Cardiopulmonary Interactions in Critical Illness
Understanding the mechanical interaction between respiration and circulation rests on understanding the individual elements. The pathophysiology of the circulation is best described by the control of the cardiac output by the systemic vessels, by the relationship between ventricular mechanics and cardiac output, and the coupling of the mechanical properties of systemic vessels with ventricular pumping function. Similarly, understanding the pathophysiology of respiration in critical illness is aided by a review of lung mechanics, pulmonary blood flow, and shunt in conditions of airflow obstruction and pulmonary edema. Because the central circulation including the heart is encased in a potential space between the lungs and chest wall (the pleural space), alterations in lung mechanics affecting intrathoracic pressure directly impinge on the central circulation and influence factors determining venous return. Furthermore, since pulmonary blood flow traverses small vessels surrounded by alveolar pressure, changes in alveolar pressure alter the distribution of pulmonary blood flow. On the other hand, vascular pressures in the central circulation in fluence lung liquid flux, cardiac output, and so indirectly influence intrapulmonary shunt. It is the purpose of this session to review aspects of these interactions, emphasizing the effects of alveolar pressure and the effects of vascular pressure on cardiorespiratory functions in critical illness. There will necessarily be some small overlap with the previous lecture discussing effects of intrathoracic pressure. The conceptual framework for this lecture is outlined below, including appropriate references.
KeywordsCardiac Output Critical Illness Pulmonary Blood Flow Intrathoracic Pressure Central Circulation
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