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.
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Measuring the inferior vena cava's (IVC) variation with respirations gives us a dynamic metric to assess volume status. The valveless IVC correlates with right atrial pressure which can also be increased by tricuspid regurgitation, pulmonary hypertension, right ventricular dysfunction, etc. IVC measurements are highly dependent on the operator and, like most measurements used to assess volume status, only a tool to be interpreted with the overall clinical picture. The high collapsibility with respirations seen here SUGGESTS intravascular volume depletion whereas a less distensible IVC suggests adequate volume or elevated RA pressures.