Ventilation has profound effects on the cardiovascular system through almost every aspect of its control. Four concepts determine most acute aspects of mechanical ventilation and its withdrawal. First, ventilation is exercise. A cardiovascular system unable to meet the increased metabolic demand of breathing will prevent successful weaning. Second, hyperinflation and increasing lung volume per se, and not the increase in airway pressure, determine the hemodynamic effect of ventilation. However, in a given patient, if ventilatory modes are matched for similar tidal volumes and mean airway pressures, then the hemodynamic effects of these modes of ventilation will also be similar. Third, although decreases in intrathoracic pressure increase left ventricular afterload, of more clinical relevance is the transition from spontaneous ventilatory efforts to positive pressure ventilation because the fall in venous return is much less, whereas the decrease in left ventricular afterload is identical for the same relative increase in intrathoracic pressure. Fourth, the decrease in venous return seen during positive pressure ventilation probably reflects an impairment in left ventricular filling rather than a reduction in the pressure gradient for venous return because of the associated increase in intra-abdominal pressure during inspiration.
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