Glomerular filtration and tubular activity are decreased during hypothermic cardiopulmonary bypass (CPB). The role of vasoconstriction to explain these changes is not known. The calcium entry blocking drug felodipine dilates constricted arterioles and reduces renal vascular resistance during noncardiac surgery. The present study was initiated to evaluate the effects of felodipine on renal perfusion and function during hypothermic, low pressure CPB. Twenty-two male patients (aged 61.7 +/- 2.3 y) were included in a prospective, randomized, controlled study. Renal blood flow was measured with thermodilution technique; renal extraction of 51Cr-EDTA and p-aminohippurate (PAH) were used to evaluate glomerular and tubular function. Systemic blood flow during hypothermic CPB was varied experimentally between 1.45 and 2.2 L.min-1.m-2. Felodipine reduced systemic vascular resistance but did not reduce the total renal vascular resistance during CPB. On the contrary, renal vascular resistance was increased at low CPB flow rates. The extraction of PAH (signifying tubular activity) was higher during felodipine infusion (0.74 +/- 0.04 vs 0.64 +/- 0.03 during low CPB flow, and 0.64 +/- 0.03 vs 0.57 +/- 0.05 during high CPB flow), whereas 51Cr-EDTA extraction was not influenced. The mechanism of enhanced PAH extraction may involve reduced regional vasoconstriction in PAH-extracting areas.
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