Mechanical respiratory support with positive-pressure ventilation is commonly used in critically ill patients with associated acute lung injury. Elevated intrathoracic pressure affects regional hemodynamics and may lead to dysfunction of other organs. The interactions of lung, gut, liver, kidney, and brain are of particular importance in critical illness.

The mechanisms by which elevated intrathoracic pressure may affect local circulations are as follows:

1. a global reduction in cardiac output and systemic arterial pressure;

2. impairment of venous return and elevation of venous pressure;

3. modification of local vasoactive status;

4. alterations in the distribution of microvascular flow.

The overall effect of elevated intrathoracic pressure on other organs depends on the magnitude of pressure transmission, the patient's volemic status, and the integrity of autoregulatory mechanisms in the regional circulation. Improvements in pulmonary function and arterial oxygenation due to positive-pressure ventilation should counteract the deleterious effects on local hemodynamics. However, the lowest possible peak inspiratory and end-expiratory pressures should be employed in patients with severely impaired autoregulatory mechanisms.

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