Ultrasound evaluation of the inferior vena cava (IVC) provides rapid, noninvasive assessment of a patient's hemodynamic status at the bedside. The size of the IVC and its respiratory variability has been shown to correlate with right atrial pressure (RAP) and intravascular volume. These observations are valuable in estimating RAP, detecting changes in intravascular volume, and monitoring a patient's response to volume resuscitation.
Structurally, the IVC is a thin-walled, highly compliant vessel. Its size and dynamics vary with respiration and changes in intravascular volume. The development of negative intrathoracic pressure during inspiration increases the venous blood return from the extrathoracic veins into the right heart. This leads to an increase in the blood flow through the IVC and a subsequent decrease in its blood volume, resulting in a reduction in intraluminal pressure. These changes decrease the diameter of the IVC during inspiration relative to expiration. These observations are reversed with positive pressure ventilation in which IVC diameter increases during inspiration.
In patients with a low RAP and/or intravascular volume, the IVC size is relatively decreased and its respiratory variability is increased. If RAP is very low, the IVC can collapse completely during spontaneous inspiration. In patients with high RAP and/or intravascular volume, the IVC size is increased and its respiratory variability is decreased. The IVC is very compliant, but its capacity to distend is not unlimited and is restricted by connective tissue in its walls and surrounding structures.
Traditionally, central venous pressure (CVP) and volume status in the acute care setting have been measured by placing a central line. Central lines are invasive, time consuming to insert, and may cause significant complications. Bedside ultrasound has been shown to provide a good estimation of CVP in place of more invasive methods. The clinician can perform serial IVC measurements on an ill patient in order to guide their decision in providing more intravenous fluids or to administer more aggressive medication therapy.
Bedside ultrasound evaluation of the IVC should be performed in:
- The patient who requires an estimation of intravascular volume status, who does not have a central line, or is at a facility that does not have the ability to measure CVP
- Any patient undergoing fluid resuscitation in order to monitor their response and need for additional fluids or medications
Curvilinear or Phased-Array Probe
To visualize the IVC, a phased-array (frequency of 2.0–4.0 MHz) or curvilinear probe (frequency of 3.5–5.0 MHz) should be used. These relatively low-frequency probes provide better penetration and visualization of deep structures.
Depth of field should be adjusted to allow complete visualization of the IVC and its entrance into the right atrium. The depth needed will mostly depend on the habitus of the patient. Obese patients will naturally have deeper vessels and therefore require an increased depth setting.