Acute kidney injury (AKI) has no uniform criteria, but is commonly defined as an increase in serum creatinine concentration by at least 25% from baseline. It occurs in 30% of patients following cardiac surgery, and at least 50% of patients with underlying renal insufficiency. Patients who have a reduced creatinine clearance pre-operatively are at the greatest risk of developing post-operative AKI. The purpose of the current study is to determine if intravenous hydration with either isotonic saline or sodium bicarbonate 150 mEq/L is effective at preventing post-operative AKI in patients with baseline kidney insufficiency and who are undergoing cardiac surgery using cardiopulmonary bypass. The study hypothesis is that an infusion of sodium bicarbonate 150 mEq/L will be more effective than isotonic saline in reducing the incidence of post-operative AKI in cardiac surgery patients with a preoperative glomerular filtration rate (GFR) less than 60 ml/min/1.73m2.