Cardiovascular fluctuations during spontaneous breathing are well recognized; it is not surprising that mechanical ventilation can have profound effects on the circulation. Mechanical ventilation, through its effects on both lung volume and intrathoracic pressure, has an influence on the determinants of stroke volume for both right and left ventricles. It is now appreciated that intermittent positive-pressure ventilation has a more complex effect on cardiac output rather than simply decreasing venous blood flow as suggested originally.
When lung volume increases, pulmonary vascular resistance changes. Pulmonary blood vessels are of two major anatomical types. Alveolar blood vessels are closely related to the alveoli and are affected by changes in alveolar pressure. During inflation, the resistance of alveolar blood vessels increases and their capacity decreases. In contrast, extra-alveolar vessels are exposed to intrathoracic pressure changes and volumes. During inflation, the caliber of these vessels increases, their resistance falls, and their capacitance rises. Clearly, the net effect on pulmonary vascular resistance during changes in lung volume is a balance; in health pulmonary vascular resistance increases with lung inflation. In lung diseases characterized by hyperinflation, pulmonary vascular resistance is often elevated.
With lung deflation, there is little change in pulmonary vascular resistance in the alveolar vessels, whereas the reduced lung volume compresses and reduces the caliber of the extrathoracic vessels. The net effect is to increase pulmonary vascular resistance at low lung volumes. In the critically ill, diseases which lead to a reduced lung volume (e.g. acute respiratory distress syndrome) will tend to increase pulmonary vascular resistance. This effect will be exacerbated with other changes in the pulmonary vasculature. One goal of ventilatory therapy is to restore lung volume to functional residual capacity to normalize pulmonary vascular resistance in hyper- and hypoinflated lungs.
Lung volume will also impact on the cardiac system through direct mechanical compression of the heart. This may cause a restrictive effect similar to tamponade when pre-existing lung expansion occurs. As inflation pressures are transmitted to the cardiac chambers, the measurement of cardiac filling pressures becomes inaccurate. Additionally, the cardiac septum may become deviated. This shift in the position of the septum may occur towards or away from the left ventricle, impairing the function of either the right or the left ventricle. As the volume of the lungs increases, so intrathoracic pressure changes. During mechanical ventilation, intrathoracic pressure impedes blood flow to the right atrium and decreases right ventricular diastolic filling.
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