Urine Biochemistry in the Early Postoperative Period after Cardiac Surgery: Role in Acute Kidney Injury Monitoring

We have recently suggested that sequential urine electrolyte measurement in critically ill patients may be useful in monitoring kidney function. Cardiac surgery is one of the leading causes of acute kidney injury (AKI) in the intensive care unit (ICU). In this paper, we describe the sequential behavior of urine electrolytes in three patients in the early (first 60？hours) postoperative period after cardiac surgery according to AKI status: no AKI, transient AKI, and persistent AKI. We have found that the patient with no AKI had stable and high concentrations of sodium (NaU) and chloride (ClU) in sequential spot samples of urine. AKI development was characterized in the other two patients by decreases in NaU and ClU, which have started early after ICU admission. Transient AKI was marked by also transient and less severe decreases in NaU and ClU. Persistent AKI was marked by the less favorable clinical course with abrupt and prolonged declines in NaU and ClU values. These electrolytes in urine had a behavior like a “mirror image” in comparison with that of serum creatinine. We suggest that sequential urine electrolytes are useful in monitoring acute kidney injury development in the early postoperative period after cardiac surgery. 1. Introduction Acute kidney injury (AKI) is frequent among patients undergoing cardiac surgery [1, 2]. It seems to be an independent risk factor for increased intensive care and hospital mortality [3]. Serum creatinine level and urine output are still the cornerstones for AKI diagnosis in all settings, including postoperative AKI. Urine biochemistry, although a major tool in AKI diagnosis and management in the past, is nowadays considered not useful [4] especially due to evidence showing its dissociation from renal hemodynamics [5]. However, sequential evaluation of urine electrolytes (basically, sodium, potassium, and chloride) in the course of early postoperative period has never, to our knowledge, been performed. We have recently observed that alterations in the concentration of these electrolytes measured in spot urine samples may be related to kidney function and AKI development, sometimes preceding elevations in serum creatinine [6, 7]. In this paper, we report the sequential behavior of urine electrolytes in a 60？h period in three patients after undergoing cardiac surgery. 2. Case Presentation We will briefly present the 3 cases separately. In all cases, there was no previous history of kidney disease, and the surgical procedure consisted in on-pump coronary artery bypass graft (CABG), and all patients had their serum