CHAPTER 108 Renal Artery Scintigraphy
In patients with a hemodynamically significant reduction in renal artery caliber, there is a reduction in renal perfusion pressure distal to the stenosis. This results in the activation of the renin-angiotensin-aldosterone system, whereby renin is released from the juxtaglomerular apparatus, and a cascade of events occurs that ultimately leads to peripheral vasoconstriction, blood volume increase, and an elevation in blood pressure. Notably, renin converts angiotensinogen to angiotensin I and angiotensin I is then converted to angiotensin II through a process that requires ACE. In the kidney, angiotensin II results in the preferential constriction of efferent arterioles, which raises the pressure gradient across the glomerular capillary membrane and maintains the glomerular filtration rate (GFR). In patients with RVH, the administration of an ACEI blocks the conversion of angiotensin I to angiotensin II, thereby lowering the degree of vasoconstriction in the efferent arterioles, dropping the transcapillary pressures, and resulting in decreased GFR. There is also an increase in the creatinine level, which is the most common reason for pursuing a diagnosis of renal vascular disease. Decreased GFR can be assessed using nuclear scintigraphy and is the underlying mechanism for the diagnosis of RVH by ACEI scintigraphy.1
TECHNIQUES
Indications
ACEI renography can be used to assess for the presence of renovascular hypertension caused by renal artery stenosis. It is most cost-effective when used in a patient population with a high prevalence of RVH.2 Many imaging modalities exist for the evaluation of renal artery stenosis, but renography may be particularly useful for those with known contrast allergy and in whom assessment of functional significance of a stenotic vessel is desired. The success of this technique is based on the inhibition of the conversion of angiotensin I to angiotensin II following the administration of an ACEI, with subsequent reduction in the GFR, and consequently a change in the radiopharmaceutical pattern of uptake and clearance in comparison to a baseline study.
Contraindications
Patients receiving an ACEI can experience significant hypotension; administration of an ACEI warrants proper clinical monitoring. Blood pressure should be carefully measured, with a baseline blood pressure established prior to the administration of the ACEI; subsequent blood pressure measurements should be performed at 5- to 15-minute intervals following drug administration and prior to discharge. Patients should not be discharged home unless their blood pressure is at least 70% of their baseline blood pressure. Chronic ACEI use can reduce the sensitivity of the study and a short-acting ACEI should be held for 3 days before the study and a longer acting ACEI should be held for 5 to 7 days. Chronic diuretic use may be associated with dehydration, leading to an increased risk of hypotension when the ACEI is administered; it should therefore be stopped a few days before the study. Calcium channel blockers have been reported to cause false-positive results and cessation prior to the study should also be considered.3 Many patients referred for this study will be at high risk for cardiovascular disease, however, and if hypertension is severe, antihypertensive use can be maintained with the understanding that there may be a mild reduction in the overall sensitivity of the study.
DESCRIPTION OF THE PROCEDURE
Patients should be appropriately hydrated, because dehydration can affect renal perfusion curves.4 Patients with renin-dependent RVH may experience a drop in systolic blood pressure when the ACEI is administered, a drop that may be more severe if the patient is dehydrated. Hydration will also help decrease radiation dose to the bladder wall5
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