The risk is a sudden fall in filtration pressure: renal artery narrowing reduces blood flow into the kidney, so filtration may depend on angiotensin II constricting the efferent arteriole—the vessel carrying blood away from the glomerulus NICE CKS. An ACE inhibitor removes that compensatory constriction; pressure within the glomerulus falls, lowering the glomerular filtration rate and potentially causing acute kidney injury NICE CKS.
Why bilateral disease matters: if only one kidney is affected, the other may maintain overall kidney function, whereas narrowing affecting both kidneys or a solitary functioning kidney leaves no unaffected kidney to compensate NICE CKS. This is a haemodynamic effect of reduced kidney perfusion, and it may improve when the cause is addressed NICE CKS.