Shunting is the abnormal flow of blood from one part of the circulation to another, i.e. a "leak" (Table 4.06).
The shunt bypasses a portion of the circulation. Shunts may be normal or abnormal. Shunts exhibit several characteristics, all of which have been used in their diagnosis: reduction in perfusion pressure, alteration of blood oxygen content and circulation times, non-laminar or turbulent blood flow leading to murmurs, and resulting in compensatory heart and vessel growth (i.e. hypertrophy).
Left to right shunting allows blood to move from the left side of the heart to the right side of the heart (Table 4.07). It usually involves right ventricle enlargement (hypertrophy), pulmonary hyper-tension, and normal arterial oxygen content (acyanosis). Depending upon severity, this can lead to tricuspid incompetence and right heart failure.
In contrast, right to left shunting causes cyanosis and eventually, polycythemia. It also involves right ventricle and pulmonary congestion. It may also involve left ventricle enlargement and mitral valve incompetence.
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