What are the alterations in urine output during Cardiopulmonary Bypass (CPB)?

From the Guidelines

Urine output during cardiopulmonary bypass (CPB) typically decreases due to multiple physiological alterations, and maintaining a normal urine output of 0.5-1 ml/kg/hr is crucial to prevent acute kidney injury (AKI), which occurs in approximately 30% of cardiac surgery patients 1. To manage decreased urine output, diuretics like furosemide (10-20mg IV) or mannitol (0.25-0.5 g/kg) can be administered. Furosemide works by inhibiting sodium reabsorption in the loop of Henle, while mannitol creates an osmotic gradient that pulls fluid into the tubular lumen. Some key points to consider in managing urine output during CPB include:

• Maintaining adequate mean arterial pressure at 50-70 mmHg to ensure renal perfusion 1
• Avoiding excessive hemodilution (hematocrit <21%) to prevent compromising oxygen delivery 1
• Minimizing hypothermia to reduce its negative effects on renal blood flow and glomerular filtration rate 1
• Considering the use of ultrafiltration to remove excessive fluids, but avoiding excessive ultrafiltration (>30 ml kg−1) to prevent postoperative AKI 1
• Monitoring urine output closely post-CPB, as renal function typically improves with restoration of pulsatile flow 1 It is essential to maintain optimal perfusion pressures and oxygen delivery to prevent AKI, and the most recent guidelines recommend against excessive ultrafiltration during CPB 1.

From the Research

Alterations in Urine Output During Cardiopulmonary Bypass (CPB)

• Urine output during CPB has been found to be associated with renal function and can be used as a predictor of postoperative acute kidney injury (AKI) 2.
• A study found that the amount of urine output during CPB had a biphasic association with the incidence of AKI, with 4 mL/kg/h being a boundary value 2.
• Low-dose dopamine infusion during CPB has been shown to increase water and sodium excretion, possibly due to a renal vasodilator effect 3.
• Urine output during CPB has been found to correlate with mean arterial pressure (MAP) rather than perfusion flow 4.
• Significant differences in urine output have been observed before CPB, with dopamine seeming to partially revert renal vasoconstriction 5.

Factors Influencing Urine Output During CPB

• Mean arterial pressure (MAP) has been found to correlate with urine flow rate during CPB 4.
• Perfusion flow has not been found to correlate with urine flow rate during CPB 4.
• Dopamine infusion has been shown to increase urine output during CPB 5, 3.
• The use of pulsatile or nonpulsatile perfusion during CPB has not been found to influence urine output 6.

Clinical Implications

• Monitoring urine output during CPB may be useful in predicting postoperative AKI 2.
• Maintaining adequate MAP during CPB may be important for preserving renal function 4.
• The use of dopamine infusion during CPB may be beneficial in increasing urine output and preserving renal function 5, 3.