Aortic impedance and compliance in hypertensive rats

We compared aortic impedance and compliance in normotensive control and hypertensive Wistar rats. Hypertension was induced by unilateral nephrectomy plus steroid and salt water administration. After at least 8 wk of sustained hypertension (tail-cuff systolic pressures greater than 172 mmHg), open-chest ascending aortic micromanometric pressures and electromagnetic flows were measured. We used a frequency-modulated pacing method to enhance the energy content of the pressure and flow signals at specific low frequencies and their multiples. Impedance spectra were calculated using both Fourier series and spectral analysis methods. Compliance was calculated from the low-frequency impedance moduli, assuming a windkessel model for the arterial system. During pentobarbital sodium anesthesia under baseline conditions, the hypertensive rats had higher total resistance (219,000 vs. 126,000 dyn.s.cm-5), higher characteristic impedance (7,334 vs. 5,377 dyn.s.cm-5), larger first zero crossing of impedance phase angle (13.9 vs. 10 Hz), larger ratio of backward to forward pressure waves (0.67 vs. 0.48), and lower compliance (0.00498 vs. 0.00720 ml/mmHg) than controls. The differences between the groups were eliminated when the blood pressures of the hypertensive rats were normalized by vasodilation with nitroprusside or when the control rats were made hypertensive by vasoconstriction with phenylephrine. Thus the hemodynamic alterations appear to be secondary to the increased blood pressure. These base-line differences and the responses to vasoactive drugs are similar to findings in humans, suggesting that this rat model is a good hemodynamic model of human hypertension.