IVC and right atrial pressure

Another heated topic. Well, things seem to be clear. When you read the guideline, say the one from ASE in 2010 below, they are quite clear:

(1) IVC diameter is normal (<2.1 cm) and collapses  >50% with a sniff: 3 mm Hg (0 - 5 mm Hg)
(2) IVC diameter is large (>2.1 cm) and collapses <50% with a sniff: 15 mm Hg (10-20 mm Hg)
(3) Either IVC diameter is normal (<2.1 cm) but collapses <50%, OR IVC diameter is large (>2.1 cm) but collapses >50%, 8 mm Hg (5-10 mm Hg)

Lang RM, Badano LP, Mor-Avi V, Afilalo J, Armstrong A, Ernande L, Flachskampf FA, Foster E, Goldstein SA, Kuznetsova T, Lancellotti P, Muraru D, Picard MH, Rietzschel ER, Rudski L, Spencer KT, Tsang W, Voigt JU. 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. 2015 Jan;28(1):1-39.e14. doi:10.1016/j.echo.2014.10.003. PubMed PMID: 25559473.

Questions

Which one could have higher RAP?
a. Large IVC diameter (>2.1 cm) but collapses > 50%
b. Normal IVC diameter (<2.1 cm) but collapses < 50%

When you read the article below, an impression is that b has a higher RAP. 

Brennan JM, Blair JE, Goonewardena S, Ronan A, Shah D, Vasaiwala S, Kirkpatrick JN, Spencer KT. Reappraisal of the use of inferior vena cava for estimating right atrial pressure. J Am Soc Echocardiogr. 2007 Jul;20(7):857-61. PubMed PMID: 17617312.

Two historical articles worth mentioning:

(1) Moreno FL, Hagan AD, Holmen JR, Pryor TA, Strickland RD, Castle CH.  Evaluation of size and dynamics of the inferior vena cava as an index of right-sided cardiac function. Am J Cardiol. 1984 Feb 1;53(4):579-85. PubMed PMID: 6695787.

(2) Kircher BJ, Himelman RB, Schiller NB. Noninvasive estimation of right atrial pressure from the inspiratory collapse of the inferior vena cava. Am J Cardiol. 1990 Aug 15;66(4):493-6. PubMed PMID: 2386120.

You are right, evidence has been there for more than 30 years. Yet, there seems to be some confusion about this. Why is that? I plan to talk more about IVC. Stay tuned.