The effect of distal pulmonary obstruction or elevated atrial pressures on pulmonary flow and pulmonary vascular disease: a mathematical flow circuit analogy model-based analysis.

BACKGROUND: Late presentation of large left to right shunts is a major problem in developing countries and evaluating the operability is a challenge during preoperative evaluation. The factors that determine the development and progression of the pulmonary vascular disease are still largely unknown, though there is evidence to show that increased pulmonary flow is the main contributing factor. We have found cases of operability despite late presentation of ventricular septal defect (VSD). In some of these cases, the reason for operability may lie in associations. We herein attempt to provide a mathematical explanation for their operability. AIMS AND
OBJECTIVES: To assess the effect of different levels of obstruction and elevated ventricular end-diastolic pressure on pulmonary flow.
MATERIALS AND METHODS: Mathematical flow analogy models were constructed and analysed based on Ohm's law and the laws governing flow in series and parallel flow circuits.
RESULTS: Any narrowing in the pulmonary circulation produces a reduction in pulmonary blood flow and the pulmonary to systemic blood flow ratio, irrespective of its position. This increases the systemic flow and blood pressure if the ventricular performance remains constant. A rise in ventricular end-diastolic pressure reduces the total cardiac output, pulmonary flow and systemic blood flow with a preserved pulmonary to systemic flow ratio. The absolute reduction in pulmonary flow will exceed the absolute reduction in systemic flow, with its ratio being in proportion to the ratio of systemic to pulmonary vascular resistance. However, a higher pressure will be transmitted distally in these settings. These mechanisms of flow reduction may prevent irreversible pulmonary vascular disease.
CONCLUSION: Distal pulmonary obstruction or elevated ventricular end-diastolic pressure can reduce pulmonary blood flow and may prevent irreversible pulmonary vascular disease. Clinical studies in this direction are recommended for validation. © Indian Association of Cardiovascular-Thoracic Surgeons 2019.