16 The inferior vena cava – not just a homogenous tube but a complex structure which may hold useful information when interpreted correctly

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Abstract

Introduction

The inferior vena cava (IVC) is a pliable capacitance vessel for circulating volume. Its current use is as per the American society of echocardiography (ASE) guidelines as an estimate of central venous pressure (CVP) based on maximal diameter (IVCD) and collapsibility index (IVCCI). The use of CVP to estimate volume status is useful in a range of conditions including heart failure and sepsis. Further consideration should be given to where and how these metrics are obtained, either on deep inspiration or sniff as well as location on the IVC where measurements are taken. More detailed, quantitative analysis rather than binary cutoffs may yield more clinically useful information, especially in patients who would be considered borderline volume overloaded by ASE criteria.

Methods

This was a sub study of IVC behaviour in heart failure patients. In addition to the standard measurements of maximal diameter and collapsibility index, we looked to see if there was variation in IVCCI by method of inspiration – deep inspiration vs sniff – as well as looking for variation in IVC behavior by station along the IVC where measurements were taken – between the hepatic vein / IVC junction and right atrium vs 1–2 cm caudad to the hepatic vein. IVC metrics were compared to serum B-type natriuretic peptide (BNP) and N-terminal pro-B-type natriuretic peptide (NT pro-BNP) as crude surrogates for volume status. As this was a small substudy, IVC metrics were detailed using descriptive statistics only.

Results

14 patients were included in total (table 1). 3 subgroups were identified:

Results

1) Patients with IVCCI >40% with both inspiration and sniff with minimal difference (<19%), n=4 2) Patients with IVCCI <40% with both inspiration and sniff with minimal difference (<19%), n=4 3) Patients with difference in IVCCI on inspiration and sniff of >19%, n=6

Results

As expected, the group with IVCCI >40% appeared to have a lower BNP and better renal function than the group with a low IVCCI. However, interestingly, the group with a low IVCCI with deep inspiration and a more normal IVCCI with sniff appeared to behave similarly to the volume overloaded patients with a higher NT pro-BNP.

Results

When comparing position of where IVCCI was calculated along the IVC, patients with similar degrees of collapse at both sites (i.e. difference < +/-9%) had a higher BNP – average of 803 pg/ml, compared to those where there was a more marked discrepancy between IVCCI between the two stations – average of 208 pg/ml, suggesting reduced pliability due to increased circulating volume.

Conclusion

Though a small observational substudy, there is potential to suggest that the IVC is not a homogenous, predictable collapsible tube. There may be a role for greater understanding of IVC anatomy and behaviour to more accurately assess volume status which could provide a useful tool for heart failure monitoring in volume-sensitive patients.

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