From the Guidelines
Inspiration decreases left heart filling due to the increase in right heart filling and the subsequent bulging of the interventricular septum into the left ventricle, as well as the increased pulmonary vascular capacity and compression of the heart by the expanded lungs. During inspiration, the decrease in pleural pressure (Ppl) increases the gradient for venous return to the right heart, leading to increased right ventricular (RV) filling 1. This increased RV filling causes the interventricular septum to bulge into the left ventricle, reducing left ventricular compliance and filling. Additionally, the decrease in Ppl increases pulmonary vascular capacity, temporarily pooling blood in the lungs and reducing pulmonary venous return to the left heart 1. The expanded lungs also compress the heart, particularly the thin-walled atria, further impeding left heart filling.
Some key points to consider include:
- The decrease in Ppl during inspiration increases the gradient for venous return to the right heart, leading to increased RV filling 1
- The increased RV filling causes the interventricular septum to bulge into the left ventricle, reducing left ventricular compliance and filling 1
- The increased pulmonary vascular capacity during inspiration temporarily pools blood in the lungs and reduces pulmonary venous return to the left heart 1
- The compression of the heart by the expanded lungs further impedes left heart filling 1
These combined effects result in decreased left ventricular preload during inspiration, leading to reduced stroke volume and systolic blood pressure. This mechanism is a normal physiological response to inspiration, but can be exaggerated in certain cardiac conditions such as cardiac tamponade or constrictive pericarditis.
From the Research
Mechanisms of Decreased Left Heart Filling during Inspiration
- The decrease in left heart filling during inspiration can be attributed to several mechanisms, including:
- Increased left ventricular outflow impedance 2
- Ventricular interdependence, where the increase in right ventricular volume affects left ventricular function 3, 4
- Reduced pulmonary venous flow 4
- Increased afterload on the left ventricle due to reduced intrathoracic pressure 4
- Alterations in left ventricular geometry and filling caused by acute changes in intrathoracic pressure 5
Effects of Intrathoracic Pressure Changes on Left Ventricular Function
- Negative intrathoracic pressure (NITP) decreases left ventricular filling and emptying independently 3
- NITP confined to diastole decreases left ventricular filling and stroke volume, likely due to ventricular interdependence 3
- NITP confined to systole also decreases left ventricular stroke volume, possibly due to increased afterload on the left ventricle 3
- Acute increases or decreases in intrathoracic pressure alter left ventricular geometry and filling 5
Hemodynamic Effects of Cardiac Cycle-Specific Increases in Intrathoracic Pressure
- Cardiac cycle-specific increases in intrathoracic pressure can affect cardiac performance by altering ventricular loading conditions 6
- Pulses timed to occur in early diastole can decrease left ventricular filling pressure and stroke volume 6
- In the setting of acute ventricular failure, pulses can increase left ventricular performance by selectively affecting left ventricular preload and afterload 6