Renal Pathophysiology

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RENAL PATHOPHYSIOLOGY: THE ESSENTIALS

The following clinical case highlights how a careful review of the clinical course can identify the etiology of AKI. An otherwise healthy patient who had severe herpes zoster infection was being treated with intravenous acyclovir, a drug that can precipitate in the tubules if an adequate urine flow rate is not maintained. The plasma creatinine concentration began to rise rapidly on day 6; no cause other than acyclovir was apparent even though the patient had been receiving this drug for 6 days. A careful review of the chart revealed that intravenous fluids had been discontinued on day 4. Oral intake and therefore urine output were relatively low on the ensuing days, thereby creat ing an environment favoring acyclovir precipitation.

What finding on urinalysis would be definitive for establishing the diagnosis of acyclovir toxicity? If this was not seen, what other test might confirm the diagnosis?

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Exclusion of Urinary Tract Obstruction Unless the diagnosis is clear from the history or urinalysis, urinary tract ob struction should always be ruled out because it is a relatively common (espe cially in men) and rapidly reversible cause of AKI. An enlarged bladder with urethral obstruction caused by prostatic disease can often be palpated, and passage of a catheter into the bladder will both relieve the obstruction and improve renal function. Obstruction at the level of the ureters can be detected only by a radio logic procedure such as ultrasonography. This should reveal dilation of the renal pelvises and calyces due to obstructing lesion. However, early in the course of obstruction, these radiologic changes may not be apparent. Glo merular filtration is significantly reduced when the hydraulic pressure in Bowman space (increased from distal obstruction to urine flow) exceeds the glomerular capillary hydraulic pressure (see Chapter 1, Eq. 1). Again, unilat eral obstruction will not manifest with a severe decline in GFR due to com pensation from the unobstructed kidney.

A patient with AKI has a urine output of 1,500 mL/day. Does the relatively normal urine output exclude the diagnosis of urinary tract obstruction?

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Urinalysis The urinalysis may reveal findings that are suggestive of a particular type of disease in patients with AKI (see Table 8.2 and Plates 8.1 and 8.2): „ Red cells (particularly if dysmorphic in shape), red cell casts, and protein uria are virtually diagnostic of glomerulonephritis or vasculitis.

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