Hemodynamic Impacts of Mechanical Ventilation: Assessment and Intervention

The position of the pulmonary circulation in the chest with blood flowing between the two ventricles has a lot of physiologic advantages but this positioning becomes disadvantageous for the heart and lungs when ventilation is produced with positive pressure ventilation instead of normal negative pressure ventilation. The two primary issues are that positive pressure ventilation increases pleural pressure and thereby reduces the potential venous return of blood to the heart in contrast to the normal potential increase in venous return with the negative pleural pressure of spontaneous breaths; secondly the increased forces needed for lung inflation can collapse vessels passing between alveolae and thereby increase pulmonary arterial pressure, increasing the load on the right heart, and ultimately leading to right heart failure. These issues are more problematic when the lungs are injured. Higher airway pressures are required to ventilate stiffer lungs to keep them open. Managing mechanically ventilated patients thus requires a good understanding of these heart-lung interactions and what tools can be used to optimize ventilator settings. A decrease in total lung compliance is a particularly difficult problem. Optimizing positive end-expiratory airway pressure can often help by opening collapsed lung units, but there is a price to pay for cardiac output. When the process decreasing lung compliance is not reversible, the high pressures needed to maintain ventilation can lead to greater lung injury and right ventricular failure. Therapeutic options become very limited.