This study will examine how heart stiffness and a weak atrium affect exercise capacity and symptoms in patients with hypertrophic cardiomyopathy (HCM). The atrium is the booster pumping chamber of the heart that helps the ventricle (main pumping chamber), to fill properly. HCM is an inherited disease in which the ventricle becomes thickened and, in some patients, stiff. The stiffness makes it difficult for the ventricle to fill and empty, causing breathing difficulty, fatigue, and reduced exercise capacity. Scar formation and a weakened atrium can cause the heart to stiffen. Information gained from this study may guide doctors in prescribing medicines to reduce scarring or improve atrial function. Patients 21 years of age and older with hypertrophic cardiomyopathy may be eligible for this study. Candidates will be screened with a medical history and physical examination, electrocardiogram (EKG), blood tests, Holter monitor, and echocardiogram. A Holter monitor is a device about the size of a Walkman that is connected to three wires that are attached to the chest. It is worn for 24 hours to provide continuous monitoring of heart rhythm. An echocardiogram uses a small probe that emits sound waves to produce images of the heart. The probe is moved across the chest and the reflection of the sound waves from the chambers of the heart produce images showing the heart's thickness and function. Participants will undergo the following tests and procedures over 3 days: * Physical examination and echocardiogram. * Intravenous cannula insertion: A plastic tube is inserted into an arm vein for collecting blood samples to measure substances that the heart and circulatory system release at rest and during exercise. * Impedance cardiography: A small current of electricity is passed across the chest and electrodes similar to those used for an EKG test are placed to measure blood flow in the area of the current. * Pulmonary artery catheterization: A catheter (plastic tube) is inserted into a vein either in the arm, under the collarbone, or in the neck and advanced to the right atrium and ventricle. The catheter remains in place during the echocardiogram tilt and bicycle exercise tests (see below). * Echocardiogram tilt test: The patient lies flat on a table. After a few minutes, the table is tilted so that the patient's head is just above his or her feet for a short while, then is positioned flat again, and then tilted so the feet are just above the head. Echocardiographic measurements and blood samples are taken at intervals to examine heart function during changes in posture. * Echocardiogram bicycle stress test: The patient exercises for as long as possible on a bicycle-like machine while lying on his or her back. Echocardiographic measurements and blood samples are taken at intervals during the test. * Treadmill stress test: The patient runs for as long as possible on a treadmill that increases in difficulty. The patient wears a facemask or mouthpiece through which small amounts of gases are added in order to measure the ability of the heart and lung to increase their effectiveness with exercise. * Digoxin loading: Only patients who demonstrate limited exercise capacity and for whom digoxin is not a risk will undergo this procedure. A medicine that makes the heart contract more strongly, digoxin is used to treat certain heart abnormalities. Patients are given doses of either digoxin or placebo (a look-alike injection with no active ingredient) at 4-hour intervals over a 24-hour period and then repeat the tilt test and the bicycle and treadmill exercise tests
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