With inspiration, a Doppler signal from the subclavian vein will usually:

During inspiration, the phenomenon known as intrathoracic pressure changes occurs. When a person inhales, the diaphragm contracts and moves downwards, creating a negative pressure in the thoracic cavity. This negative pressure facilitates the return of blood from the upper body, including the subclavian vein, back to the heart.

As the blood volume returning to the heart increases during this phase, the Doppler signal from the subclavian vein typically shows an increase in velocity, reflecting the augmented blood flow. Therefore, the signal can indeed appear to augment with inspiration due to this enhanced venous return.

In contrast, when examining the other potential responses, a decrease in the Doppler signal would indicate a reduction in blood flow, which does not align with the physiological changes during inspiration. No change would imply that blood flow remains constant, also contrary to the expected dynamics during inhalation. The suggestion that the signal becomes triphasic is more characteristic of arterial flow rather than venous, particularly in the context of venous Doppler waveforms, which typically have a different pattern.