Mannitol Increases Renal Blood Flow and Maintains Filtration Fraction and Oxygenation in Postoperative Acute Kidney Injury: A Prospective Interventional Study

INTRODUCTION:

Acute kidney injury (AKI), which is a major complication after cardiovascular surgery, is associated with significant morbidity and mortality. Diuretic agents are frequently used to improve urine output, and to facilitate fluid management in these patients. Mannitol, an osmotic diuretic, is used in the perioperative setting in the belief that it exerts reno-protective properties. In a recent study on uncomplicated post-cardiac surgery patients with normal renal function, mannitol increased glomerular filtration rate (GFR), possibly by a de-swelling effect on tubular cells. Furthermore, experimental studies have previously shown that renal ischemia causes an endothelial cell injury and dysfunction followed by endothelial cell oedema. We studied the effects of mannitol on renal blood flow (RBF), glomerular filtration rate (GFR), renal oxygen consumption (RVO2) and extraction (RO2Ex) in early, ischaemic AKI after cardiac surgery.

METHODS:

Eleven patients with AKI were studied during propofol sedation and mechanical ventilation 2-6 days after complicated cardiac surgery. All patients had severe heart failure treated with one (100%) or two (73%) inotropic agents and intra-aortic balloon pump (36%). Systemic haemodynamics were measured by a pulmonary artery catheter. RBF and renal filtration fraction (FF) were measured by the renal vein thermo-dilution technique and by renal extraction of chromium-51- ethylenediaminetetraacetic acid (51Cr-EDTA), respectively. GFR was calculated as the product of FF and renal plasma flow RBF x (1-hematocrit). RVO2 and RO2Ex were calculated from arterial and renal vein blood samples according to standard formulae. After control measurements, a bolus dose of mannitol 225 mg/kg, was given followed by an infusion at a rate of 75 mg/kg/h for two 30-minute periods.

RESULTS:

Mannitol did not affect cardiac index or cardiac filling pressures. Mannitol increased urine flow by 61% (P<0.001). This was accompanied by a 12% increase in RBF (P<0.05) and 13% decrease in renal vascular resistance (P<0.05). Mannitol increased the RBF/cardiac output (CO) relationship (P=0.040). Mannitol caused no significant changes in RO2Ext or renal FF.

CONCLUSIONS:

Mannitol treatment of postoperative AKI induces a renal vasodilation and redistributes systemic blood flow to the kidneys. Mannitol does not affect filtration fraction or renal oxygenation, suggestive of balanced increases in perfusion/filtration and oxygen demand/supply.