Heart Failure: Compensation by the Heart and Body
Heart failure means that your heart muscle doesn't pump as much blood as your body needs. Because your heart cannot pump well, your heart and your body try to make up for it. This is called compensation.
Your body has a remarkable ability to compensate for heart failure. The body may do such a good job that many people don't feel symptoms in the earlier stages of heart failure. It is only when your body isn't able to compensate enough that you will begin to experience symptoms.
Compensation may help your body adjust to the effects of heart failure in the short term. But over time it can make heart failure worse by further enlarging the heart and reducing the pumping ability of the heart.
How does the body compensate?
With heart failure, the heart doesn't pump as well as it should. So your body doesn't get enough blood and oxygen. When this occurs, the body believes that there isn't enough fluid inside its vessels. The body's hormone and nervous systems try to make up for this by increasing blood pressure, holding on to salt (sodium) and water in the body, and increasing heart rate. These responses are the body's attempt to compensate for the poor blood circulation and backup of blood.
- The nervous system. If your body senses that the brain and vital organs aren't receiving enough blood, the sympathetic nervous system starts working to get more blood to your brain and organs. This system releases substances called catecholamines into the bloodstream. These substances cause the blood vessels to constrict and speed up the heart rate. At the same time, the arteries supplying the brain and vital organs widen to carry the increased blood flow.
- Hormone systems. When the body thinks it needs more fluid in its blood vessels, it releases specific chemicals (renin, angiotensin, and aldosterone) that cause the blood vessels to constrict. In addition, these hormones cause the body to retain more sodium and water. This adds fluid to your circulatory system. This fluid becomes part of the blood circulating throughout your system.
How does the heart compensate?
Your heart's goal in compensating for heart failure is to maintain your cardiac output. Cardiac output is the amount of blood your heart is able to pump in 1 minute. The problem in heart failure is that the heart isn't pumping out enough blood each time it beats (low stroke volume). To maintain your cardiac output, your heart can try to:
- Beat faster (increase your heart rate).
- Pump more blood with each beat (increase your stroke volume).
How does the heart know to beat faster? Your brain signals your heart to beat faster by sending messages to your heart's electrical system, which controls the timing of your heartbeat. When your cardiac output is low, your adrenal glands also release more norepinephrine (adrenaline), which travels in the bloodstream and stimulates your heart to beat faster. Although beating faster helps to maintain cardiac output as the stroke volume falls, a faster heart rate can be counterproductive because it allows less time for the ventricle to fill with blood after each heartbeat. Also, a very fast heart rate can itself weaken the heart muscle over time.
How does the heart increase its stroke volume? To increase its stroke volume, your heart can try to:
- Get more blood into your heart. If your left ventricle isn't doing a good job pumping blood out, your heart can try to compensate by allowing more blood to fill the ventricle before it pumps by expanding its size (dilating) to increase its volume. This form of compensation may be helpful at first, but as the heart gets bigger and bigger, there is more and more tension on the walls of the heart to pump out the blood inside it. This increases the strain on the heart, making its function worse over time.
- Pump harder. Your heart can pump harder by developing stronger, thicker muscle. This thickening of your heart muscle is called hypertrophy, and it can help your heart pump more forcefully and increase your stroke volume. But hypertrophy of the heart muscle increases the heart's need for oxygen and other nutrients. These requirements can eventually outstrip the blood supply to the heart, leading to further weakening of the heart muscle. In addition, hypertrophy of the walls of the heart can make diastolic function worse by impairing the ability of the heart to relax properly. This limits the heart's ability to fill with blood, which can also further reduce cardiac output.
What happens when your body can no longer compensate?
If your body can no longer compensate for heart failure, you will begin to have symptoms, which consist of two major types:
- Congestive symptoms, which are caused by the backup of blood into the lungs and the other organs of the body. These symptoms include shortness of breath and swelling in the ankles and abdomen.
- Low-output symptoms, which are caused by the inability of the heart to generate enough cardiac output, leading to reduced blood flow to the brain and other vital organs. These symptoms may include lightheadedness, fatigue, and low urine output. If the cardiac output is very low, this can damage organs, particularly the kidneys.
How long does it take before the body stops compensating for heart failure? Your body can compensate for heart failure for a long time, often for many years. But the duration of compensation can be extremely variable and depends on the cause of your heart failure and whether you have other medical problems.