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
with this problem. God equipped us with these reflexes
to help survive injury and blood loss until our heart
has had a chance to recover. These reflexes were not
designed as a long term measure to support a very sick
heart. It is very important to remember this concept!.
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. The heart muscle is composed of units
known as actin and myosin. These units serve as the
contracting mechanism of the heart, very similar in
its action to spring devices that are used in a gym.
In the example shown above, the spring
would be expected to contract more vigorously if it
was stretched a little more. Conversely, the spring
would have a weaker recoil or contraction if it was
stretched less. In medical terms, this is known as Frank-Starling's
mechanism (more about this later).
Therefore, it comes as no
surprise that decreased filling of the heart would be
expected to cause 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 very 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.
First, the veins that return blood
to the heart is constricted or reduced in caliber (as
shown above). This shunts more blood to the heart.
Secondly, the kidney releases chemical
substances that causes 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 (exercise spring example)..
However, the Frank-Starling's law
also points out that increased stretch 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
looses its recoil or elasticity. You can compare this
to an exercise spring (above) 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 it. This causes fluid build up and
swelling of the feet, ankles, legs, liver, lungs, etc.,
as previously discussed.
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 (contraction)
of the arteries that supply blood to the less vital
organs, while the arteries to the brain, etc. are left
wide open.
This is shown in the example (above),
where the failing heart is represented by a pump and
different parts of the body are depicted by the colored
balloons. The clothes pins show how the tube (arteries)
to the skin and muscle is clamped down (two pins) to
a greater degree than the tube to the brain (no pins)
and other parts of the body (one pin). The clamping
down (clothes pin effect along the entire length of
the affected artery) is induced by the release of chemical
substances such as angiotensin converting enzyme (ACE)
and catecholamines (adrenaline and similar chemicals).
The above stop gap 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
or clamping down of the arteries creates a new problem
by increasing the resistance against which the failing
heart has to pump against. 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 turn into a self-destruct mechanism. The
fittest survive, 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.
