In renal artery stenosis, what effect do angiotensin-converting enzyme (ACE) inhibitors have on glomerular filtration rate (GFR)?

Effect of ACE Inhibitors in Renal Artery Stenosis

In renal artery stenosis, angiotensin-converting enzyme (ACE) inhibitors will result in a decrease in glomerular filtration rate (GFR). This effect occurs due to the specific hemodynamic changes that ACE inhibitors induce in the stenotic kidney 1.

Pathophysiology of ACE Inhibitor Effects in Renal Artery Stenosis

The mechanism behind this decrease in GFR involves several key factors:

1. Normal Compensatory Mechanisms: In renal artery stenosis, the kidney maintains filtration through angiotensin II-dependent efferent arteriolar vasoconstriction 1. This compensatory mechanism helps maintain glomerular hydrostatic pressure and GFR despite reduced renal perfusion.

2. ACE Inhibitor Interference: ACE inhibitors block the formation of angiotensin II, which leads to:

   • Dilation of the efferent arteriole (more than the afferent arteriole)
   • Reduction in glomerular capillary hydrostatic pressure
   • Consequent decrease in filtration pressure and GFR 1

3. Hemodynamic Consequences: When ACE inhibitors are administered in renal artery stenosis:

   • Blood is shunted from the afferent arteriole to the efferent arteriole
   • Inadequate hydrostatic pressure to maintain filtration occurs
   • GFR falls while renal blood flow changes very little
   • Filtration fraction decreases 1

Clinical Implications

The decrease in GFR with ACE inhibitors in renal artery stenosis can manifest as:

• Acute renal failure, especially in patients with:

  • Bilateral renal artery stenosis
  • Stenosis of a solitary functioning kidney
  • High-grade stenosis 1

• Significant azotemia (defined as >50% rise in serum creatinine) that persists or worsens after correction of hypoperfusion states 1

• This effect is more pronounced in patients with:

  • Volume depletion from diuretic therapy
  • Preexisting hypotension
  • Low cardiac filling pressures 1

Diagnostic Value

The GFR reduction following ACE inhibitor administration can actually serve as a diagnostic clue for renal artery stenosis:

• A significant decrease in GFR after ACE inhibitor administration suggests hemodynamically significant renal artery stenosis 1
• This principle is used in captopril renal scintigraphy to diagnose renovascular hypertension 2
• Studies show that captopril induces a more pronounced decrease in GFR in hypertensive patients with renal artery stenosis compared to those without stenosis 2

Important Distinctions

• While ACE inhibitors cause efferent arteriolar dilation, they do not cause afferent arteriolar constriction (option D is incorrect)
• They do not increase GFR in renal artery stenosis (option B is incorrect)
• They primarily dilate the efferent arteriole, not constrict it (option E is incorrect)
• They do not primarily cause afferent arteriolar dilation in this context (option C is incorrect)

Understanding this mechanism is crucial for clinical practice as it explains why monitoring renal function after initiating ACE inhibitors is essential, particularly in patients at risk for renal artery stenosis.