Heart disease is a leading cause of poor health and death worldwide.

One of the lesser-known consequences of heart disease is its impact on kidney function. The kidneys are essential organs responsible for filtering waste products and excess fluid from the body. They also regulate blood pressure, produce hormones that control red blood cell production and maintain electrolyte balance.

When the heart is healthy, it pumps blood efficiently, providing the kidneys with the necessary oxygen and nutrients to function adequately. However, in people with heart disease, this physiology is compromised, which can have negative effects on kidney function. This decrease in kidney performance that is directly related to heart dysfunction is known as “cardio-renal syndrome”. Studies have shown that as many as 40 per cent of patients with cardiac disease, notably those with heart failure, have some degree of kidney dysfunction.

How does the heart exactly impact the kidneys?

The mechanisms by which heart disease leads to kidney dysfunction are complex and multifaceted.

Shared atherosclerosis risk factors

High blood pressure and diabetes are very common conditions, and constitute major risk factors for heart and kidney diseases. In fact, in the western hemisphere, they are the two leading causes of cardiac disease and kidney failure! Other risk factors include smoking, obesity and high cholesterol levels. All these risk factors generate atherosclerosis or plaque build-up in the vessels, leading to vascular narrowing or occlusion, and consequent coronary and renal ischemia, with ultimate dysfunction of both organs.

Heart failure

In this disorder, the pumping function of the heart is compromised, leading to less kidney perfusion. In addition, the sympathetic nervous system known as RAAS is on overdrive. These two mechanisms can cause the kidneys to suffer and their function to decline transiently or permanently.

Atrial fibrillation

It can lead to inefficient heart-pumping performance, with a similar outcome as above, but also to clot development in the heart, then showering of these clots to the kidneys, killing a portion of the renal tissue and leading to function decline.

Cardiac drugs

Many medications used to treat cardiac disease have a negative effect on the kidney function. Their usage would require a fine balance between their benefits and side-effects, and a close co-ordination between the heart and kidney specialists.

Cardiovascular procedures

Many invasive cardiac and vascular interventions and surgeries take a significant toll on the kidneys, more so the ones that require a transient drop in circulation such as open-heart surgery or aorta repair, or those where a contrast dye is used since the latter can damage the kidney cells, such as in a cardiac catheterisation or an ablation.

Healthy kidneys help to maintain an environment that allows the heart to work optimally. However, as kidneys become diseased, a number of changes occur that promote the development of cardiovascular disease. Studies have shown that patients with significant kidney disease are more than three times more likely to die from cardiovascular disease than someone without kidney disease.

How exactly do the kidneys affect the heart?

Diseased kidneys can lead to abnormal heart function in a number of ways:

Anaemia

The kidneys produce a hormone called erythropoietin which stimulates the bone marrow to produce more red blood cells. When the kidneys become damaged, they produce less erythropoietin and, therefore, the bone marrow produces fewer red blood cells. Also, the build-up of waste products caused by kidney failure shortens the survival of the red blood cells that have already been produced. This results in anaemia (low blood haemoglobin level) which means that the blood is less efficient at delivering oxygen to the organs and tissues around the body. This includes the coronary vessels in the heart and so can lead to worsening angina and heart failure, and can increase the risk of heart attacks.

Increased renin levels

The kidneys also produce a hormone called renin which is important for blood pressure control. Damaged kidneys often produce too much renin, which can cause a significant increase in blood pressure. High blood pressure is an important cause of angina, heart attacks and heart failure.

Decreased salt and water clearance

One of the main jobs of the kidneys is to regulate the amount of salt and water in the body. When the kidneys are damaged salt and water builds up in the body; this is often first noticed as swelling around the ankles. It also causes an increase in the blood pressure and can also “overload” the heart, resulting in heart failure, angina and heart attacks.

Abnormal calcium and phosphate levels

Kidney disease results in the loss of the normal balance of calcium and phosphate levels in the blood. Over time this can lead to a particular type of bone disease but it has also been shown to cause a build-up of calcium in the walls of blood vessels including the coronary arteries. This can directly lead to angina and heart attacks but in addition, the calcified blood vessels become very stiff, which causes the blood pressure to rise, further increasing the risk.

High homocysteine levels

Damaged kidneys cannot remove extra homocysteine, an amino acid in the blood. Homocysteine is a potent risk factor for cardiovascular disease, and high levels of homocysteine can lead to coronary and cerebrovascular diseases.

In many patients, the interplay between the heart and kidneys can be quite complicated, setting the stage for a vicious cycle of negative feedbacks where one organ’s decline harms further the other and vice versa. However, trials have conclusively shown that early detection and aggressive treatment of cardiac and kidney diseases along with tight control of their risk factors may markedly slow the progression of both conditions.

• Peter Topham is a consultant nephrologist at the Bermuda Hospitals Board. He is also co-editor of the textbook Oxford Desk Reference: Nephrology. Joseph Yammine is a consultant cardiologist at the Bermuda Hospitals Board. The information herein is not intended as medical advice nor as a substitute for professional medical opinion. Always seek the advice of your physician.