Cardiomyopathy means “heart muscle disease,” so cardiomyopathy is a broad term used to describe a variety of issues that result from disease of the myocardium, or heart muscle.
When cardiomyopathy develops as a way to compensate for some other underlying disease, like hypertension or valve diseases, it’s called secondary cardiomyopathy; when it develops all by itself, it’s called primary cardiomyopathy.
Now, hypertrophic cardiomyopathy is when the walls get thick, heavy, and hypercontractile; essentially, the muscles grow a lot larger because new sarcomeres are being added parallel to existing ones.
Usually, the left ventricle is affected, and in most cases, this muscle growth is asymmetrical, meaning that the interventricular septum grows larger relative to the free wall.
These larger muscles do two things: the walls take up more room, so less blood is able to fill the ventricle; and they become more stiff and less compliant, so they can’t stretch out as much, again, leading to less filling. When the ventricles don’t fill as much, they don’t pump out as much blood, and so stroke volume goes down. Thus, the heart can fail to pump enough blood to the body; this is called heart failure. Because this is due to a dysfunction in filling, which happens during diastole, this is a type of diastolic heart failure.
In some patients, the muscle growth of the interventricular septum essentially gets in the way of the left ventricular outflow tract during systole, or ventricular contraction. This increases blood velocity through the smaller opening, and pulls the anterior leaflet of the mitral valve toward the septum. This is called the venturi effect, which further obstructs the left ventricular outflow tract. For this reason, hypertrophic cardiomyopathy is sometimes called hypertrophic obstructive cardiomyopathy.