Our hearts are vital pumps. Just like any pump they work best at the pressures for which they are designed. But unlike most mechanical pumps, the heart does adapt to working under pressures for which it was not necessarily designed. It does this by thickening to accomplish work at a higher pressure, just as other muscles in our body thicken when we are forcing them to do heavy labor on a chronic basis. Just think of the thick and heavy arms of the weight lifter. But there is a problem with this adaptive mechanism. The thicker the muscle becomes, the more oxygen it needs to sustain itself, and this is a limited commodity for heart muscle. As the oxygen supply is outstripped by the heart's demand, problems inevitably occur. The heart begins to stiffen, abnormal heart rhythms become more common, the chambers dilate and ultimately the heart begins to fail. Excess thickening of the left ventricle is called hypertrophy (hyper = too much trophy = growth) and can increase the possibility of sudden death.
Excessive thickening of the heart is related to two general mechanisms. The first is a need to generate high pressure either due to hypertension, or a mechanical blockage (stenosis of a blood vessel or valve). Through a complex mechanism of chemical interactions the body is able to stimulate muscle growth dependant upon its need. In the case of essential hypertension (the most common variety of high blood pressure), the walls of the heart thicken to provide for the chronic pressure overload. This is accomplished by series of chemicals called growth factors (local growth hormones). Some people produce more of these then others, some are more resistant to their actions for many reasons, some of which are genetic. Thus, some people with very high blood pressure will have hearts that have not thickened, others with more modest blood pressure will have responded by more thickening of the muscle. Generally, when the need for higher pressures is relieved (such as in the replacement of a mechanical blockage, i.e. aortic valve replacement) the process is reversed, and the heart thickness returns toward more normal. This is also what we normally expect to happen with the control of high blood pressure.
The second mechanism is totally due to heredity. There are probably several genes responsible for this propensity to increase the thickness of the heart. In each, the heart responds to local growth hormones to thicken. In some, it is likely due to an excess of locally produced growth hormones, in others due to an oversensitivity of the body to normal amounts of such hormones causing the muscle to thicken. The most common example of hereditary causes for heart muscle thickening is what we call hypertrophic cardiomyopathy (other names for the same problem are HOCM: hypertrophic obstructive cardiomyopathy or IHSS idiopathic hypertrophic subaortic stenosis); another less common problem is called Fabry's disease. Although these problems have a genetic cause they are medically treatable conditions.