Increasing renal blood flow: Low-dose dopamine or medium-dose norepinephrine

Background and objectives: Many clinicians believe that low-dose dopamine (LDD) [2 μg/kg/min] increases renal blood flow (RBF) and medium-dose norepinephrine (MD-NE) [0.4 μg/kg/min] decreases RBF. They also believe that MD-NE might induce mesenteric and/or coronary ischemia. In fact, the effects of these drugs on renal and vital organ blood flow are poorly understood. The aim of this study was to compare the effects of 6 h of IV LDD and MD-NE infusion on mammalian renal, coronary, mesenteric, and sagittal blood flow. Design: Randomized, controlled, experimental animal study. Setting: Animal laboratory of tertiary physiology institute. Subjects: Seven Merino cross sheep were studied. Measurements and results: We performed a staged insertion of transit-time flow probes around ascending aorta, sagittal sinus and circumflex coronary, superior mesenteric, and left renal arteries. We then randomized these animal with long-term embedded flow probes to either 6 h of placebo (saline solution) or drugs (MD-NE at 0.4 μg/kg/min or LDD at 2 μg/kg/min), and performed continuous measurement of systemic pressures, cardiac output (CO), and flow to vital organs. We also sampled blood and urine for the measurement of lactate, creatinine, and creatinine clearances at preset intervals. Results: Compared to placebo, LDD did not affect systemic hemodynamics. However, it increased mean RBF by 20% (267.3 ± 87.6 mL/min vs 222.0 ± 74.4 mL/min, p = 0.028) without a detectable effect on other vital regional circulations. MD-NE, however, increased mean arterial pressure (101.0 ± 8.3 mL/min vs 84.2 ± 5.2 mL/min, p = 0.018) [mean ± SD] and CO (4.93 ± 1.45 L/min vs 3.81 ± 0.57 L/min, p = 0.028). It also increased coronary blood flow (36.0 ± 15.7 mL/min vs 23.0 ± 10.7 mL/min, p = 0.018) and RBF (286.5 ± 79.0 mL/min vs 222.0 ± 74.4 mL/min, p = 0.018). MD-NE had no detectable effect on mesenteric or sagittal sinus flow. LDD infusion increased urine output, but did not change creatinine clearance. MD-NE infusion increased urine output significantly more than LDD but not creatinine clearance, Conclusions: Both LDD (2 μg/kg/min) and MD-NE (0.4 μg/kg/min) increased RBF and urine output. However, the effect of MD-NE was more pronounced. LDD did not affect other vital organ flows, but MD-NE increased coronary blood flow without any changes in mesenteric and sagittal sinus blood flow.