SVC provides 25% of venous return, IVC 75%
SVC is collapsible, to remain open CVP must be greater than closing pressure. Distending pressure=RAP-pleural pressure. Distending pressure is reduced by lung inflation and may become insufficient in hypovolemic patients. In other words, if pleural pressure is greater than CVP, SVC closes.
(Intensive Care Med. 2006 Feb;32(2):203-6.)
SVC collapsibility Index=SVC max-min/max >36% predicts positive response to fluids
12% increase in IVC diameter during lung inflation predicts need for fluids
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