Renal Artery Scintigraphy

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CHAPTER 108 Renal Artery Scintigraphy

Renovascular hypertension (RVH) secondary to renal artery stenosis (RAS) can be a challenging diagnosis to make. The presence of RAS does not necessarily imply RVH. There are many radiologic methods of investigating RAS, including Doppler ultrasound, computed tomography, magnetic resonance imaging, nuclear renography, and angiography. Most imaging modalities assess renal artery stenosis, but the functional information provided by nuclear renography augmented with an angiotensin-converting enzyme inhibitor (ACEI) allows for the detection of renovascular hypertension.

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.

DESCRIPTION OF THE PROCEDURE

The initial patient screening should include a thorough patient history that includes listing of all medications and a review of any relevant imaging studies. Whenever possible, patients who have a history of chronic use of ACEI and diuretics should have these medications held prior to the study.

Oral captopril or intravenous enalaprilat may be used, although captopril is used more commonly. The oral captopril dose is 25 to 50 mg in adults, and 1 mg/kg in children, with a maximum dose of 50 mg. The captopril tablet is generally crushed and dissolved in water to ensure better gastric absorption. Peak systemic concentrations of captopril occur around 60 minutes after administration and then begin to decrease; therefore, the radiopharmaceutical is injected 60 minutes following captopril administration. Patients receiving oral captopril should avoid solid foods for at least 4 hours prior to administration to aid gastric absorption, although normal hydration should be continued.

Enalaprilat is administered intravenously over 3 to 5 minutes, with a dose of 40 µg/kg and a maximum dose of 2.5 mg. The radiopharmaceutical may be given 15 minutes following enalaprilat administration and the intravenous line should be maintained, because enalaprilat administration may be associated with more significant hypotension. The use of enalaprilat avoids the problem of uneven or incomplete gastric absorption, which could be an issue with captopril. In addition, the shorter waiting period prior to injection may also lead to a shorter study duration than when using oral captopril. Although enalaprilat and captopril are both thought to be suitable for the evaluation of RVH, enalaprilat is associated with a greater risk of hypotension. Captopril is generally more widely used and therefore will be discussed in this chapter.

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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