From the Guidelines
Renal function typically shows signs of decreased perfusion within 24 to 48 hours after a significant reduction in blood flow to the kidneys, as evidenced by rising serum creatinine levels, which is a standard marker of kidney function 1. This timeframe is supported by the most recent and highest quality study, which defines acute kidney injury (AKI) as an increase in serum creatinine by ≥0.3 mg/dL within 48 hours or is ≥50% from baseline 1.
Key Points to Consider
- The kidneys are particularly vulnerable to perfusion changes because they normally receive about 20-25% of cardiac output despite comprising only 0.5% of total body weight.
- When perfusion drops below a critical threshold, the kidneys cannot maintain glomerular filtration rate (GFR), leading to reduced urine output (oliguria) and accumulation of waste products in the blood.
- Early detection is crucial, which is why monitoring urine output and serum creatinine is standard practice in critically ill patients.
- Restoration of adequate perfusion within this timeframe can often reverse early kidney injury, but prolonged hypoperfusion may lead to acute tubular necrosis and more lasting damage.
Important Considerations for Clinical Practice
- AKI can progress to more severe forms with oliguria and need for renal replacement therapy, emphasizing the need for early detection and intervention.
- The definition of AKI relies on biomarker change from an established baseline, and true risk of renal decline must take into account baseline renal function, which is most commonly defined using eGFR formulae 1.
- Chronic kidney disease (CKD) is common, affecting 10% of the world population, and is defined as an abnormality of kidney structure or function, present for >3 months, with health consequences 1.
From the Research
Renal Function and Perfusion
- The time it takes for renal function to show loss of perfusion is not explicitly stated in the provided studies.
- However, it is mentioned that acute kidney injury can be characterized by an abrupt deterioration in kidney function, manifested by an increase in serum creatinine level with or without reduced urine output 2.
- The studies suggest that renal perfusion is crucial, and its assessment is necessary and prioritized in shock resuscitation 3.
- Renal function can be affected by various factors, including neurohormonal factors, functional changes, and decreased renal blood flow or perfusion pressure 4.
- The effects of decreased renal perfusion on renal function can be seen in changes in urine output, serum creatinine levels, and electrolyte imbalances 5, 6, 2.
Factors Affecting Renal Perfusion
- Decreased renal blood flow or perfusion pressure can lead to a decrease in glomerular filtration rate and urine output 4.
- Neurohormonal factors and functional changes can also influence diuresis and natriuresis in critically ill patients 4.
- The use of certain medications, such as dopamine and fenoldopam, can affect renal perfusion and function 5, 6.
- Critical-care ultrasonography can provide the best evaluation of renal perfusion 3.
Assessment and Management
- The diagnostic evaluation of acute kidney injury includes a patient history, physical examination, laboratory tests, and ultrasonography of the kidneys 2.
- Management of acute kidney injury involves fluid resuscitation, avoidance of nephrotoxic medications and contrast media exposure, and correction of electrolyte imbalances 2.
- Renal replacement therapy (dialysis) may be indicated in severe cases of acute kidney injury 2.