Renovascular HTN
Renovascular hypertension is a form of secondary high blood pressure caused by narrowed renal (kidney) arteries. This condition can affect one kidney (unilateral) or both kidneys (bilateral). On auscultation, you are able to appreciate a bruit.
How Kidney Size Changes with Arterial Narrowing
When renovascular hypertension is unilateral, you'll notice an important physical change: the kidney receiving blood from the narrowed artery becomes smaller and more atrophic compared to the healthy kidney. This happens because reduced blood flow leads to tissue shrinkage over time.
In bilateral cases, where both renal arteries are narrowed, both kidneys will show signs of atrophy and become smaller than normal. This bilateral involvement often makes the condition more challenging to diagnose since there's no "normal" kidney for comparison.
The Body's Misguided Response
Here's where renovascular hypertension becomes particularly problematic. When blood flow to the kidneys decreases due to arterial narrowing, the kidneys interpret this as a sign that the body needs higher blood pressure. In response, they release hormones designed to raise blood pressure throughout the body.
However, this creates a frustrating cycle: the real problem isn't low blood volume (hypovolemia) but rather the physical blockage in the renal arteries. So while the body's compensatory mechanism successfully raises blood pressure, it does absolutely nothing to address the underlying arterial stenosis. This is why renovascular hypertension can be so persistent and difficult to control with standard blood pressure medications alone.
The Renin-Angiotensin-Aldosterone System in Action
The kidney's response follows a well-established hormonal pathway called the renin-angiotensin-aldosterone system (RAAS):
- Renin release: The affected kidney releases renin into the bloodstream
- Angiotensin I formation: Renin converts angiotensinogen (a protein made by the liver) into angiotensin I
- Lung conversion: Angiotensin I travels to the lungs, where an enzyme called ACE (angiotensin-converting enzyme) transforms it into the more potent angiotensin II
- Dual blood pressure effect: Angiotensin II raises blood pressure through two mechanisms:
- Vasoconstriction: It directly narrows blood vessels throughout the body.
- Aldosterone release: It triggers the release of aldosterone, which increases sodium reabsorption (and potassium excretion) in the kidneys, leading to fluid retention.
Both of these effects work together to significantly elevate blood pressure, often to dangerous levels.
Common Underlying Causes
Two conditions account for the majority of renovascular hypertension cases:
Atherosclerosis: This involves the buildup of fatty plaques in the renal arteries, similar to what happens in heart disease. It's more common in older adults and those with cardiovascular risk factors.
Fibromuscular dysplasia: This is a condition where the artery walls develop abnormal cellular growth, leading to narrowing. It tends to affect younger patients, particularly women, and often has a distinctive "string of beads" appearance on imaging studies.