Ultrasound of the inferior vena cava (IVC) provides very little benefit in evaluating a patient’s fluid status.
The IVC is Trash (for volume assessment)
This will certainly come as a shock to many clinicians who for years have casually placed a probe on a patient’s epigastrium and based on IVC collapsibility concluded that the patient either needed fluids or was adequately ‘tanked up.’ Doesn’t that seem too easy? The human body is about 60% water with fluid distribution managed by multiple organs and gradients influenced by electrolyte and protein concentrations. In critically ill patients, capillary leakage, vasoplegia, post-surgical status, vasopressors and/or mechanical ventilatory support can further complicate the picture. The IVC’s collapsibility with respirations has been considered the key to estimating fluid status, but the tidal volumes in spontaneously breathing patients, especially critically ill patients, change from breath to breath. There are just too many variables affecting IVC collapsibility for it to be a reliable tool to assist with determining fluid status and fluid responsiveness.
There are, however, some uncommon clinical scenarios where IVC respirovariability, when combined with the evaluation of a patient’s biomarkers, echocardiogram, physical exam, vitals, pulmonary status, and presenting symptoms, can contribute to the fluid status assessment
1) The IVC almost entirely collapses with respirations. This MAY indicate an under-resuscitated, hypovolemic patient.
2) The IVC is plethoric with minimal respirovariability. This MAY indicate a resuscitated or even hypervolemic patient.
3) The patient is compliant on a ventilator with no patient-triggered breaths and is receiving at least 8 cc/kg tidal volume. This is an uncommon occurrence and present in only ~2% of patients in an Intensive Care Unit. However, if present, the IVC distensibility index can be used: IVCmax diameter – IVCmin diameter / IVCmax diameter x 100. If this value is >12-18%, then the patient MAY be fluid responsive.
The IVC, despite its limitations in measuring fluid status and fluid responsiveness, remains useful in calculating pulmonary artery systolic and diastolic pressures, ruling out tamponade physiology, and confirming device placement.
- Use ultrasound of the IVC in conjunction with other variables (labs, exam, vitals, pulmonary status, thoracic ultrasound, echocardiography & LVOT VTI) to help determine fluid status and fluid responsiveness and whether or not a patient needs crystalloid.
- IVC respirovariability by itself is a poor tool for determining fluid status
- Ultrasound of the IVC is more useful for PA pressure calculations, ruling out cardiac tamponade, and confirming device placement.
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