This week Shane O'Donnell and I discuss how we can use the IVC as a secondary data point in patient assessment. There are a lot of myths about utilizing the IVC to guide fluid management. Like any data we gather, the way we interpret and synthesize our findings will be predicated on a clear understanding of what the IVC assessment is and isn't.
What the IVC isn't
A way to decide between fluid or pressors
What the IVC is
A representation of the pressure gradient between systemic venous pressure and right atrial pressure.
Topics discussed in this episode
True vs. False Diameter
The IVC is a cylinder that we bisect to view collapsibility and size. If we have our probe too close to the edge of that cylinder, it can appear that the diameter of the IVC is smaller than it actually is.
Utilizing M- Mode to measure collapsibility
To quantify the collapsibility, you can drop an M-line down the center of the IVC and measure the diameter on inhalation and exhalation. It is important to note that the collapse % of the IVC is a representation of right atrial pressure to systemic venous pressure gradient and the patient's work of breathing.
Normal collapsibility is roughly 50%. However, if you are measuring this on yourself, relaxing calmy in the station recliner, it is unlikely you will see 50% collapse.
Flat IVC and Hyperkinetic Heart Does Not Always Equal Hypovolemic
When resuscitating a patient in a vasodilatory shock, assessment of the heart and IVC will likely show a high ejection fraction and collapsing IVC. The natural instinct is to think this correlates with hypovolemia. However, if the patient has already received a moderate amount of fluid resuscitation, it is important to not withhold pressors to restore the vascular tone that is lost with a distributive shock. Waiting on the IVC to get plump before starting pressors is the wrong way to address distributive shock. Dr. Farkas explains this very well in this blog https://emcrit.org/pulmcrit/mythbusting-empty-ivc-hyperkinetic-heart-≠-volume-depletion/
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