From the Guidelines
The increased gradient for venous return during inspiration does not lead to increased left-sided filling due to the effects of pulmonary blood pooling, interventricular dependence, and the time delay in blood traversing the pulmonary circulation. During inspiration, the decrease in pleural pressure (Ppl) increases the gradient for venous return to the right heart, as explained by experts in the field of intensive care medicine 1. However, this increased blood volume must first pass through the pulmonary circulation, which acts as a reservoir, temporarily holding the blood before it reaches the left heart.
The interventricular septum shifts toward the left ventricle as the right ventricle fills more during inspiration, due to their shared pericardial space, reducing left ventricular compliance and filling 1. This phenomenon, known as ventricular interdependence, contributes to the characteristic respiratory variation in left ventricular filling. Furthermore, there is a time delay as blood traverses the pulmonary circulation, meaning the increased right-sided filling during inspiration will not immediately translate to increased left-sided filling.
Key factors influencing this process include the effects of mechanical ventilation on hemodynamics, particularly how changes in Ppl and transpulmonary pressure impact cardiac function 1. The increase in Ppl during positive airway pressure decreases LV afterload but also decreases venous return to the RV and eventually to the LV, highlighting the complex interplay between respiratory and cardiac mechanics. Ultimately, these mechanisms explain why, despite increased venous return during inspiration, left ventricular stroke volume and systolic blood pressure typically do not increase immediately.
From the Research
Increased Gradient and Left-Sided Filling
- The increased gradient for venous return during inspiration does not lead to increased left-sided filling due to several factors, including increased left ventricular afterload and decreased left ventricular diastolic compliance 2.
- The increase in negative pleural pressure during inspiration may increase right heart volume due to increased venous return, but this does not necessarily translate to increased left ventricular filling 2.
- In fact, studies have shown that left ventricular stroke volume can decrease during inspiratory efforts, even when increases in venous return have been prevented 2.
Venous Return and Left Ventricle
- Venous return is the flow of blood from the systemic venous network towards the right heart, and it is influenced by factors such as mean systemic filling pressure, right atrial pressure, and resistance to venous return 3.
- However, the relationship between venous return and left ventricular filling is complex, and increased venous return does not always lead to increased left ventricular filling 4.
- For example, studies have shown that venous return from the legs can actually decrease during inspiration due to the rise in abdominal pressure, which can impede blood flow from the legs to the heart 4.
Clinical Implications
- Understanding the relationship between venous return and left ventricular filling is important for the management of patients with heart failure, where increased left ventricular filling pressure is a common feature 5.
- Vasodilators, such as nitroglycerin and nitroprusside, can be used to improve hemodynamics and symptoms in patients with acute heart failure, but their effects on left ventricular filling must be carefully considered 5.
- In chronic heart failure, the use of vasodilators and inotropic agents has been shown to have limited benefits, and a more nuanced understanding of the underlying pathophysiology is needed to develop effective therapies 6.