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.

Afterload reduction has been considered to be of primary importance in the treatment of hypertensive patients. It has not been clear how the central aorta and the peripheral vasculature interact to bring this about. The current investigation examines this aspect in terms of wave reflections in open-chest anaesthetized dogs. Results show that nitroprusside effectively reduced the increased wave reflection from hypertension induced by methoxamine administration.

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Myocardial ischemia, the condition in which an insufficient supply of oxygen to the heart muscle occurs due to a decrease in coronary blood flow, damages the mechanical function of the LV and may impair pulse transmission. To investigate this, the power associated with the forward (Wf) and reflected (Wr) waves were computed for the normal and ischemia-compromised left ventricle. Results show that power generated by the ischemic heart is decreased as expected, but the pulsatile transmission of pressure and flow is little affected. I "Hydraulic power in the vascular system is made up of a steady, non-pulsatile term and an oscillatory term [ l ] W t = w s + WO (1) with the steady power equal to the product of mean pressure and mean flow, and the oscillatory term defined in terms of the pulsatile flow (Qn) and the input impedance (Zn) harmonics,This concept has been applied to the systemic circulation by many investigators, especially in studying left ventricular energetics in disease states such as congestive heart failure [2] and hypertension [3,4]. By separating pressure and flow waves into their forward and reflected components the pulsatile power can be resolved into its components with respect to wave reflections, i.e the power that could be delivered in a reflectionless system (with the forward wave) and the power dissipated due to the actual state of the arterial system (with the reflected wave). Thus the power associated with a reduced-energy state such as myocardial ischemia and the effect of this decreased power on pulse wave transmission to the arterial system can be more effectively studied. METHoDSAortic flow, pressure and the standard lead I1 ECG were simultaneously measured in anaesthetized dogs. The data were recorded on an FM recorder (Vetter C-4) and on a paper recorder. Steady state recordings were made during control and after approximately one hour of mechanical occlusion of the left anterior descending (LAD) coronary artery Measured pressure and flow were digitized at 10 msec intervals. The characteristic impedance (20) was estimated in the time domain during the first 60 msec of systole as before [3,41where AF' is the instantaneous aortic pressure above end diastole and Q is the instantaneous flow. Compliance (C) and peripheral resistance (Rs) were determined in the time domain as before [4]. Measured pressure (P) and flow (Q) waves were separated into their forward (Pf, Qf) and reflected (Pr, Qr) components. Fourier analysis was performed on all the pressure and flow waveforms. In-phase pulsatile power was defined by Wn = I Pn I I Qn I COS 8 (4)where n represents the harmonic value and 8 the phase angle.Similarly, the power associated with the forward pulse wave can be expressed asand the power dissipated along the transmission path due to reflections can be considered as Wm = I Pm I I Q n I COS (e m + Q fn) (6)Only the power contained in the first three harmonics, which accounts for more than 90% of total power, are presented.