Figure 1.

Heart–lung interaction, and hemodynamic effects of mechanical ventilation (a) and weaning/spontaneous breathing trial (b). During mechanical ventilation, the global effect of positive intrathoracic pressure is to reduce cardiac and global oxygen demand. During weaning and especially during spontaneous breathing trials, the reduction or abolition of positive intrathoracic pressure produces an increase in cardiac workload, while increased work of breathing, stress, and anxiety raise the global oxygen demand. Ppl, intrapleural pressure; Palv, alveolar pressure; RV, right ventricle; LV left ventricle.