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The amount of work that the right ventricle needs to do to pump blood through the lungs is mostly determined by the pulmonary artery pressure. Since the right ventricle is "designed" to pump at low pressure, if it needs to pump at significantly higher pressure it can have a difficult time pumping an adequate amount of blood through the lungs. It also can become weakened if the pressure is high enough for a long period of time. If the right ventricle becomes sufficiently weakened, symptoms of "congestive heart failure" may develop: very limited capacity for exercise, breathlessness with little exertion, and fluid retention.
Fortunately, as it turns out, the right ventricle can adapt to increased pressure, at least up to a point. The right ventricle becomes thicker than normal, allowing it to do more work than normal. To the right are two pictures of the heart, obtained using an MRI scanner. The one on the left shows a normal right ventricle; note how much thinner the wall of the right ventricle is compared to the left ventricle (since the left ventricle normally pumps blood at much higher pressure). On the right, with pulmonary hypertension the right ventricle pumps blood at much higher pressure than normal, and has become as thick as the left ventricle.
Because the right ventricle's capacity to adapt to increased pressure, mild or even moderate elevations in pulmonary arterial pressure may cause few or no symptoms or problems. However, because the right ventricle's capacity to adapt to higher than normal pressure is limited, and because pulmonary artery hypertension may increase with time, it is important to diagnose and treat this problem early on.
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