I work with left ventricular assist devices in patients with heart failure. These devices operate by maintaining a fixed speed of a centrifugal or axial rotor. The native heart continues to contract creating pressure differences between the pump inflow (heart) and outflow (aorta).
The device and the heart are in parallel by my reckoning. Volume A enters the heart. Volume B exits the aortic valve. Volume C exits the pump. Flow in the body is A= B+C. If the pump is off, flow in the body is A=B. If the aortic valve is sewn shut, the flow in the body is A=C. It is considered dogma that when flow through the device increases, device power consumption increases. The heart and the pump share the pulsatility of the heart and are subject to the same afterload in most situations.
I want to understand why power increases when pressure across the pump decreases. I understand that there will be more mass-flow through the pump with a lower delta-P across the pump. What I don't understand is why this mass-flow slows the pump, ostensibly requiring more power to overcome apparent resistance. It seems more logical that increased flow decreases energy of rotor rotation as the reduction in pressure difference would seem to have the action of imparting energy to the rotor.
Why is more power required to turn the rotor with a lower delta P across the pump and what principles/equations explain this effect?