Effects of mannitol alone and mannitol plus furosemide on renal oxygen consumption,blood flow and glomerular filtration after cardiac surgery

Objective

Imbalance of the renal medullary oxygen supply/demand relationship can cause hypoxic medullary damage and ischaemic acute renal failure (ARF). The use of mannitol for prophylaxis/treatment of clinical ischaemic ARF is controversial and the effect of mannitol on renal oxygenation in man has not yet been investigated. We evaluated the effects of mannitol on renal oxygen consumption (RVO₂)_, renal blood flow (RBF) and glomerular filtration rate (GFR) in postoperative patients.

Design

Prospective interventional study.

Setting

University hospital cardiothoracic ICU.

Patients

Ten uncomplicated mechanically ventilated and sedated postcardiac surgery patients with preoperatively normal renal function.

Interventions

Mannitol infusion (225 mg/kg + 75 mg/kg/h) and combined mannitol and furosemide infusion (0.25 mg/kg + 0.25 mg/kg/h).

Measurements and results

Systemic haemodynamics were evaluated by a pulmonary artery catheter. RBF and GFR were measured by the renal vein thermodilution technique and by renal extraction of ⁵¹Cr–EDTA, respectively. Mannitol increased urine flow (60%), GFR (20%) and filtration fraction (FF) (20%) with no change in RBF. This was accompanied by an increase in renal sodium reabsorption (18%), RVO₂ (19%) and renal oxygen extraction (21%). When combined with mannitol, furosemide normalised sodium reabsorption, RVO₂, renal oxygen extraction with no change in RBF, while GFR and FF were still elevated compared to control.

Conclusions

In patients with normal renal function, mannitol increases GFR, which increases tubular sodium load, sodium reabsorption and RVO₂ after cardiac surgery. The lack of effect on RBF, indicates that mannitol impairs the renal oxygen supply/demand relationship. Furosemide normalised renal oxygenation when combined with mannitol.