IVC Distensibility Index vs Collapsibility Index: Using the Correct Index

Background 

In 1979, Hiroshi Natori was the first to appreciate the sonographic changes that occur in the inferior vena cava (IVC)’s diameter with ventilation in spontaneously breathing patients, mechanically ventilated patients, and those with carcinogenic and tuberculoid cardiac tamponade.¹ They noticed how spontaneously breathing patients had their IVC collapse with inspiration; and ventilated patients had their IVC dilate during positive pressure ventilation. The use of point-of-care ultrasonography (POCUS) to evaluate the IVC as a marker for fluid responsiveness has spread from cardiology and nephrology into critical care and emergency medicine, transforming medical practice.

To go back to basics, position your probe approximately 1.5cm from the right atrium-IVC junction. Use M-mode Doppler to acquire information on IVC variability with at least three respiratory cycles.² In spontaneously breathing patients, calculate the IVC collapsibility index (cIVC) with the formula below:

Collapsibility IVC = ((IVC max – IVC min) / IVC Max ) * 100

Airapetian, et al in 2015 found that amongst 59 non-ventilated, non-intubated individuals, a collapsibility index of greater than 42% had a specificity of 97% and a positive predictive value of 90% for the likelihood of fluid response.³ Cutoff CI of 15% has a negative predictive value (NPV) of 100%. All the data ultimately comes down to: Very collapsed, probably fluid responsive. Very dilated, probably not fluid responsive. 

Positive pressure ventilation, with transmission of pressure through the thorax, will distend the inferior vena cava. As renowned educator and intensivist, Alex Yartsev, puts it: “The IVC is a blood-filled manometer”.⁴ Increased intrathoracic pressure will result in increased extra-thoracic venous volume by way of a laplacian effect on the right ventricle.⁴ 

intrathoracic pressure = increased extra-thoracic venous volume

Crudely put, a spontaneously breathing individual will almost always pull blood into their chest with inspiration, driven by the negative thoracic pressure created. This might appear as a “kissing” of the IVC’s walls during inspiration, a view that most emergency medicine physicians are likely to be familiar with in 2024. But with positive pressure ventilation, the IVC will distend with each breath applied. For more than just the correctness of terminology, the distensibility index should then be calculated. 

RESUS SCENARIO 

Picture this: you just arrived at your shift at the local emergency department. You’re barely logged in, coffee still in hand, when you get sign out on your soon-to-be newest patient. You’re told they were in respiratory distress, febrile, and borderline hypotensive. They failed non-invasive ventilation (NIV) and were then intubated with the aid of push-dose pressors. Do they need fluids, you think to yourself? Their capillary refill is quite sluggish. You astutely place a phased array probe on their upper abdomen to assess the likelihood of fluid responsiveness… and thus evaluate the IVC. 

Distensibility index (DI) 

Distensibility IVC = ((IVC max – IVC min) / IVC min ) * 100

The IVC distensibility index is used for IVC assessment in mechanically ventilated patients. Why does it matter? Well, the established cutoff for the distensibility index is 18%. A cutoff of 18% has a positive predictive value (PPV) of 92% and NPV of 90% for fluid responsiveness.5,6 As such, the cutoff ranges you are familiar with in spontaneously breathing individuals should not be applied to your mechanically ventilated patients. 

We see this physiological principle applied to patients when they become hypotensive after increasing positive-end expiratory pressure (PEEP).⁷ This effect is likely to worsen if the venous system is underfilled. Those little thin capillaries surrounding the alveoli are more likely to get squished with less volume in them. This results in increased pressure in those vessels, which will get transmitted to the right ventricle by way of increasing pulmonary artery pressures.^8,9 This may then create a devastating effect on systemic blood pressure. As such, ‘underfilled’ patients are more prone to hypotension with increased PEEP. 

Of course, IVC variability with ventilation is one part of the whole picture. An astute resuscitationist will likely deploy several instruments from their grab bag when assessing fluid status.   

Discussion

IVC’s diameter has been classically correlated with right atrial pressure (RAP). The classical teaching of IVC ultrasonography is simplified and summarized in the table below, thanks to the work of the American Society of Echocardiography.¹⁰

Put simply, we are playing a game of surrogates. The IVC diameter is used as a surrogate for RAP, which is one of the determinants of preload. Further complicated, the right atrial pressure and the IVC’s dance in the thoracoabdominal cavity is influenced by numerous factors. Some factors affecting venous return are summarized below:⁹

1. Afterload
2. Contractility of the myocardium
3. Total volume of blood in the venous system
4. Venous vessel tone
5. Venous obstruction (Thrombus)
6. Intrathoracic compliance (different in spontaneous vs positive pressure ventilation)
7. Pericardial compliance and Right atrial compliance
8. Atrial contractility
9. Valvular disease
10. Posture and positioning
11. Pregnancy

Multiple studies have looked at IVC ultrasound and its correlation with increased cardiac output, measured one way or another. As with most things in medicine, it is important to understand the pitfalls. Most measurements are somewhere around the hepatic confluence. M-mode measurements are prone to error because of a cylindrical effect.^11,12 Lastly, there is significant breath-to-breath variation in spontaneously breathing individuals.¹³ That is to say nothing of the effect that the type and response to shock has on the individual patients involved in these studies. 

Of course, there are other methods of assessing fluid ‘tolerance’:  Capillary refill evaluation, passive leg raise, central venous pressure measurement, pulmonary artery wedge pressures, stroke volume variation, pulse pressure variation, etc. However, none of these alone is likely to provide the clinician with a definitive answer as to the patient’s response to a fluid challenge. The most astute clinicians will likely use a combination of these tools to make a confident decision on their sickest patients. 

So, what is in your grab bag? You need not be sad regarding the lack of Swan Ganz catheters in your ED. There is evidence that pulmonary artery wedge pressures and central venous pressures fail to predict response to volume infusion.^14,15 You might use a passive leg raise, which carries a roughly 250cc fluid challenge. This has a pooled sensitivity of 89% and specificity of 91% ..just be careful with pelvic fractures or any femoral arterial punctures/ devices.16 Or maybe your patient has an arterial line, and if sedated/ mechanically ventilated, one can measure the pulse pressure variation. Simply put, a difference greater than or equal to 12% between the largest and smallest pulse pressure can predict fluid responsiveness.¹⁷ Of course, do not forget your physical exam. As seen in the Andromeda Shock Trial, and multiple other trials involving shocked patients, capillary return also reigns supreme regarding physical examination.^18,19 As seen in a trial involving shocked falciparum malaria patients, prolonged capillary refill has a specificity of 98% for identifying hypovolemia.¹⁹ These are all invaluable tools that providers can use at the bedside to assess the probability of a patient responding to IV fluids. 

Regarding caval indexes, the advent of artificial intelligence and advanced learning has become integrated into many ultrasound machines. Many companies have proprietary algorithms for evaluating and calculating both collapsibility and distensibility indexes for the IVC. ²⁰ However, their reliability when compared to expert analysis is yet to be widely accepted. 

Conclusion 

1. On patients receiving positive pressure ventilation, distensibility index, not collapsibility index, should be used.
2. The IVC’s appearance on ultrasonography is one part of the whole picture when assessing the likelihood of fluid responsiveness.
3. Distensibility index ≥ 18% likely predicts fluid responsiveness.

Clinical Bottom Line

But now, astute resuscitationist, you know that it is erroneous to use collapsibility index for your patient on invasive positive pressure ventilation. Instead, you calculate the Distensibility index, and factor that into your evaluation of the patient as a whole. 

Guest Post By:

Jean Carlo Medina MD
Emergency Medicine Resident (PGY-2)
Kendall Regional Medical Center

Nicole Aviles MD
Associate Program Director
Ultrasound Fellowship and Emergency Medicine Residency Program
Kendall Regional Medical Center

Jesus Seda MD
Associate Program Director
Emergency Medicine Residency
Kendall Regional Medical Center

References: 

1. Natori H, et al. Ultrasonographic evaluation of ventilatory effect on inferior vena cava configuration. Am Rev Respir Dis. Aug 1979; PMID: 475160.
2. Lang R.M, et al. Recommendations for Cardiac Chamber Quantification by Echocardiography in Adults: An Update from the American Society of Echocardiography and the European Association of Cardiovascular Imaging. J. Am. Soc. Echocardiogr. Jan 2015; PMID: 25559473
3. Airapetian N, et al. Does inferior vena cava respiratory variability predict fluid responsiveness in spontaneously breathing patients. Crit Care. Nov 2015; PMID: 26563768 
4. Yartsev, Alex. “Effects of positive pressure ventilation on cardiovascular physiology.” Mar 2024; Derangedphysiology.com.
5. Di Nicolò P, et al. Inferior Vena Cava Ultrasonography for Volume Status Evaluation: An Intriguing Promise Never Fulfilled. J Clin Med. Mar 2023; PMID: 36983218
6. Feissel M, et al. The respiratory variation in inferior vena cava diameter as a guide to fluid therapy. Intensive Care Med. Sep 2004;  PMID: 15045170
7. Luecke T, et al  “PEEP and cardiac output.” Anaesthesia, Pain, Intensive Care and Emergency Medicine—APICE:  Oct 1005; PMID: 16356246
8. Bilgili B, et al. The assessment of intravascular volume with inferior vena cava and internal jugular vein distensibility indexes in children undergoing urologic surgery. J Invest Surg Sept 2018; PMID: 28952826 
9. Robb J. Physiological changes occurring with positive pressure ventilation: Part Two. Intensive Crit Care Nurs. Dec 1997; PMID: 9564354.
10. Rudski LG, et al. Guidelines for the echocardiographic assessment of the right heart in adults: a report from the American Society of Echocardiography endorsed by the European Association of Echocardiography, a registered branch of the European Society of Cardiology, and the Canadian Society of Echocardiography. J Am Soc Echocardiogr. Jul 2010; PMID: 20620859.
11. Zhang Z, 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 Biol. May 2014; PMID: 24495437.
12. Sanfilippo F, et al. Assessment of the inferior vena cava collapsibility from subcostal and trans-hepatic imaging using both M-mode or artificial intelligence: a prospective study on healthy volunteers. Intensive Care Med Exp. Apr 2023; PMID: 37009935.
13. Muller L, et al. Respiratory variations of inferior vena cava diameter to predict fluid responsiveness in spontaneously breathing patients with acute circulatory failure: need for a cautious use. Crit Care. Oct 2012; PMID: 23043910
14. Kumar A, et al. Pulmonary artery occlusion pressure and central venous pressure fail to predict ventricular filling volume, cardiac performance, or the response to volume infusion in normal subjects. Crit Care Med. Mar 2004; PMID: 15090949.
15. Marik P, et al. Does the Central Venous Pressure Predict Fluid Responsiveness? An Updated Meta-Analysis and a Plea for Some Common Sense. Critical Care Medicine. July 2013; PMID: 23774337
16. Cavallaro, F et al: Diagnostic accuracy of passive leg raising for prediction of fluid responsiveness in adults: systematic review and meta-analysis of clinical studies. Intensive care medicine. Sep 2010; PMID: 20502865
17. Teboul JL, et al. Arterial Pulse Pressure Variation with Mechanical Ventilation. Am J Respir Crit Care Med. Jan 2019; PMID: 30138573.
18. Castro R, et al. Effects of capillary refill time-vs. lactate-targeted fluid resuscitation on regional, microcirculatory and hypoxia-related perfusion parameters in septic shock: a randomized controlled trial. Ann Intensive Care. Nov 2020; PMID: 33140173
19. Hanson J, et al. The reliability of the physical examination to guide fluid therapy in adults with severe falciparum malaria: an observational study. Malar J. Oct 2013; PMID: 24079262
20. Gohar E, et al. Artificial Intelligence (AI) versus POCUS Expert: A Validation Study of Three Automatic AI-Based, Real-Time, Hemodynamic Echocardiographic Assessment Tools. Journal of clinical medicine. Feb 2023; PMID: 36835888

Peer Reviewed By: Mark Ramzy, DO (X: @MRamzyDO), Marco Propersi, DO (X: @Marco_Propersi)