Effects of spontaneous ventilation on the circulation

Spontaneous inspiration causes an increase in right ventricular stroke volume due to a transient increase in venous return. In contrast, spontaneous inspiration causes a fall in left ventricular stroke volume which is exaggerated in conditions of accentuated pleural pressure swings and pericardial disease. This is manifested by the clinical sign of pulsus paradoxus. Recent evidence supports the idea that pulsus paradoxus is the result of two mechanisms. First, negative pleural pressure surrounding the left ventricle is equivalent to an increase in aortic pressure and impedes emptying of the left ventricle much like an increase in afterload. Second, distention of the right ventricle due to the transient increase in venous return causes an elevation of left ventricular diastolic pressure due to ventricular interdependence and therefore impedes left ventricular filling. The mechanism of negative pleural pressure "afterload" is thought to be the major cause of pulsus paradoxus when pleural pressure swings are accentuated. Ventricular interdependence is thought to predominate when pericardial disease is present.