Congestive Heart Failure 3

How Does the Body Respond to CHF?

When the body perceives a decrease in the pumping ability or output of the heart, specific reflexes are triggered to allow it to cope. These reflexes help the body survive injury and blood loss until the heart has had a chance to recover. However, they were not designed as a long term measures to support a very sick heart.

With blood loss, the heart is forced to work with a smaller volume of circulating blood. This smaller volume results in decreased filling of the heart and a reduction in the pumping ability of the heart. If the body suffered from an excessive reduction in the circulating volume, as with severe blood loss or even with marked dehydration, the output of the heart would be reduced so drastically that it would not be able to maintain the blood pressure of a patient. This would cause a low blood pressure or even result in shock.

The output of the heart can also be reduced when its muscle is weakened or failing. As less blood is pushed out, more blood is left behind. The body compensates for the reduced output by trying to increase its filling. Firstly, the veins that return blood to the heart are constricted or reduced in caliber This shunts more blood to the heart. Secondly, the kidney releases chemical substances that cause the body to retain sodium and water. This in turn increases the volume of circulating blood. Both of these mechanisms cause the heart to dilate or enlarge in an attempt to take advantage of Frank-Starling's Law. (The heart muscle is composed of units known as actin and myosin. These units serve as the contracting mechanism of the heart, similarly to a spring.) However, Frank-Starling's law points out that increased stretching of the elastic heart muscle increases its recoil within set limits. If these limits are exceeded (when the elastic heart muscle strips are stretched beyond a certain point), the heart muscle loses its recoil or elasticity. You can compare this to a spring that has been stretched too far and now cannot "spring back" like it used to.

Increased filling of the failing heart with associated decreased emptying causes "back-pressure" or congestion of all the parts of the body that have to return blood to the heart. This causes fluid buildup and swelling of the feet, ankles, legs, liver, lungs, etc.

If the body's need for nutrients cannot be met by the struggling heart, the system now calls upon another emergency measure which is a ration system. Blood supply to the brain and vital organs are given the highest priority, while supply to relatively less important parts of the body (skin, muscles of the arms and legs, etc.) is drastically reduced. This is made possible by selective constriction or contraction of the arteries that supply blood to the less vital organs, while the arteries to the brain, etc. are left wide open.

The clamping down is induced by the release of chemical substances such as angiotensin converting enzyme (ACE) and catecholamines (adrenaline and similar chemicals).

The above rationing system ensures adequate blood supply to the brain by reducing supply to parts of the body that have less or intermediate importance. However, the constriction of arteries creates a new problem by increasing the resistance against which the failing heart has to pump. This load makes the heart fail even further.

In summary, all the above mechanisms serve as a safety back-up systems that can temporarily increase the output of the heart. If the heart does not recover, or if it continues to weaken, all of these mechanisms result in further failure. Those who are more fit survive heart failure, while others need medications to counteract the above reflexes. Remember that these measures were designed to combat a low output of a normal heart (as caused by blood loss during an accident) and can only temporarily help a failing heart.

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