Fluid responsiveness is key in guiding the resuscitation of critically ill patients, and both under and over resuscitation can lead to poor clinical outcomes. Vitals and physical exam are not always reliable in determining fluid responsiveness. The search for a quick, easy and accurate diagnostic test to determine fluid responsiveness is ongoing. IVC collapsibility (cIVC) has been proposed as a helpful measure, and in ventilated patients this measurement has been validated. However, spontaneously breathing patients have different physiology, so it is unclear if cIVC is an accurate predictor of fluid responsiveness in this cohort. Previous studies have recommended a cIVC cutoff of 40-42% as a reliable predictor of fluid responsiveness in spontaneously breathing patients (1,2). This study sought to validate those findings.
Can the collapsibility of the inferior vena cava differentiate between fluid responders and fluid non-responders in non-ventilated critically ill patients?
Methods & Study Design
This was a prospective observational trial.
Inclusion: spontaneously breathing patients with signs of acute circulatory failure in the ED and ICU of 2 academic hospitals in the US.
Exclusion: primary traumatic, cardiogenic, obstructive, or neurogenic shock; age < 18 years old; incarceration; pregnancy; and/or hospitalization for >36 h; NIPPV; if the clinical team felt that they had active pulmonary edema; or that believed that further IVFs might pose a clinical risk.
A NICOM device monitored cardiac index at 1 minute intervals for the duration of the study. Patients had initial cardiac index measurements and IVC videos recorded. Then, after a 3 minute passive leg raise, an additional IVC video was recorded. Lastly, patients received a 500ml normal saline bolus and immediately had a final IVC video recorded. Images were reviewed after the study to determine the cIVC.
The primary outcome was fluid responsiveness, defined as a ≥ 10% increase in cardiac index.
A cIVC of 25% provided maximum sensitivity (87%) and specificity (81%) in identifying fluid responders. However, as you can see in the figure below, there were several patients with cIVC below 25% who were fluid responders, and several patients with cIVC above 25% who were not fluid responders.
Strength & Limitations
Strengths: The prospective study design reduces bias and confounding factors. Few studies have examined fluid responsiveness in non-ventilated patients, and this adds to the growing body of evidence in this population.
Limitations: This study primarily included patients with severe sepsis/septic shock and DKA/HHS. This limits the generalizability of the findings to other forms of acute circulatory failure. Furthermore, fluid responsiveness was measured once immediately following the bolus. Monitoring the patients’ clinical status over several hours or for the duration of the ICU admission may have provided additional, clinically relevant data regarding fluid resuscitation.
“cIVC, as measured by POCUS, is able to detect fluid responsiveness and may be used to guide IVF resuscitation among spontaneously breathing critically-ill patients.”
In spontaneously breathing patients with distributive shock, cIVC can be a useful tool in identifying fluid responsiveness, with the caveat that a minority of patients with minimal IVC collapsibility may still be fluid responders and those with significant IVC collapsibility may not be fluid responders. There is certainly a trend toward a collapsible IVC identifying fluid responders, but the outliers in this study should be taken into account. IVC, along with history, exam and bedside echo, can be used to identify which patients may need more IV fluid resuscitation.
The Bottom Line
IVC collapsibility >25% predicts fluid responsiveness in spontaneously breathing patients with distributive shock most of the time, but should not be solely relied upon.
This post was written by Aaser Ali, MS4 at UCSD School of Medicine, Charles Murchison, MD and Amir Aminlari, MD.
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