In conclusion there was a definite difference in pathologic findings between the pulmonary hypertension due to ventilatory dysfunction and the postcapillary pulmonary hypertension due to heart disease. In the cases of the pulmonary hypertension due to lung diseases it takes longer for organic changes in the pulmonary vesseles, but, once the changes occur, they become, in most cases, irreversible and its critical level is also lower (Fig. 6). It can be considered almost hopeless when the pulmonary arterial pressure is more than 30 mmHg. In contrast, the organic changes occur quite rapidly in the postcapillary pulmonary hypertension with the severe congestion and thickning of alveolar walls and/or a high pulmonary arterial pressure.The ratio of pulmonary to systemic resistance, the cardiac index, histological changes of pulmonary arteioles at biopsy, and secondary changes of the alveolar wall seem to be quite useful in judging the reversibility of the pulmonary hypertension and in establishing indication for the operability of heart and lung surgery in severs cases in addition to the general surgical judgment.
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