Basics of Renal Failure and Acute Kidney Injury

Acute kidney injury (AKI), previously known as acute renal failure, is a rapid reduction in kidney function that can represent a spectrum of injuries that involve failure to maintain fluid, electrolyte and acid-base balances. A variety of patients suffering from a variety of medical conditions can develop AKI. Patients with urological issues often experience AKI as a result of their disease, or occasionally treatments thereof. This blog will review the basics of AKI for urological patients.

Acute Kidney Injury Definitions

The hallmark of AKI is a rising creatinine value in the blood. Creatinine is a protein byproduct of muscle metabolism created at a constant rate by each individual and filtered by the kidneys at a constant rate. Decreases in kidney function lead to a rise in serum creatinine values (as the protein is not being released into the urine as readily). In general, a normal creatinine value for the average person is 1mg/dL and a 50% decrease in the filtering ability of the kidney results in a doubling of serum creatinine (see figure).

Campbell-Walsh Urology, 10th edition. Chapter 3: Evaluation of the Urologic Patient:
History, Physical Examination, and Urinalysis

The basic features of AKI are a rising serum creatinine and decreasing urine output. A number of stringent definitions have been created by nephrologists and other physicians to identify and risk-stratify patients with AKI. One such definition, created by the KDIGO (Kidney Disease: Improving Global Outcomes Research Group)[1] includes any of the following:

• An increase in serum creatinine of 0.3mg/dL or greater in a 48 hour period
• An increase in serum creatinine from baseline of 1.5x or greater within a week's time period
• Urine volume <0.5mL/kg/hour for 6 hours

Acute Kidney Injury Incidence

It is estimated that 2-5% of all patients admitted to a medical or surgical hospital unit have or will develop AKI. That number increases to 36% of ICU (Intensive Care Unit) patients, of whom 20% will progress to need renal replacement therapy (i.e. dialysis).[2]

For urological patients, many surgeries affect the urinary system and kidney surgeries have an important influence on short- and long-term kidney function. The incidence of AKI following partial nephrectomy (removing a portion of the kidney involving a tumor) is approximately 3.6%. However, looking at patients with normal pre-operative renal function, the incidence of AKI is less than 1%. For patients with existing renal disease, the risk of AKI rises from 6.2% (if CKD, chronic kidney disease, stage 3) to 34% (CKD stage 4).[3]

It should be mentioned that a rising creatinine after surgery is very different from a rising creatinine due to a medical illness or injury. Future blogs will address this important distinction.

 

Implications of Acute Kidney Injury 

AKI often results from a transient insult to the kidneys and will recover when that insult is corrected or treated (a future blog will cover the common causes of AKI). Most patients will recover from AKI without any residual or long-term deficits in kidney function or health. However, according to one study, an increase of 0.5mg/dL in serum creatinine can result in:

• 6.5 fold increase in odds of death
• 3.5 day increase in length of stay
• $7500 in excess hospital costs [3]

Therefore, prevention and awareness of AKI are important considerations for patients undergoing urological surgery. Stay tuned for future blogs regarding kidney failure as it relates to urologic patients.

 

 
[1] KDIGO Clinical Practice Guideline for Acute Kidney Injury. Kidney International Supplements. VOLUME 2 | ISSUE 1 | MARCH 2012. http://www.kdigo.org/clinical_practice_guidelines/pdf/KDIGO%20AKI%20Guideline.pdf

[2] Ostermann M and Chang RS. Crit Care Med 2007; 35 (8): 1837-43.
[3] Lane BR, Babineau DC, Poggio ED et al. Factors predicting renal functional outcome after partial nephrectomy. J Urol 2008; 180 (6): 2363-2369.
[4] Chertow GM et al. J Am Soc Nephrol 2005; 16 (11): 3365-3370.