Angiotensin converting enzyme inhibitors (ACEI) are contraindicated in bilateral renal artery stenosis. Why?
In renal artery stenosis, renal perfusion is less and hence GFR is low. In such a case, nephrons adapt the filtration by causing efferent arteriolar constriction to maintain the pressure needed for filtration.
If ACEI is given, there is dilation of efferent arteriole and renal perfusion will again decrease further and GFR will become more low. Hence, contraindicated.
In bilateral renal stenosis, the effective renal blood flow is not significantly reduced but maintained at the cost of increasing the efferent artery tone. ACEI causes inhibition of angiotensin 2, leading to efferent artery vasodilation in glomerulus. This decreases intra glomerular pressure and filtration, resulting in renal function detoriation.