One of mine just got diagnosed with the same (thanks Phil for the advice)
but the vet had no problem in draining the all fluid from his chest. He is
on Hypercard 10mg 3 times a day now for the rest of his life and he was back
to his normal self after a couple of days of pills, just as Phil advised to
me.

Worf hasn't lost any weight and it also came on pretty quickly but I imagine
the external symptoms were much to do with the fluid in his chest
compressing his lungs and making him wheeze. He's even getting used to
getting 'pilled' 3 times a day. :)

Did your vet prescribe a diuretic such as furosemide (a/k/a Lasix) to
control the fluid (pulmonary edema)?  If not, you might want to ask him
about it. Furosemide should help your cat breathe easier by getting rid of
fluid in and around your cat's heart and lungs .  After the intital fluid
resolves, your vet should instruct you to taper the dose to just enough to
control edema.  You want to keep the dose as low as possible and make sure
your cat stays well hydrated-- cats can become dehydrated quickly when
taking furosemide.  Also, if your vet does prescribe a diuretic, be sure to
ask him about a potassium supplement.  Diuretics causes rapid urine
production which in turn increases potassium excretion.

Most vet cardiologists that I know or have read use Diltiazem (Cardizem,
Dilicor) as the first-choice drug for cats with HCM. Among the other
numerous benefits of diltiazem- it might also reduce the risk of aortic
thromboembolism (saddle thrombus) which is the most devastating complication
associated with HCM in cats. Diltiazem also reverses hypertrophy in the left
atrium in some cats.  So, you might want to ask your vet about it-- or seek
as second opinion from a veterinary cardiologist.

Enacard is an ACE inhibitor and would be a good choice if your cat is in
CHF. Enacard also helps reduce diuretic-induced potassium depletion.

My best advice would be to have an echocardiogram done- Electrocardiograms
(EKG/ECG) can't distinguish the different forms of cardiomyopathy- or
hyperthyroidism from cardiomyopathy.  An echo will also let you know the
severity of your cat's condition- and if he actually has some form of
cardiomyopathy.

Best of luck,

Phil