What is the initial haemodynamic response during a Valsalva manoeuvre?

During the Valsalva manoeuvre, a forced expiration against a closed airway leads to a rapid increase in intrathoracic pressure, which significantly affects venous return to the heart. This increased pressure in the thoracic cavity compresses the large veins, reducing the flow of blood back to the heart, resulting in decreased venous return.

As the venous return diminishes, the right atrial filling decreases, which subsequently leads to reduced preload and cardiac output. This series of physiological changes explains why the choice of decreased venous return to the heart correctly represents the initial haemodynamic response during the Valsalva manoeuvre.

The other options do not accurately capture the initial response. For instance, while systolic blood pressure may decrease following the initial reduction in venous return, this is a later response rather than an immediate effect. Likewise, the jugular venous pressure (JVP) would initially increase due to the reduced venous flow, and an increase in cardiac output would only occur after compensatory mechanisms kick in, not immediately during the manoeuvre.