Because the subclavian is guarded by the clavicle, you can't compress it with the transducer, and to see the vessel walls come together it is common practice to tell the patient to "sniff" or inhale quickly. This rapid inhalation obviously reduces intrathoracic pressure. But why would reducing intrathoracic pressure cause the subclavian vein to collapse? Shouldn't it dilate when this happens?
Your thought process is correct; in the absence of all other factors, physics dictates that the reduction of pressure surrounding a flexible fluid-filled vessel would result in expansion.
But there is an even stronger force: venous return, which is the flow back to the heart from peripheral circulation. (Resource linked is from the author of the cardiology physiology text used in many med schools.)
- Inhalation = diaphragm contracts, decreasing intrathoracic pressure
- This causes the R atrium to expand, reducing pressure in the R atrium
- This increases venous return to the R atrium by pulling from the SVC and IVC.
- If this occurs rapidly, then much like sucking liquid rapidly through a flexible tube, the tube (vein) can collapse.
Also this video is a good illustration of venous return and its impact on the IVC.
I couldn't find a diagram of exactly what I wanted so I altered a graphic to illustrate.