Background

Assessing fluid responsiveness is essential to guiding resuscitation of critically ill patients. Inferior vena cava (IVC) collapsibility measured by point of care ultrasound (POCUS) has been shown to accurately predict fluid responsiveness in mechanically ventilated patients. However, it’s utility in spontaneously breathing patients is less well established.

Clinical Question

Can IVC collapsibility predict fluid responsiveness in spontaneously breathing critically ill patients undergoing resuscitation?

Population

Spontaneously breathing patients presenting with signs of acute circulatory failure (sustained hypotension or tachycardia, decreased urine output, or serum markers of hypo-perfusion) requiring ICU admission who presented to two urban adult academic hospitals in the US.

Intervention

Ultrasonographic measurement of IVC diameter with respiration before and after passive leg raise and 500cc normal saline bolus.
Measurements were obtained in subcostal long axis view while patients were supine
Calculation of collapsibility of IVC (cIVC), as defined as (IVC expiratory diameter-IVC inspiratory diameter)/IVC expiratory diameter
Determination of fluid responsiveness defined as a ≥10% increase in cardiac index following IVF bolus as measured by the Non-Invasive Cardiac Output Measurement device (NICOM™).

Outcomes

Primary Outcome: Ability of cIVC measurements to predict fluid responsiveness in spontaneously breathing critically-ill patients.
Secondary Outcomes:
Establish optimum cutoff value for cIVC, compare this value to previously suggested cutoffs.
Determine if incorporation of a passive leg raise with cIVC measurement assists in assessing fluid responsiveness.

Design

Prospective observational cohort study

Excluded

Primary traumatic, cardiogenic, obstructive, or neurogenic shock
<18 years of age, incarcerated, pregnant, or hospitalized >36 h
Receiving non-invasive positive pressure ventilation
Active pulmonary edema or estimation that further IVF would pose a clinical risk

Primary Results

  • 124 patients included
  • 61 patients (49.2%) fluid responsive based on > 10% increase in SVI as measured by NICOM™ Cheetah
  • Mean amount of IVF administered prior to enrollment: 4060ml (95% CI 3738 – 4831)

Critical Results

  • Primary Outcome (ability of cIVC to predict responsiveness)
    • Baseline measurement of cIVC predicted fluid responsiveness with AUC of 0.84 (95% CI: 0.76-0.91)
      • AUC is the area under the receiver operating characteristic curve for a given diagnostic test and measures discriminatory value
        • AUC of ≤0.75 considered of no clinical value
        • AUC of 0.97 correlates with LR of 10 and 0.1
  • Secondary Outcomes
    • cIVC cutoff (based on performance characteristics)
      • Optimal cutoff = 24.6%
      • Rounding to cIVC of 25%
        • Sensitivity: 87%
        • Specificity: 81%
        • PPV: 81.5%
        • NPV: 86.4%
        • (+) LR: 4.56
        • (-) LR: 0.16
      • Suggested cutoff produced lower misclassification rate (16.1%) than previous suggested cutoffs of 40% and 42%, which misclassified 34.7% and 36.3% of patients, respectively
    • Baseline maximum or minimum IVC diameters unable to predict fluid responsiveness
    • Change in cIVC following passive leg raise or IVF bolus was unable to predict fluid responsiveness

Strengths

  • Largest study to date examining POCUS measurement of cIVC in spontaneously breathing critically ill patients
  • Ultrasounds performed by EM and critical care physicians without formal ultrasound fellowship training
  • Ultrasound reviewers blinded to fluid responsiveness

Limitations

  • Included only medical ICU patients, no primary trauma, surgical or, cardiogenic shock patients
  • Heterogeneous patient population included septic patients, patients in DKA/HHS, and patients with GI hemorrhage
  • Respiratory effort was not standardized
  • Patients enrolled up to 36 hours post ED-triage and had already received an average of 4L of IVF
  • IVC measurements not performed in real time at bedside
  • Single subcostal long axis view of IVC and supine positioning of patients limits comparison of results to previously published literature
  • The accuracy of NICOM Cheetah device is not well established
  • The 25% cIVC correlating with the best performance characteristics was found from the data and not hypothesized up front. This decreases the likelihood that the same numbers will be found on external validation
  • Needs prospective and external validation as their cutoff is only based on their data set and can vary greatly in other cohorts.

Author's Conclusions

“IVC collapsibility, as measured by POCUS, is able to detect fluid responsiveness and may be used to guide IVF resuscitation among spontaneously breathing critically-ill patients.”

Our Conclusions

POCUS measurement of cIVC can be used to predict fluid responsiveness in spontaneously breathing patients with acute circulatory failure who have already received initial fluid resuscitation. In this study, a 25% cIVC cutoff provided acceptable test characteristics but, this value is based on the data in this study and will need external validation prior to adoption.

Potential Impact To Current Practice

There is still ongoing debate about whether using fluid responsiveness assessed by any means to guide resuscitation leads to clinically relevant and positive patient centered outcomes. However, considering qualitative assessment of IVC collapsibility using POCUS is already clinical practice of a number of EM physicians, this study adds validity to the role of those assessments. The ideal cIVC requires further study.

Bottom Line

Measurement of IVC collapsibility measured by POCUS can be a useful, quick assessment to assess fluid responsiveness in spontaneously breathing critically ill patients who have already completed initial fluid resuscitation.

Read More

Resus Room: October 2017 Papers

EMCrit: Vena Caval Ultrasound – Just Don’t Do It!

EMCrit: Podcast 86: IVC Ultrasound for Fluid Tolerance in Spontaneously Breathing Patients

Core EM: Diagnostic Accuracy of IVC Collapsablity to Predict Fluid Responsiveness in Spontaneously Breathing Patients