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In humans, the heart is composed of four chambers: The upper chambers are
called the atria (singular: atrium) and the lower two are called the
ventricles. The blood flows into the right atrium, goes through a valve into
the right ventricle, and then gets pumped from the ventricle to the lungs
where it picks up oxygen and unloads carbon dioxide, flows back to the left
atrium, down into the left ventricle, and then the left ventricle pumps the
blood into the body. The heart is supposed to pump in a slow steady rhythm.
"Tachycardia" means the heart is beating faster than it is supposed to.
"Supraventricular" means there is an abnormal electrical signal being
generated from an area of the heart that is above the ventricle (i.e. the
atrium). The result is that the heart is pumping inefficiently because the
chambers are not relaxing in between pumps and are not contracting strongly
and are not filling completely before pumping out the blood. Therefore,
there is less blood and less oxygen with each pump than there should be.
Sometimes, people get lightheaded because there is not as much oxygen going
to the brain. In humans, supraventricular tachycardia is often a chronic
condition and people live normal lives. Sometimes, a human may be given
medications or a pacemaker to regulate the heart rate. This is not a
possibility for cats but dogs can get pacemakers. Cardizem is a medication
that is normally used for humans.

I would get a second opinion. I don't have any personal experience with
supraventricular tachycardia (in my cat), but I do have experience with
cardiac hypertrophy. My cat has been on Cardizem (diltiazem) in the past and
will be going on it again. That having been said, my vet has told me that at
least in the case of cardiac hypertrophy, there is no scientific evidence to
demonstrate that (in cats), the diltiazem either halts disease progression
or prevents a crisis (it used to be thought that it would prevent a crisis).
My cat is nearly 17 years old, has had a heart murmur and cardiac
hypertrophy since the age of 2, and has not been on any heart medication for
most of that time. I am not a veterinarian, and I would never presume to
advise someone about his cat's medication, I am just relating my experience.
My cat doesn't have supraventricular hypertrophy. But if I were you, I would
get a second opinion and ask the vet what are the benefits of the Cardizem
(diltiazem) in supraventricular tachycardia and is there actual scientific
evidence that the Cardizem returns the heart to a normal rhythm *in cats*.
(I would presume the Cardizem does help to return the heart to a normal
rhythm *in humans.*)
Good luck.
Best regards,
---Cindy S.