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

Background

Background: Assessing fluid responsiveness in patients in shock is crucial as fluid balance is important in their management. Identifying patients who are fluid responsive allows us to rapidly increase their cardiac index. However, rapid infusion does not benefit all patients in shock and in some, may be harmful. As a result, it is crucial to identify parameters that can aid in identifying patients who may benefit from volume expansion. In past studies, inferior vena cava collapsibility (cIVC) of >18% in mechanically ventilated patients who were in septic shock showed both good sensitivity and specificity in identifying patients who were fluid responsive (Vieillard-baron 2004). However, a few small studies that looked at this correlation in spontaneously breathing patients, illustrating high specificity but low sensitivity of the marker (Xu 2014). This may be secondary to low tidal volumes. The researchers in this study predicted that standardization of inspiration strength may increase the sensitivity of cIVC in identifying patients who are fluid responsive. 

Primary Results

• 112 patients enrolled in the study
• 22 were excluded
  • Nonmeasurable parameters (SVI or IVC)
  • Signs of pulmonary edema due to heart failure
  • Declined post volume expansion imaging
• No significant difference in baseline demographic characteristics between the two groups
• 90/112 (80%) got pre and post volume expansion imaging
• Using 48% in cIVC in standardized breathing patients has a sensitivity of 84% and specificity of 90% in identifying patients who are fluid responsive
• A patient with a cIVC >48% is approximately 8 more likely to be fluid responsive compared to those that are not fluid responsive

Critical Findings

Hemodynamic Parameters	Area Under Receiver Operating Characteristic Curve (95% CI)	Threshold	Sensitivity (95% CI)	Specificity (95% CI)	LR+	LR-
cIVC- Standardized (%)	0.89 [0.82-0.97]	>48 >39 >48	0.84 [0.71-0.93] 0.9	0.90 [0.76-0.970] 0.90	8.4	0.18
cIVC- Spontaneous (%)	0.82 [0.73-0.91]	>31 >13 >41	0.76 [0.62-0.87] 0.90	0.88 [0.73-0.96] 0.90	3.45	0.27
Stroke volume index (ml/m^2)	0.61 [0.50-0.73]	<30 <37 <22	0.54 [0.40-0.68] 0.90	0.65 [0.48-0.79] 0.90	1.54	0.71

Strengths

• First study to report the role of IVC collapsibility in a standardized spontaneous breathing patient population
• cIVC-st has a good diagnostic value with 95% CI of its area under ROC curve greater than 0.80
• Ultrasonographic measurements were analyzed offline, on anonymous records by operators that were blinded to clinical data

Limitations

• The bedside ultrasound was performed by advanced users who underwent more training making it limited to a small number of users in the ED
• The study was conducted in 2 hospitals and it was in ICU setting. Study population comprised only septic patients with no or low dose of norepinephrine, and few or no patients with chronic heart failure, active exhalation, arrhythmia, abdominal compartment syndrome, pregnancy, and obesity. This limits generalizability
• The 48% change in 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
• Used buccal pressure measurement to insure that patient generated enough pressure during standardized breath limiting its use in the ED
• 25 patients (28%) were unable to generate an inspiratory pressure below –5mm Hg.
• Used colloid solution rather than crystalloid solution for volume expansion
• Norepinephrine infusion may decrease IVC compliance at low central venous pressure levels (i.e., and did identify doses of norepinephrine that can lead to cIVCst misinterpretations
• Needs prospective and external validation as their cutoff is only based on their data set and can vary great in other cohorts

Author's Conclusions

“The collapsibility index of the inferior vena cava during a deep standardized inspiration is a simple, noninvasive bedside predictor of fluid responsiveness in nonintubated patients with sepsis-related acute circulatory failure.”

Our Conclusions

In this study, using a cIVC cutoff of 48% in septic patients in acute circulatory failure undergoing a standardized deep inspiration improved the sensitivity in identifying patients who are fluid responsive.

Potential Impact To Current Practice

Being able to assess fluid responsiveness non-invasively in spontaneously breathing patients would be very helpful in clinical practice. cIVC with a standardized deep inspiration is promising but prospective and external validation studies are needed before this approach is embraced.

Bottom Line

Using POCUS to look at cIVC in spontaneously breathing patients may allow us to identify those patients who are fluid responsive. However, performance of standardized breathing, obtaining high-quality images and accurate measurements on US as well as the need for external and prospective validation limit the usefulness of these results.

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References

Vieillard-baron A et al. Respiratory changes in inferior vena cava diameter are helpful in predicting fluid responsiveness in ventilated septic patients. Intensive care med. 2004 30(9): 1740-6. PMID: 15034650

Xu L et al. Ultrasonographic measurement of the respiratory variation in the inferior vena cava diameter is predictive of fluid responsiveness in critically ill patients: systematic review and meta-analysis. Ultrasound med biology 2014; 40(5): 845-53. PMID: 24495437