Hemodynamic fluctuations and baroreflex sensitivity in humans: a beat-to-beat model

Abstract

A beat-to-beat model of the cardiovascular system is developed to study the spontaneous short-term variability in arterial blood pressure (BP) and heart rate (HR) data from humans at rest. The model consists of a set of difference equations representing the following mechanisms: 1) control of HR and peripheral resistance by the baroreflex, 2) Windkessel properties of the systemic arterial tree, 3) contractile properties of the myocardium (Starling's law and restitution), and 4) mechanical effects of respiration on BP. The model is tested by comparing power spectra and cross spectra of simulated data from the model with spectra of actual data from resting subjects. To make spectra from simulated data and from actual data tally, it must be assumed that respiratory sinus arrhythmia at rest is caused by the conversion of respiratory BP variability into HR variability by the fast, vagally mediated baroreflex. The so-called 10-s rhythm in HR and BP appears as a resonance phenomenon due to the delay in the sympathetic control loop of the baroreflex. The simulated response of the model to an imposed increase of BP is shown to correspond with the BP and HR response in patients after administration of a BP-increasing drug, such as phenylephrine. It is concluded that the model correctly describes a number of important features of the cardiovascular system. Mathematical properties of the difference-equation model are discussed.

Validity and variability of xBRS: instantaneous cardiac baroreflex sensitivity

“…If one considers the baroreflex to be the main underlying factor for HRV (DeBoer et al. 1987), the same should be found here. And indeed, as Figure 7 shows, this also holds for xBRS: longer heart periods go along with higher xBRS.…”

Blood pressure control and correlation dimension

“…Model Design: The beat-to-beat model consists of a set of difference equations [1,3]. These equations describe the temporal evolution of the following state variables: systolic pressure (S), diastolic pressure (D), RR intervals (I), and arterial decay time constant (T).…”

“…The diastolic pressure (D i ) of each beat is expressed in terms of the previous beat parameters (denoted X i-1 ), in accordance with the Windkessel properties of the systemic arterial tree [1]:…”

“…al. [1]. Though the original model was based on nonlinear equations, it was studied after linearization.…”

“…The sigmoidal nature of the baroreceptors sensitivity is accounted for by defining an effective systolic pressure (S') (3) [1], upon which the control over the heart rate (4) and peripheral resistance (6) depends.…”