Renal Pathophysiology

CHAPTER 11 Acute Kidney Injury

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decreased phosphate excretion and reduced calcitriol production) and ane mia (due largely to less erythropoietin secretion—see Chapter 12). Early Renal Disease In addition to reflecting the loss of functioning renal mass, a reduced GFR may be one of the only signs of mild-to-moderate or even severe renal disease. For example, a patient with a GFR of 40 mL/min (roughly 40% of normal) may have no edema, normal plasma sodium and potassium concentrations, and a normal hematocrit. Only an elevated plasma creatinine concentration and possibly an abnormal urinalysis may point to the presence of underlying renal disease.

CHAPTER 11 Acute Kidney Injury

Sodium and potassium balance can be maintained (ie, urinary excretion equals intake) even in some patients who have a GFR < 20 mL/min. How might these adaptations occur?

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For reasons that are not well understood, the intrarenal adaptations that allow the maintenance of fluid and electrolyte homeostasis are more likely to occur with chronic (long-standing) renal disease. At the same reduction in GFR, patients with AKI are more likely to develop edema, hyponatremia, and hyperkalemia due to sodium, water, and potassium retention, respectively. The amount of intake as well as reduced excretion determines the likelihood

of these problems being seen. Acute Kidney Injury

The definitions of AKI are somewhat arbitrary and differ slightly. One widely used definition is (1) an increase in the plasma creatinine concentration of ≥ 0.3 mg/dL within 48 hours, (2) an increase in creatinine of ≥ 1.5 times base line creatinine within 7 days, or (3) urine volume of < 0.5 mL/kg/h for 6 hours. Although a 0.3-mg/dL elevation in creatinine is numerically small, it usually represents a large fall in GFR when the baseline plasma creatinine concentra tion is < 1.5 mg/dL (see Chapter 1 for a discussion of the relationship between the GFR and the plasma creatinine concentration). In comparison, large increases in the plasma creatinine concentration ( > 1 mg/dL) represent relatively small reductions in GFR in patients with ad vanced renal disease who begin with a low GFR. Consider a patient with underlying renal disease and a baseline plasma creatinine concentration of 4 mg/dL, which reflects a GFR of 20 mL/min. Assuming no change in creatinine secretion, what is the approximate new GFR if the plasma creatinine concentration rises to 6 mg/dL the day after surgery? How are the results different if the plasma creatinine concentration remains at this level for several days? 2

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