The kidney-heart connection

More than one in seven adults have chronic kidney disease, which means their kidneys aren't working as well as they should. Yet many of them aren't aware of the problem. Early-stage kidney disease often has no symptoms, but the condition slowly and silently worsens over time — and is closely connected to cardiovascular disease.

The two most common causes of chronic kidney disease — high blood pressure and diabetes — are also leading risk factors for heart disease. "The underlying causes and the treatment approaches for kidney and heart disease often overlap," says Dr. Martina McGrath, a transplant nephrologist at Harvard-affiliated Brigham and Women's Hospital. In fact, most people with chronic kidney disease will die of heart disease before they develop kidney failure and require dialysis, (the use of a machine to do the kidneys' job of filtering the blood), she adds.

Often, people don't experience symptoms until the kidneys start to fail. This usually doesn't occur until kidney function drops below 15%, which typically takes years. The resulting accumulation of waste products and fluid in the body can lead to loss of energy, trouble concentrating, poor appetite, swollen feet and ankles, and more frequent urination, especially at night.

A vicious cycle

The kidneys — bean-shaped organs about the size of a fist — sit just below your rib cage on either side of your spine. They play an essential role in regulating blood pressure by controlling fluid and sodium levels in the circulation. When blood pressure rises, the kidneys move water and sodium out of the bloodstream and into the urine for excretion. This brings pressure back down by stimulating fluid loss through urination, which lowers the volume of circulating blood. By contrast, when blood pressure falls, the kidneys retain water and sodium to conserve blood volume and raise pressure.

People with high blood pressure tend to retain too much water and sodium. This puts added stress on the circulation within the kidneys, scarring and weakening blood vessels. "It can become a vicious cycle, because kidney disease drives high blood pressure in itself, and uncontrolled high blood pressure is a cause of progressive kidney failure," says Dr. McGrath.

Likewise, prolonged exposure to high blood sugar, as occurs in diabetes, causes the membranes of tiny blood vessels known as capillaries to thicken. This damages and distorts the capillaries within the kidneys, reducing their filtering capacity.

Detecting kidney disease

Most people know their blood pressure and cholesterol values, but few are familiar with the tests used to assess kidney health. They include these:

Serum creatinine. Creatinine, which is released from muscle cells into the bloodstream, is one of the substances the kidneys filter from the blood. If kidney function falters, blood creatinine levels rise. Levels greater than 1.2 milligrams per deciliter (mg/dL) in women or 1.4 mg/dL in men may suggest early kidney disease.

Glomerular filtration rate (GFR). This test uses the serum creatinine level (along with age and gender) to estimate how well your kidneys are working. A value below 60 is a sign that the kidneys are not working properly.

Urine protein tests. Protein in the urine is an early sign of kidney disease, which can be detected with a simple test using chemically treated paper dipped in a urine sample. A more sensitive test, the urine albumin-to-creatinine ratio (UACR), can detect very small amounts of the protein albumin, a condition known as microalbuminuria. A UACR above 30 milligrams per gram (mg/g) may suggest kidney disease. "Having small amounts of protein in your urine is an important signal that you're at higher risk of cardiovascular disease," says Dr. McGrath.

Who should be tested?

People with cardiovascular disease, diabetes, and high blood pressure face the highest risk for chronic kidney disease and should be checked at least once a year. Other conditions that increase the risk are obesity, chronic viral infections such as HIV and hepatitis C, autoimmune diseases such as lupus, and cancer. In addition, certain medications have been linked to kidney damage. People who take daily, high doses of non-steroidal anti-inflammatory drugs (NSAIDs) — such as ibuprofen (Motrin) and naproxen (Aleve) — are especially vulnerable to kidney injury.

Slowing kidney disease

"The main thing I emphasize to my patients is that everything we know about a heart-healthy lifestyle, especially a low-sodium diet, is also good for your kidneys," says Dr. McGrath. If you have diabetes and high blood pressure, doctors usually prescribe blood pressure drugs known as ACE inhibitors or ARBs. These medications help slow the progression of kidney disease in people with diabetes. Use a home blood pressure monitor to make sure your blood pressure is well controlled, she advises.

A newer class of medications originally designed to treat diabetes, called SGLT2 inhibitors, work by blocking your kidneys from reabsorbing sugar back into your body. They help prevent kidney damage even in people who do not have diabetes.

In addition to lowering blood pressure and promoting weight loss, these drugs also have cardiovascular benefits, especially in people with heart failure. They include canagliflozin (Invokana), dapagliflozin (Farxiga), empagliflozin (Jardiance), and ertugliflozin (Steglatro). Another medication, finerenone (Kerendia), also helps slow the progression of diabetic kidney disease and prevents heart-related complications.

Damaged kidneys might not clear medications from the body efficiently, so if you have chronic kidney disease, it's very important to follow your doctor's dosing instructions for all prescribed medications. And always check with your doctor before taking any over-the-counter pain relievers and herbal supplements, since some may harm kidney function.

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