Regulation of the Pulmonary Circulation
Individual investigations have shown that a wide variety of factors influence the pulmonary circulation. These factors include mechanical ventilation, hydration, cardiac output, pH, PCO2,PO2, age, sex, and many drugs and diseases. In practice, however, the changes are often quite subtle and this paradox between the expected changes and the apparent observed outcome can be understood by the recognition of two basic peculiarities of the pulmonary circuit. The first is the fact that the pulmonary circuit is quite unlike the systemic circulation in almost every way (1), it is a high conductance (low resistance), low pressure, high flow system. Large changes in flows can be accommodated with small changes in pressure and conversely small changes in pressure or vascular conductance can represent large changes in physiological or pathophysiological conditions. A 5 mmHg change in blood pressure, which in the systemic circulation would be trivial, in the pulmonary circulation represents the pressure change that accompanies a two-fold increase in cardiac output. The second peculiarity derives from the first; the low hydrostatic pressure, the thin walled vasculature, the presence of the alveolar gas space and the requirements for ventilation all interact with each other in non-linear and unexpected ways (2). The result has often been that the fundamental actions of a drug, disease or ventilatory technique observed in vivo are difficult to interpret.
KeywordsLeft Atrial Pulmonary Artery Pressure Pulmonary Circulation Hypoxic Pulmonary Vasoconstriction Flow Diversion
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