It's anatural phenomenan because you are using artificial Kidney for filtration and that will reduce the load on the natural kidney leading to decreae in urineoutput.

Once the kidneys see pulsatile blood flow, you probably see the output return to normal or even increase. You could employ zero balance UF during rewarming phase to continue the positive effects of UF without excessive fluid removal.

Ultrafiltration acting as additional kidney reduces tissue edema by drawiing water from both intravasclar space and also the third spaces, hence less volume left for kidney filteration

Fluid balance should be the key point. The more you get the pt negative, the more urine output will decrease.

Sure pulsatile flow will enhance perfusion, thus urine output, but in our experience fluid balance remains the main issue.

Do not use the ultrafiltration continuously. Allow the patient's kidney to do the job! Just use it when the urine output decrease during CPB and /or if the hamatocrit is less that 23, to hemoconcentrate de patient.

The continuous ultrafiltration is rather a simple filtration technique against the pressure gradient between the two fluids (dialysate and the blood) and electrolyte composition. Flow of the two fluids and contact period are the other factors to determine the removed fluid volume. So it is a plain a simple mechanical way of body fluid removal from the blood and has no biofeedback regulatory mechanism playing role in urine formation from the kidney. Thus a feeling that the urine output is reduced probably is reflection that we do not give any diuretic when ultrafiltration is applied on CPB. Even the biofeedback and urine out put from the kidney may not efficient in anesthetized patient during CPB.

As you increase the osmolarity with ultrafiltration, receptors in the hypothalamus stimulate ADH secretion which reduces diuresis in the kidneys. So you would expect to see a decrease in urine output.

I think it has nothing to do with blood volume (volemia is trapped anyway in cardiotomy reservoir) or arterial pressure or blood flow (controlled by the pump) nor osmolarity (little changes observed), I Think research should be oriented to neurohumoral factors, (elimination of natriuretic factors through filter)

I have observed that the cardiac function is better after MUF , which does eventually translate to improved overall perfusion, kidneys included.