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IVC
I really like the IVC. I know many people don't. But I think those people try to push the IVC into giving them a concrete answer; whereas the IVC is more nuanced and needs to be interpreted with all the other findings. I use the IVC to confirm the pathology which I have seen. Eg pericardial effusion with signs of tamponade, acutely dilated RV ?PE - if the IVC is not dilated and non collapsible as I would expect, it would make me question my other findings. Similarly, hyperdynamic LV ?hypovolaemia, if the IVC isn't flat and collapsing, I would consider other things like ?PE as a cause of the hyperdynamic LV.
The IVC is best viewed from the subxyphoid with the transducer marker to 12 o clock and the transducer fanning towards the liver. To make sure it is the IVC, ensure that you also visualise the IVC opening into the RA. Too much rotation and you will be viewing the aorta,
Ideal image of IVC
It is important to be able to differentiate the IVC from the aorta. Otherwise you may think the aorta is a dilated non collapsing IVC.
Subxyphoid view of the aorta. Note how there is only spine poeterior to the aorta. IVC would have liver.
MEASURING THE IVC
The IVC is measured in inspiration and expiration from inner wall to inner wall at least 2 cm from the RA opening. This is usually distal to the hepatic vein. This is because the IVC is tethered at the RA by the pericardium and so there is less respiratory variation close to the RA.
M mode of IVC
IVC measurement in 2D
The measurement can be done in 2D or using M mode. It is probably easier to get an accurate reading with 2D. On M mode: make sure the m mode line is perpendicular to the axis of the IVC otherwise the measurement will be incorrect.
In the unventilated patient, the negative intrathoracic pressure generation with patent inspiration increases RV filling leading to IVC collapse with inspiration. Normally, the IVC will collapse ≥ 50%. In the ventilated patient, the IVC will increase in size with inspiration >12%.
In formal echo, the IVC size and collapsibility is used to estimate RA pressure.
IVC
>2.1cm <50% collapse = elevated RAP
FLUID RESPONSIVENESS
In critical care, IVC has traditionally been used as a marker of fluid responsiveness. Respiratory variation >50% was considered to indicate fluid responsiveness. However, a recent systematic review and metanalysis highlights the heterogeneity of the data (see table below).
Studies of IVC to assess fluid responsiveness from Long et al 2017 (1)
Long et al (1,2) found that the overall sensitivity and specificity of IVC collapse for fluid responsiveness is only 64% and 76% respectively. An increase in IVC size with inspiration in the mechanically ventilated patient is much more predictive of fluid responsiveness (3).
In general, the consensus with IVC is: A very small IVC <1cm with >50% collapse is highly sensitive for hypovolaemic shock (4).
For all other values and permutations, use multiple IVC measurements to determine a trend. A patient with an IVC which is collapsing >50% may have a fluid challenge. Reassess after the fluid to see if there is a change in collapsibility.
IVC <1cm + >50% collapse is more predictive of hypovolaemic shock
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REFERENCES
1. Long E, Oakley E, Duke T, Babl FE; Paediatric Research in Emergency Departments International Collaborative (PREDICT). Does Respiratory Variation in Inferior Vena Cava Diameter Predict Fluid Responsiveness: A Systematic Review and Meta-Analysis. Shock. 2017 May;47(5):550-559. doi: 10.1097/SHK.0000000000000801. PMID: 28410544.
2. Orso D, Paoli I, Piani T, Cilenti FL, Cristiani L, Guglielmo N. Accuracy of Ultrasonographic Measurements of Inferior Vena Cava to Determine Fluid Responsiveness: A Systematic Review and Meta-Analysis. J Intensive Care Med. 2020 Apr;35(4):354-363. doi: 10.1177/0885066617752308. Epub 2018 Jan 17. PMID: 29343170.
3. Zhang Z, Xu X, Ye S, Xu L. 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. 2014 May;40(5):845-53. doi: 10.1016/j.ultrasmedbio.2013.12.010. Epub 2014 Feb 2. PMID: 24495437.
4. Yanagawa Y, Sakamoto T, Okada Y. Hypovolemic shock evaluated by sonographic measurement of the inferior vena cava during resuscitation in trauma patients. J Trauma. 2007 Dec;63(6):1245-8; discussion 1248. doi: 10.1097/TA.0b013e318068d72b. PMID: 18212645.