A baroreflex model of short term blood pressure and heart rate variability.

Parameters based on heart rate variability alone or on both heart rate and blood pressure variability (baroreflex sensitivity) are becoming increasingly clinical relevant. Nevertheless a complete insight in physiological mechanisms underlying these parameters is still lacking. A computer model may help to fill up some of the gaps. We present a model which consists of a simple beat-to-beat hemodynamic part (Starling heart and Windkessel) linked to a detailed continuous modelled neural control part. The intermediate between continuous and beat-to-beat part is an integral pulse frequency modulator (IPFM) acting as cardiac pacemaker. Input for the IPFM is a "sympathovagal" balance signal, with different dynamics for sympathetic and vagal branches. Low-frequency variability is supposed to arise from resonance of existing noise, while high-frequency variability (respiration) is assumed to enter the closed loop at the hemodynamic (blood pressure) site. Results of three studies have been used for validation: (1) spontaneous variability in heart rate and blood pressure (baroreflex transfer functions), (2) vagal blockade with atropine, (3) a modified Valsalva manoeuvre performed in normal and quadriplegic man. Steady state as well as dynamic properties of the model reasonably well fitted to these experimental data.