The subcostal view on transthoracic echocardiography can be used to examine the inferior vena cava (IVC), a large valveless vein which returns blood from the lower body to the right atrium (RA).
With spontaneous inspirations, the diaphragm contacts downward generating a negative intrathoracic pressure and drawing blood from the superior vena cava (SVC) and IVC into the RA. This, in return, will partially collapse the IVC as it is emptied. In comparison, patients who are intubated and on positive pressure ventilation will increase their intrathoracic pressure during inspiration and push blood out of the heart into the IVC. In either situation, the magnitude of IVC change during breathing is said to roughly correspond to intravascular volume status. In other words, a patient is who intravascularly overloaded will not have significant variability (collapse nor distension) during breathing.
Keep in mind that anything that increases right atrial pressure can be backtransmitted into the IVC (ie, pulmonary hypertension, tricuspid regurgitation, thrombus, etc.) Additionally, the ultrasonographer’s experience and skill level affect the quality of IVC measurements.