Pulse pressure, arterial compliance and wave reflection under differential vasoactive and mechanical loading

Similar pulse pressure increases and flow reductions have been reported by many investigators, despite dissimilar forms of arterial loading applied. Increased vascular load is most commonly observed due to mechanical and vasoactive interventions. The present study intended to differentiate the hemodynamic contributions of these two forms of arterial loading at closely matched blood pressure levels. To accomplish this, proximal aortic characteristic impedance (Z_o), total arterial compliance (C), peripheral vascular resistance (R_s) and time-domain resolved forward (P_f) and reflected (P_r) waves were obtained in six anesthetized, thoracotomized and ventilated dogs. Acute loading was accomplished by brief descending thoracic aorta (DTA) occlusion or by intravenous bolus infusion of methoxamine (MTX:5 mg/ml) Systolic pressure increases were matched to a similar extent. Results showed that pulse pressures were drastically increased, reflecting large increases in wave reflections and decreases in arterial compliances. Changes in Z_o, R_s and C were quantitatively different between the two forms of loading. DTA occlusion primarily increased Z_o and R_s with a concurrently large reduction in C. MTX infusion significantly increased small vessel R_s to the same extent as DTA occlusion, but with a slight decrease in C secondary to an increase in pressure, with Z_o unchanged. Examination of dynamic loading showed similar increases in reflection coefficients, but P_f and P_r were qualitatively different. We conclude that vasoactive methoxamine infusion provides primarily an increased resistive load, while mechanical DTA occlusion provides an increased complex load to the left ventricle. These loads also occur earlier and variably during ventricular ejection.