What is the effect of inspiration on venous return to the right atrium?

From the Guidelines

Inspiration increases venous return to the right atrium by decreasing pleural pressure, which enhances the gradient for blood to flow back to the heart. This effect is primarily due to the decrease in pleural pressure (Ppl) during spontaneous inspiratory efforts, whether in a ventilated or non-ventilated patient, which effectively lowers the pressure baseline of the heart relative to the rest of the body 1. The decrease in Ppl increases the gradient for the return of blood to the right ventricle (RV), and this transient inspiratory increase in RV filling is then passed to the left ventricle (LV) over the next few beats.

Key Mechanisms

• Decrease in pleural pressure (Ppl) during inspiration, which lowers the pressure baseline of the heart relative to the rest of the body
• Increase in the gradient for blood to flow back to the heart, specifically to the right ventricle (RV)
• Transient inspiratory increase in RV filling, which is then passed to the left ventricle (LV) over the next few beats

Clinical Implications

• In certain clinical conditions, such as cardiac tamponade or constrictive pericarditis, the normal inspiratory augmentation of venous return may be impaired, leading to pulsus paradoxus or Kussmaul's sign respectively
• The respiratory pump mechanism is crucial during exercise when increased venous return is needed to support higher cardiac output
• Understanding the effects of inspiration on venous return is essential in managing patients with respiratory and cardiac conditions, such as ARDS, where mechanical ventilation can significantly impact hemodynamics 1

Important Considerations

• The pressure gradient from the upstream large venous reservoir to the heart is normally only 4–8 mmHg, making small changes in Ppl relatively large effects on cardiac output 1
• Increased LV afterload and increased RV filling during repetitive large negative swings in Ppl can increase pulmonary blood volume considerably, even producing pulmonary edema when LV function is decreased and capillary permeability is increased 1

From the Research

Effect of Inspiration on Venous Return

The effect of inspiration on venous return to the right atrium is a complex phenomenon that has been studied in various contexts.

• Research has shown that venous return may change in opposite direction according to the type of ventilation (spontaneous or assisted) 2.
• A study found that quiet inspiration was associated with a fall in femoral venous blood flow, which averaged 65 +/- 11% of the end-expiratory value 3.
• The decrease in femoral venous blood flow during inspiration was found to be related to the rise in abdominal pressure, with a greater decrease in flow observed when the diaphragm contributed more to inspiration and the rise in abdominal pressure was greater 3.
• In contrast, a study found that breathing through a low level of added resistance can increase circulation by enhancing venous return and cardiac stroke volume 4.
• Another study found that augmented respiratory muscle pressure production can affect locomotor limb venous return during calf contraction exercise, with selectively augmenting inspiratory intrathoracic pressure excursion during rib cage breathing augmenting inspiratory facilitation of femoral venous blood flow 5.
• A recent study found that negative pressure breathing, which involves breathing with decreased pressure in the respiratory tract without lowering pressure acting on the torso, can increase venous return to the heart and is considered as a countermeasure against redistribution of body fluids toward the head during spaceflight 6.

Mechanisms Involved

The mechanisms involved in the effect of inspiration on venous return are complex and multifactorial.

• The decrease in intrathoracic pressure during inspiration can increase venous return by increasing the pressure gradient between the peripheral veins and the right atrium 4.
• The rise in abdominal pressure during inspiration can decrease venous return from the legs by compressing the inferior vena cava 3.
• The type of ventilation, whether spontaneous or assisted, can also affect venous return, with spontaneous breathing resulting in a decrease in venous return and assisted breathing resulting in an increase 2.
• The level of rarefaction during inspiration can also affect the cardiorespiratory response, with higher levels of rarefaction resulting in greater changes in circulatory parameters 6.