Download ABOUT THE HEART

Appendix B
ABOUT THE HEART
The heart must maintain a steady rhythm in order to pump properly. Excessively slow
heart rhythms cause the cardiac output to be insufficient, and this can cause symptoms of
fatigue, weakness, lightheadedness, fainting, shortness of breath, and fluid accumulation
(i.e. "congestive heart failure"). On the other hand, if the heart beats too quickly it doesn't
get enough time in between beats to fill up with blood. As a result, only a small amount
of blood gets squirted out with each beat, and the cardiac output drops significantly. In
order to understand this, imagine trying to put out a large fire with a shot glass. You can
fill this tiny glass very quickly and splash the fire with lots of little drops, but it's much
more efficient to fill a large bucket and douse that fire with only a couple of trips to the
faucet. Rapid heart rhythms decrease the efficiency of the heart's pumping action, and
this can cause the same symptoms as a slow heart rhythm, namely, weakness, shortness
of breath, and fainting. In addition, rapid heart rhythms can cause a symptom known as
"palpitations."
The word "palpitation" refers to any abnormal or inappropriate feeling in the chest caused
by the heart's beating. It's a generic term, and people can even feel palpitations when their
heart is beating normally. Some palpitations can feel like a fluttering, a racing, or a
pounding in the chest. Others may feel like a heavy or hollow feeling in the chest, or a
feeling of a "skipped" or "missed" beat. For some people they can even be perceived as
painful. Sometimes palpitations make you feel like you can't catch your breath.
Palpitations can be brief - only a fraction of a second, like when the heart "skips a beat."
Then again, palpitations can last for several minutes or even hours. Sometimes, brief
flutter (skipped beat) type palpitations may occur over and over every few seconds. This
can cause an irregular pulse that can last for hours and can be very annoying.
You are probably wondering where all these abnormal rhythms come from and why the
heart sometimes beats too fast or too slow. Arrhythmias result from the electrical signal
of the heart getting fouled up by a "wiring" problem. Sometimes the signal gets blocked
by a "loose connection" and the heart doesn't beat when it is supposed to (because the
muscle is not getting the signal). This can cause slow heart rhythms. Sometimes there is a
"short circuit" in the wiring which causes the electrical signal to take an abnormal route
or to get stuck traveling around and around in a circular pathway. This problem can force
the heart to beat rapidly. Sometimes, an abnormal signal will arise from somewhere in the
heart and cause the electrical system of the heart to fire prematurely. This is what causes
a premature beat. There are many different kinds of rhythm problems caused by many
different electrical disturbances. Some cause fast beats and some cause slow ones. Each
one of these problems may require different treatment, and some may not require
treatment at all. One medication may be good for one kind of rhythm problem and bad
for another. Therefore, it is important for your doctor to determine the exact type of
rhythm problem that you may have.
THE CARDIAC CYCLE
Let's take a deeper, more detailed look at how the heart beats, and what controls the
rhythm. The various and complex set of events that occur with each heart beat are
referred to as the "cardiac cycle." As mentioned in the previous section, the top chambers
of the heart are called the atria and the bottom chambers (that do most of the work of
pumping) are called the ventricles. The job of the atria is to receive blood from the veins
and store it prior to each heart beat. The left atrium receives blood from the lungs via four
separate pulmonary veins, and the right atrium receives blood from the main body veins
called the "vena cava" (the superior vena cava carries blood from the head, upper chest
and arms, while the inferior vena cava carries blood from the lower chest, abdomen and
legs). The normal heart beat begins in the top chambers. Both atria contract and push the
stored blood across a pair of one-way valves into the ventricles, to help fill them. The
valves are known as atrio-ventricular (AV) valves and include the mitral valve located
between the left atrium and the left ventricle, and the tricuspid valve which separates the
right atrium from the right ventricle. A split second after the atria contract, the ventricles
begin to contract. As the pressure in the ventricles increases, the AV valves close (to
prevent blood from flowing backwards). The sound made by the AV valves closing is
known as the first heart sound (the lub of "lub-dub"). With continued contraction of the
ventricles the blood is ejected out the arteries though another pair of valves - the aortic
and the pulmonic valves (on the left and right sides of the heart respectively). At the end
of the cardiac cycle, all the chambers relax, the aortic and pulmonic valves close (making
the dub sound) which keeps blood from flowing backwards into the heart, and the atria
begin to fill up with blood again to start the next cycle.