Beat-to-beat estimation of windkessel model parameters in conscious rats

A windkessel model was applied on a beat-to-beat basis to evaluate the arterial mechanical characteristics in seven conscious rats. Ascending aortic arterial pressure (AP) and blood flow were recorded during steady-state in basal conditions, during infusions of isoprenaline, sodium nitroprusside, and phenylephrine, and after intravenous atenolol injection. For each cardiac cycle the exponential decay time constant (tau) was estimated from the aortic AP curve, peripheral resistances (R) were taken as the ratio of mean AP to cardiac output, and systemic arterial compliance (C) was calculated as tau/R. In all conditions, mean correlation coefficients of the exponential regression and approximately 70% of values in each rat were > 0.99, demonstrating the model validity. In all conditions tau and C exhibited a large spontaneous variability over time, and beat-to-beat correlations were high between tau and C (0.83 +/- 0.03). C was increased by sodium nitroprusside, decreased by isoprenaline, but not significantly decreased by phenylephrine [5.1 +/- 0.2, 3.2 +/- 0.3, and 3.9 +/- 0.2 microliters/mmHg, respectively, vs. 4.2 +/- 0.3 microliters/mmHg (baseline)]. In conclusion, the windkessel model enables tau and C to be reliably estimated in conscious rats during spontaneous and drug-induced hemodynamic variations.