l). Time series were analysed by autoregressive transfer function anal ysis. beta-blockade failed to induce systematic changes on the power o f the LF peak of RR, in any condition. The coherence between RR and SA P ill the same region remained high (0.77 +/- 0.03) and a constantly n egative phase (approximate to 50-60 degrees, corresponding to a delay of 1-2 heart beats of RR on SAP) was always seen. beta-blockade decrea sed the power of the LF peak of SAP, increased the transfer function g ain between SAP and RR at LF, and the HF power of RR. We conclude that LF oscillations of RR are not directly generated by the sympathetic d rive to the heart but reflect mainly the parasympathetic activity, The results suggest that the LF oscillations of the vagal outflow, and of RR, are generated by the baroreceptor reflex, driven by sympathetical ly-induced blood pressure LF waves.