Effects of vasoconstriction and vasodilatation on LV and segmental circulatory energetics

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Abstract
Although the hydraulic work generated by the left ventricle (LV) is not disputed, how the work was dissipated through the systemic circulation is still subject to interpretation. Recently, we proposed that the systemic circulation should be considered as waves and a reservoir system (Wk). By combining the arterial and venous reservoirs, the systemic vascular resistance can be viewed as a series of resistors, which in sequence are the large-artery resistance, arterial reservoir resistance, the microcirculatory resistance, venous reservoir resistance, and large-vein resistance, and propelling blood through these resistance elements represents resistive losses. We then studied the changes in the fraction of the work consumed by each element when infusing methoxamine (MTX), a vasoconstrictor, and sodium nitroprusside (NP), a vasodilator. Results show that, under control condition, ∼50% of the LV stroke work was dissipated through arterial reservoir resistance (NP, ∼36%; MTX, ∼27%), another ∼25% was dissipated by the microcirculation (NP, ∼20%; MTX, ∼66%), and ∼20% of work by the large-artery resistances (NP, ∼37%; MTX, ∼6%). The energy dissipated by the venous resistances was small and had limited variation with NP and MTX, where the large-vein and venous reservoir resistances shared ∼1 and ∼3% of LV stroke work, respectively. Approximately 60% of LV stroke work is stored as the potential energy during systole under control, and the ratio decreases to ∼45% with NP and ∼80% with MTX.