Beat-to-beat determination of peripheral resistance and arterial compliance during +Gz centrifugation.

This study focused on the problem of describing changes in total peripheral resistance (TPR) and systemic arterial compliance (SAC) under time-varying +Gz acceleration stress. Nonsteady-state measures of peripheral resistance can only be derived when arterial compliance is taken into account. We have developed a successful analytical model to track simultaneous changes in peripheral resistance and systemic arterial compliance during non-stationary periods of increased gravitational load on a beat-to-beat basis. Using a 2-element windkessel model, aortic flow into an input node was defined as equal to the sum of a capacitative (Cao) and a resistive (Rarterial) flow leaving the node such that: Iao = Caod(Pao - Ppleural)/dt + (Pao - Pra)/Rarterial We made the assumption that Cao and Rarterial were constant over a cardiac cycle, and divided the pressure and flow signals for each beat of a record into two different intervals, integrating this equation over each, giving two equations in two unknowns. Cao and Rarterial were then obtained from the matrix solutions. To test the model, we used recordings from chronically instrumented baboons subjected to a 10 s rapid onset +Gz (head-to-foot) stress. Beat-to-beat calculations of peripheral resistance and systemic arterial compliance from our model were compared to values obtained from a previously reported 3-element wind-kessel model.